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An automatic scanning system for nuclear emulsion ... - opera - Infn

for oscillation in atmospheric neutrinos has been confirmed independentlyby K2K [8], which is the first long baseline experiment.The Standard Model is in a paradox situation: it is extremely successfulin describing elementary particles and their interactions but it isbelieved to be still incomplete. The discovery of neutrino oscillations requiresa change in order to accommodate three massive neutrinos, threemixing angles and one Dirac-type CP-phase. Neutrinos are the only neutralfundamental fermions and therefore a Majorana mass term can appearin the Lagrangian. In that case also two Majorana phases enter into theextension of the Standard Model.The mixing of neutrinos is very different from that of quarks, since thereare two large mixing angles, which were completely unexpected: in analogywith the quark sector the common belief was that mixing angles should besmall, if any. Neutrino masses are also peculiar because they are at leastfive order of magnitude smaller than the mass of the electron. These factspose a major challenge to any theory of neutrino masses and mixing: whyare neutrino masses so small? Why is the neutrino mixing pattern so differentfrom that of the quarks? What is the pattern of neutrino masses? Theobservation of neutrino oscillations has motivated fundamental researchesto answer these questions.The smallness of neutrino masses is usually accounted for by the socalledsee-saw mechanism [9] [10], i.e. heavy right handed singlet neutrinoswith masses around GUT-scale suppress the neutrino mass to very smallvalues. In such a scenario neutrino masses are a probe of very high energyscales which may otherwise be not accessible. It turns out that it is farfrom trivial to construct a theory which can account for the observed mixingpattern, i.e. predict two large mixing angles.This chapter will give in its first part an introduction to the basic formalismof neutrino oscillation. First the simple case of two flavor oscillationsin vacuum will be introduced. The two neutrino framework is thengeneralized to the three neutrino case where also the standard parametrizationof the leptonic mixing matrix is presented. The second part of thischapter deals with the experimental evidence for neutrino oscillations andthe current global understanding of oscillations. Finally the most recentbest fit values and errors on the neutrino mixing parameters are given.1.2 Neutrino oscillations theory1.2.1 Two flavorsNeutrino oscillation is a quantum mechanical effect, which can be illustratedby a two-state system and it was first discussed in [11]. Let us4


assume |ν i 〉 to be the stationary eigenstates of the free Hamiltonian thetime evolution of the state |ν i (t)〉 is given by|ν i (t)〉 = e −iE it |ν i 〉 (1.1)In the context of neutrino oscillations the eigenstates of the Hamiltonianare called mass eigenstastes since the Hamiltonian is proportional to thediagonal matrix diag(m 2 1, m 2 2), where m 1 and m 2 denote the masses of thestates |ν 1 〉 and |ν 2 〉. Neutrinos are produced and detected with a definiteflavor, i.e. as eigenstates of the weak interaction, which are denoted by|ν α 〉. These weak eigenstates do not necessarily coincide with the masseigenstates. Therefore a linear superposition of mass eigenstates may becreated in the neutrino production and the transformation between thetwo basis is determined by a unitary matrix [12], which is 2 × 2 in the twoneutrino case|ν α 〉 = ∑ Uαi|ν ∗ i 〉 (1.2)iThe general vacuum oscillation probability is then given by 1P(ν α → ν β ) = |〈ν β |e −iE it |ν α 〉| 2 = ∑ ijU αj U ∗ βjU ∗ αiU βi e −i ∆m2 ij L2E (1.3)where ∆m 2 ij = m 2 i − m 2 j is called mass splitting. Thus neutrino oscillationare only sensitive to mass differences but not to the absolute neutrino massscale.In the case of two neutrinos the unitary matrix U can be parameterizedin the following way: ( )cos θ sin θ(1.4)− sin θ cos θwhere θ is called mixing angle. Inserting this in eq.1.3 yields the followingsimple expressions for the oscillation probabilities( ) ∆mP(ν α → ν β ) = sin 2 2θ sin 2 2 L4E( ) (1.5)∆mP(ν α → ν α ) = 1 − sin 2 2θ sin 2 2 L4Ewhere L is the distance travelled by the neutrinos, usually called baseline,and E is the neutrino energy. P(ν α → ν β ) is called appearance probability,since the flavor β appears in the final state while P(ν α → ν α ) is calledsurvival probability. Moreover P(ν α → ν β ) is invariant under time reversaland CP-conjugation, since in the two flavors case there is no CP violationin neutrino oscillations.1 In the ultra-relativistic limit m i ≪ E ν .5


due to the CP-asymmetry of matter.In the case of three neutrinos there are two independent mass splittingsdenoted by ∆m 2 12 and ∆m 2 23. Oscillations in a three-generation scenarioare consequently described by six independent parameters: two mass differences(∆m 2 12 and ∆m 2 23), three Euler angles (θ 12 , θ 23 and θ 13 ) and oneCP-violating phase δ.The mixing matrix U can be rewritten as:⎛⎞ ⎛⎞ ⎛⎞1 0 0 c 13 0 s 13 e iδ c 12 s 12 0U = ⎝ 0 c 23 s 23⎠ ⎝ 0 1 0 ⎠ ⎝ −s 12 c 12 0 ⎠0 −s 23 c 23 −s 13 e −iδ 0 c 13 0 0 1(1.10)Therefore, in the ideal limit of exactly vanishing s 13 the oscillations decoupleand depend on two separate sets of two-flavor parameters.The present experimental knowledge on neutrino oscillation parametersindicates ∆m 2 12 ≪ ∆m 2 23 and small values for θ 13 , if non-vanishing. For anoscillation measurement with E ν ≃ ∆m 2 23 · L the oscillation probabilitiesrelated to ν µ transitions can be expressed in the following approximateformulation:P(ν µ → ν e ) ≃ sin 2 Φ 23 sin 2 θ 23 sin 2 2θ 13P(ν µ → ν τ ) ≃ sin 2 Φ 23 sin 2 2θ 23 cos 4 (1.11)θ 13where Φ ij ≡ ∆m 2 ij · L/4E ν .Because of θ 13 smallness we have P(ν µ → ν e ) ≪ P(ν µ → ν τ ). The subleadingchannel ν µ → ν e is a possible way for θ 13 measurement.1.3 Present status of neutrino oscillationsAll existing data for neutrino oscillation can separately be analyzed in aneffective two neutrino framework, i.e. the data do not indicate any genuinethree flavor effects, in particular all data can be accommodated withoutany complex entries in the mixing matrix. The reason for this is that thereis a pronounced hierarchy of mass splittings |∆m 2 12| ≪ |∆m 2 23| and thatthe angle θ 13 which couples the two oscillations is small. Therefore the presentationof the current knowledge on neutrino mixing parameters can bedivided into three subsets: oscillations associated with |∆m 2 12|, oscillationsassociated with |∆m 2 23| and the non-observation of θ 13 . Finally, the statusof four neutrino scenario will be discussed.1.3.1 Solar parameters: ∆m 2 12 and θ 12Historically the first hint for neutrino oscillations came from the observationof neutrinos from the Sun. In 1968 Ray Davis and his collaborators7


Figure 1.1: Energy distribution of the flux of solar neutrinos predicted bythe standard solar model, as computed by J. Bahcall [20]. The ranges ofenergies associated with the various experiments are indicated at the topof the figure.published the first result of the Homestake chlorine experiment [18] showingthat the measured flux of solar electron neutrinos was significantly lowerthan the expected value. It was the beginning of the solar neutrino puzzleand it took nearly forty years to resolve it.According to the Solar Standard Model [19] all the solar energy is producedin a series of thermonuclear reactions and decays at the center ofthe sun. The two reaction chains responsible for the energy productionare known as the pp-cycle and the CNO-cycle. For both cycles the overallresult is the fusion of hydrogen nuclei into helium, with the emission ofelectron neutrinos:4p + 2e − → 4 He + 2ν e + 26.73 MeV (1.12)The energy distribution of the flux of solar neutrinos predicted by the SSMis showed in fig. 1.1.The experiments to detect solar neutrinos can be subdivided intotwo categories: radiochemical experiments (Homestake [21], SAGE [22],GALLEX [23] and its successor, GNO [24]) and water-Cherenkov experiments(Kamiokande [25], Super-Kamiokande [26] and SNO [27]). One ofthe main differences between these two categories is the fact that, while for8


Experiment Reaction E thr (keV) φ meas (SNU) φ pred (SNU)Homestake ν e + 37 Cl → e − + 37 Ar 814 2.56 ± 0.23 7.6 +1.3−1.1SAGE 70.8 +5.3−5.2GALLEX ν e + 71 Ga → e − + 71 Ge 233.2 77.5 +7.6−7.8 128 −7+9GNO 62.9 +5.4−2.5Table 1.1: Solar neutrino flux measured by radiochemical experiments.The neutrino reaction, the energy threshold and the SSM prediction arealso reported.radiochemical experiments only an integrated measurement of the fluxes ispossible, water-Cherenkov detectors can perform real-time measurementsand therefore can study differential distribution (i.e. energy spectrum, angulardistribution, correlation with the Sun position in the sky).In tab. 1.1 results reported by the various radiochemical experimentsare summarized: for all of them, a substantial lack in the measured neutrinoflux with respect to the SSM prediction is observed.Kamiokande [25] was the first water-Cherenkov detector, located 1000meters underground (2700 m.w.e.) in the Kamioka mine in Japan. Itstarted its operation in 1984 and ended physics data-taking at the beginningof 1995. Since then its physics programme has been continued by itssuccessor experiment, Super-Kamiokande.Super-Kamiokande consist of a huge cylindrical tank (volume 50 kton)filled with pure water. An inner volume (16.9 m diameter, 36.2 m height,volume 32.5 kton) is defined by an inner surface equipped with 11146PMTs. An outer volume, 2 m thick, equipped with 1185 PMTs surroundsthe inner detector and serves as an active veto counter against gamma-rays,neutrons and through-going cosmic muons.In both Kamiokande and Super-Kamiokande the solar neutrinos aredetected through the observation of the Cherenkov rings produced by theelectrons emitted in the elastic process:ν x + e − → ν x + e − (x = e, µ, τ) (1.13)Although the cross-sections for this process are very small, elastic interactionsturn out to be very useful, thanks to the nice correlation between therecoil electron momentum direction and the direction of the incoming neutrino.Therefore the direction of the recoil electron can be used to correlatethe direction of the impinging neutrino with the Sun’s position in the sky.Moreover the energy of the recoiling electron can be used to obtain a lowerlimit on the incoming neutrino energy.The energy threshold for Kamiokande and Super-Kamiokande is determinedby the threshold for detecting the recoil electron in the elasticscattering (1.13), which is about 5 − 7.5 MeV. Thus the solar neutrino flux9


measured in those experiments is essentially the 8 B flux, plus a small componentfrom the hep reaction in Super-Kamiokande. The 8 B solar neutrinoflux observed in Kamiokande is 2.8±0.19 (stat.)±0.33 (syst.)×10 6 cm −2 s −1which is about 64% of the standard solar models. The Super-Kamiokandecollaboration has reported the result [26] 2.32±0.03 (stat.)±0.08 (syst.)×10 6 cm −2 s −1 .The results of both radiochemical and water-Cherenkov experimentsindicated that if one believes in the standard solar models, there must besome modification in the neutrino flavor composition while they travel betweenthe center of the Sun to the Earth. Neutrino oscillation appeared tobe a possible solution to the puzzle.The definitive confirmation of neutrino oscillation hypotesis came out in2002, when SNO published its neutral current data, showing the clear presenceof ν µ and ν τ in the solar neutrino flux. SNO [27] (Sudbury NeutrinoObserver) is a 1 kton heavy-water Cherenkov detector located 2 km underground(6010 m.w.e.) in the Creighton mine, Sudbury, Ontario, Canada.The detector is made of a spherical acrylic vessel, 12 m diameter, containingultra-pure D 2 O and surrounded by an ultra-pure H 2 O shield, inturn contained in a cylindrical cavity (34 m height, 22 m maximum diameter)4 . The light produced in the water is detected by 9456 PMTs (20 cmdiameter) with light concentrator, installed on a stainless steel structuresurrounding the acrylic vessel.As well as using the elastic scattering (ES) reaction (1.13) as Kamiokandeand Super-Kamiokande do, SNO can detect 8 B solar neutrinos via chargedcurrent(CC) and neutral-current (NC) interactions on deuterium:and:ν e + d → p + p + e − (threshold = 1.4 MeV) (1.14)ν x + d → n + p + ν x (threshold = 2.2 MeV) (1.15)The charged current reaction is sensitive exclusively to electron-type neutrinos,while the neutral current reaction is equally sensitive to all activeneutrino flavors. The elastic scattering reaction is sensitive to all flavorsas well, but with reduced sensitivity to ν µ and ν τ . Sensitivity to thesethree reactions allow SNO to determine the electron and non-electron activecomponents of the solar flux.In order to test the non oscillation hypothesis, the assumption thatthere are only electron neutrinos in the solar neutrino flux, the data areresolved into contributions from CC, ES and NC events, normalizing the4 In 2001, 2 tons of NaCl were dissolved in the heavy water in order to increase theneutron capture efficiency and the associated Cherenkov light, obtaining in this way anenhancement for the sensitivity and the signature for neutral-currents interactions.10


integrated rates above the kinetic energy threshold of 5 MeV. Fluxes observed[28] (in units of 10 6 cm −2 s −1 ) are:φ CC = 1.70 ± 0.07(stat) +0.09−0.10(syst)φ ES = 2.13 +0.29−0.28(stat) +0.15−0.08(syst)φ NC = 4.90 ± 0.24(stat) +0.29−0.27(syst)(1.16)Because φ CC is due only to electron neutrinos, the excess of φ NC is a clearconfirmation of neutrino flavor changing.A further confirmation of the oscillation hypothesis was produced byKamLAND (Kamioka Liquid scintillator Anti-Neutrino Detector [6]), a reactorexperiment in the site of the earlier Kamiokande 5 .The KamLAND experiment consists of 1 kton of ultra-pure liquid scintillator(LS) contained in a transparent nylon-based balloon suspended innon-scintillating oil. The balloon is surrounded by an array of 1879 photomultipliertubes mounted on the inner surface of an 18 m diameter sphericalstainless-steel containment vessel.Electron anti-neutrinos are detected via the inverse β-decay reaction:¯ν e + p → e + + n (1.17)with a 1.8 MeV ¯ν e energy threshold. The prompt scintillation light fromthe e + gives an estimate of the incident ¯ν e energy, E¯νe = E prompt + E n + 0.8MeV, where E prompt is the prompt event energy including the positronkinetic energy and the annihilation energy, and E n is the average neutronrecoil energy. The ∼ 200 µs delayed 2.2 MeV γ-ray from neutron capture onhydrogen is a powerful tool for reducing backgrounds. A 3.2 kton water-Cherenkov detector surrounds the containment sphere, absorbing γ-raysand neutrons from the enclosing rock and tagging cosmic-ray muons.KamLAND is surrounded by 53 power reactor units in Japan. Thereactor operation data are given to calculate the time dependent fissionrate of each isotope. The expected ¯ν e flux is calculated using the fissionrates and anti-neutrino spectra taken from the literature.The fraction of ¯ν e events observed by KamLAND [29] with respect to theexpected value is 0.582 ± 0.069(stat) ± 0.039(syst): this was the first proofof neutrino oscillation with reactor neutrinos by detecting anti-neutrinointeraction and constituted an important confirmation of the CPT theorem.KamLAND has recently published [7] data showing distortion in the energyspectrum (fig. 1.2), in agreement with oscillation theory predictions.The information collected in solar neutrino experiments, KamLANDand CHOOZ [30] (a reactor neutrino experiment) allow a very precise determinationof mixing parameters, whose best fit values and 3 σ ranges are5 A nuclear reactor produces electron anti-neutrinos. The assumption of P (ν α →ν β ) = P (¯ν α → ¯ν β ) is based on CPT conservation hypothesis.11


Figure 1.2: Prompt event energy spectrum of the ¯ν e candidate events.In the upper side the spectrum of all expected neutrino events is showedwhile in the lower Kamland result are overlapped, confirming the neutrinooscillation hypothesis.[31] [32]:∆m 2 12 = 6.9 +2.6−1.5 × 10 −5 eV 2 sin 2 θ 12 = 0.30 +0.09−0.07 (1.18)Among the previously proposed solutions only the so called Large MixingAngle (LMA) survives after KamLAND data (fig. 1.3).1.3.2 Atmospheric parameters: ∆m 2 23 and θ 23Atmospheric neutrinos are produced in the decay of secondary particlescreated in the interaction of primary cosmic rays 6 with the Earth atmosphere.If the energy of the secondary particles is sufficiently low ( 2GeV) to let all of them decay, we have:p + N → π ± + Xπ ± → µ ± + ν µ (¯ν µ )µ ± → e ± + ν e (¯ν e ) + ν µ (¯ν µ )(1.19)6 Mainly protons (∼ 80%) and α-particles (∼ 15%) plus a small contribution fromheavier nuclei.12


Figure 1.3: Result of a combined two-neutrino oscillation analysis of Kam-LAND, CHOOZ and the observed solar neutrino fluxes under the assumptionof CPT invariance.Assuming that no effect can change the flavor composition of the showerbefore it is measured on Earth, eq. 1.19 implies that:R = N ν µ+ N¯νµN νe + N¯νe∼ 2 (1.20)where N νe (N νµ ) and N¯νe (N¯νµ ) are the number of muon (electron) neutrinosand anti-neutrinos respectively. The exact value of R can in principlebe affected by several effects, such as the primary spectrum composition,the geomagnetic cut-off, solar activity, and, of course, the details of themodel for the development of the hadronic shower. However, although theabsolute neutrino fluxes are rather badly known (predictions from differentcalculations disagree by ∼ 20 − 30%) the ratio 1.20 is to first order insensitiveto such uncertainties and is known to ∼ 5%. Atmospheric neutrinooscillation first manifested themselves as a discrepancy between the measuredand the expected value of the ratio R.13


In order to quote a number which is independent of experimental parameters,such as energy thresholds and detector acceptances for signal andbackground, the result is generally presented in terms of the double ratio:R ′ ≡ R DAT AR MC(1.21)Assuming a correct modeling for R MC , R ′ < 1 could either mean thatthere is a deficit of measured muon-like events, or that an excess of electronlikeevents has been observed. R’ cannot discriminate between these twopossibilities. Moreover, a low value of R’ is not by itself a proof for neutrinooscillations, since one could imagine other mechanism inducing a similareffect.Another more sensitive way to detect neutrino oscillations is the studyof the zenith angle distribution of the incoming atmospheric neutrino.Downward-going neutrinos, produced in the atmosphere above the detector,will travel a path of the order of 10 − 20 km, while upward-goingneutrinos, produced at the opposite side of the globe, will have travelledup to l2000 km before detection. This means that, by studying the neutrinoflux as a function of the zenith angle, one has access to baselines spanningthree orders of magnitude and can therefore observe the modulation inducedon the flux by neutrino oscillations. In the no-oscillation hypothesis,the zenith angle distribution must be up-down symmetric, conversely, anydeviation from up-down symmetry could be interpreted as an indicationfor neutrino oscillations.Historically, the first results on what was referred as the atmosphericneutrino anomaly came from experiments originally designed to searchfor proton decay, for which atmospheric neutrino flux constituted a backgroundsource. The first experiment to report, in 1986, a discrepancybetween the observed and the predicted number of atmospheric neutrinowas IMB [33], a water-Cherenkov detector located in the Morton mine,Cleveland, Ohio, USA. Two years later Kamiokande confirmed that themeasured deficit of muon-like events was the order of 30% [34]. However,the same effect was not observed by two other proton decay experiments,NUSEX [35] and Fréjus [36], both using fine-grained iron calorimeters.This discrepancy led for some time to the belief that poorly understoodsystematic effects, mainly related to an incomplete description of neutrinointeractions in iron and water, were including an intrinsic difference betweenthe two experimental techniques. The issue was definitively resolvedwhen the Soudan2 experiment [37], another fine-grained iron calorimeter,located in the Soudan mine (2100 m.w.e.), Ely, Minnesota, USA, confirmedKamiokande and IMB results.14


Figure 1.4: Distribution of the cosine of the zenith angle obtained by Super-Kamiokande for electron-like and muon-like contained events. The solidline is the distribution expected in absence of oscillation, while the hatchedhistogram is for ν µ → ν τ oscillation in the two-flavor mixing scheme, withmaximal mixing and ∆m 2 23 = 2.5 × 10 −3 eV 2 .The big change in the scientific community’s perception of the atmosphericneutrino anomaly certainly came from the results obtained by theSuper-Kamiokande experiment [4] which in summer 1998 announced evidencefor neutrino oscillation in the atmospheric neutrino flux sample. Astrong discrepancy with respect to no-oscillation prediction was observedin the distributions of the cosine of the zenith angle for the sub-GeV 7 andthe multi-GeV muon-like events, while the electron-like events were in goodagreement with predictions in absence of oscillation. As shown in fig. 1.4the results obtained were rather described by the oscillation hypothesis,by assuming sin 2 2θ 23 > 0.92 and 1.4 × 10 −3 < ∆m 2 23 < 3.7 × 10 −3 : neutrinooscillation became the most reasonable explanation of atmosphericneutrino anomaly.Further confirmation of Super-Kamiokande results were obtained inthe last years from K2K (KEK to Kamioka) experiment [8], the firstaccelerator-based long-baseline neutrino oscillation experiment ever built,which makes use of an almost pure ν µ beam (98.2%) directed from KEK, inJapan, to the Super-Kamiokande detector, 250 km away. K2K, which has7 Super-Kamiokande data sample is often subdivided into two subsets, called sub-GeVand multi-GeV, with energies below and above 1.33 GeV respectively.15


een taking data since June 1999, is a disappearance experiment, aimingto measure a deficit of muon neutrinos in the far detector at Kamioka comparedto the initial beam intensity measured in the near detector at KEK.The K2K experiment confirms indications of neutrino oscillations: the totalnumber of ν µ collected is 108, while the number of expected events withoutoscillations is 150.9 +11.6−10.0. Furthermore, the observed spectrum reveals thetype of distortion expected from oscillations, as shown in fig. 1.5. The allowedregion for neutrino mixing parameters obtained by K2K, comparedwith last Super-Kamiokande results are reported in fig. 1.6.Figure 1.5: Distortion of neutrino energy spectrum observed at Super-Kamiokande (data points), compared to scaled KEK beam spectrum (blackhistogram), which represents expectation in the absence of neutrino oscillation.Also shown is the expected spectrum taking into account neutrinooscillation effects (red histogram), applying the K2K best-fit oscillationparameters.1.3.3 The global pictureThe two instances of a very strong evidence for neutrino oscillations havebeen presented and discussed in the previous two sections. There remainsnow the combinations of these two pieces with the other existing data to aglobal picture.The LSND (Liquid Scintillator Neutrino Detector) experiment [38] atthe Los Alamos Meson Physics Facility, designed to search for ¯ν µ → ¯ν ereported [39], in 1995 reported an excess of electron-like events above back-16


Figure 1.6: Contours enclosing the 90% confidence regions for two-flavorν µ → ν τ neutrino oscillation parameters for K2K results (green), comparedto results from Super-Kamiokande atmospheric neutrino (black)ground and gave an interpretation of that result in terms of neutrino oscillations.The results of this experiment indicate that there is a third masssplitting ∆m 2 LSND in the range 0.2 − 10 eV2 .The Karmen experiment [40] at the ISIS neutron spallation facility atthe Rutherford Appleton Laboratory in the United Kingdom on the otherhand excludes a large part of the parameter region claimed by LSND, butin a combined analysis of both data set there still remains a combined allowedregion.The third mass splitting cannot be accommodated within a three neutrinoflavor framework. Basically two possible solution exist: either thereare more than three neutrinos, which means that the additional neutrinosare sterile in order not to create a conflict with the decay width of theZ 0 measured at LEP [14], or there is a huge CPT-violation, which wouldmake the mass splittings of neutrinos and of anti-neutrinos independent ofeach other. In any case, the LSND results are going to be tested by theMiniboone experiment [41].Existing neutrino oscillation data are used to perform global analysisin a three neutrino framework. The basic trend is that the oscillation ofthree active flavors describes very well all data except for LSND and that17


the corrections to the two neutrino analysis of each of the data sets arenon-negligible but still small. The best fit values and ranges at 3σ for theoscillation parameters are [31] [32]:|∆m 2 23| = 2.6 +1.1−1.2 × 10 −3 eV 2 sin 2 θ 23 = 0.52 +0.20−0.21∆m 2 12 = 6.9 +2.6−1.5 × 10 −5 eV 2 sin 2 θ 12 = 0.30 +0.09−0.07(1.22)Even if these mixing parameters are determined with high accuracy,the whole scenario is far to be complete. The value of θ 13 , if non-vanishing,is known to be small thanks to CHOOZ, a reactor neutrino experimentin the Ardennes region of France, whose measured value is sin 2 2θ 13


Chapter 2The OPERA experiment2.1 IntroductionThe neutrino oscillation hypothesis as explanation of both solar neutrinopuzzle and atmospheric neutrino anomaly has been confirmed by severalexperiments, as shown in the previous chapter. Anyway, so far, the oscillationsignal has been only produced through the disappearance of theinitial neutrinos, while the direct observation is still missing. As far asatmospheric neutrinos are concerned, Super-Kamiokande and K2K dataare well fitted by the ν µ → ν τ transition, while transitions of ν µ ’s into ν e ’sor sterile states are disfavored. By assuming ν µ → cos ξν τ + sin ξν s , thefraction sin 2 ξ of atmospheric ν µ ’s that transform into sterile ν s is limitedat 90% C.L. by sin 2 ξ < 0.19 [42].OPERA (Oscillation Project with Emulsion-tRacking Apparatus) [45]is a long baseline experiment designed to be the conclusive test of theν µ → ν τ oscillation hypothesis by means of the direct observation of ν τ inan initially pure ν µ beam.The OPERA apparatus, located at the Gran Sasso underground laboratory,will detect neutrinos coming from CERN SPS (the CNGS, CERNNeutrino beam to Gran Sasso), 732 Km downstream. The beam energyhas been tuned to be well above the τ lepton production threshold andin the oscillation parameter region indicated by the atmospheric neutrinoexperiments. In case of a positive signal, the observation of a few ν τ eventswill be already significant, because of the very low expected background.Due to its electron identification capability for the reconstruction of theτ → e decay, OPERA is able to perform oscillation searches in the subdominantchannel ν µ → ν e , even if the CNGS beam has been optimized forν τ appearance. The sensitivity to θ 13 of ICARUS [43] (the other experimentlocated in Gran Sasso underground laboratory on the CNGS neutrinobeam line) and OPERA data combined [44] can improve the current limit19


of the CHOOZ experiment of a factor of about 5.OPERA is a massive lead/emulsion target. It exploits nuclear emulsionsfor the unambiguous detection of the decay of the τ’s produced inν τ charged-current (CC) interactions. A detector target mass of ∼ 1 ktonis needed to be sensitive at ∆m 2 of 10 −3 eV 2 . Such a mass, too expensiveif made of pure emulsions, is achieved by means of the Emulsion CloudChamber (ECC), a sandwich of dense passive material (Pb) plates andthin emulsion sheets. The emulsion is used only for tracking and τ decaysare identified through the kink detection. The whole detector structureincludes magnetized iron spectrometers for muon charge and momentummeasurement, and electronic detectors for event location inside the emulsiontarget (Target Trackers). Moreover, a veto detector system is requiredto flag events from neutrino interactions in the rocks surroundingthe OPERA detector.The ECC combines in one cell the high precision tracking capabilitiesof 300 µm thick nuclear emulsion films and the relatively large mass givenby 1 mm thick lead plates. By piling-up a series of cell in a sandwich-likestructure one obtains a brick, which constitutes the fundamental detectorelement. Bricks are assembled to form walls, while a wall and its relatedelectronic tracker plane constitute a module. A sequence of modules witha downstream spectrometer constitutes a supermodule. The final detectorbaseline consists of two supermodules, for a total mass of ∼ 1.8 kton.The detector installation started in 2003 and the first neutrino beamdelivery is foreseen for May 2006.2.2 The CNGS neutrino beamThe West Area Neutrino Facility (WANF) on the CERN 450 GeV SuperProto Synchrotron (SPS) has been operating since 1976, providing a veryintense beam of high-energy muon neutrinos to the particle physics community.The CNGS is a long baseline neutrino beam connecting two researchfacility, the CERN SPS and the Gran Sasso INFN laboratory, after a 732km underground travel.The original CNGS neutrino beam is described in Ref. [46]. In November2000 a new version of the CNGS beam was released, which gives ∼ 8%more ν µ CC events and ν τ CC events. The beam energy has been optimizedto have the desired ν energy and the proton energy has been fixed to 400GeV.A schematic overview of the CNGS neutrino beam is shown in Fig. 2.1.SPS protons hit a graphite target made of a series of rods, for an overall20


Figure 2.1: Layout of the CNGS neutrino beam. The coordinate origin isthe focus of the proton beam.target length of 2 m, producing secondary pions and kaons. The targetrod diameter is 4 mm so that the proton beam is well contained within thetarget. The first coaxial lens, the horn, is placed 1.7 m downstream of theproton beam. The second one, the reflector, is located 43.4 m downstreamof the focus.Helium tubes are placed in the free spaces of the target chamber inorder to reduce the interaction probability for secondary hadrons. A firsttube is located between the horn and the reflector, while a second one fillsthe gap between the reflector and the decay channel placed downstream.Pion and kaons focused by the optics are then directed towards the decaytunnel to produce the neutrino beam. The typical π decay length (2.2km at 40 GeV/c) makes a long decay tunnel justified. Given the angulardistribution of the parent mesons, the longer the decay tunnel the largermust its diameter be. A tunnel of 2.45 m diameter and 1000 m length hasbeen chosen for the CNGS. A massive iron hadron stopper is situated atthe exit of the decay tunnel.The signal induced by muons (from meson decays) in two arrays of silicondetector placed in the hadron stopper is used for the online monitoringand the tuning of the beam (steering of the proton beam on target, hornand reflector alignment, etc.). The separation of the two arrays, equivalentto 25 m of iron, allows a rough measurement of the muon energy spectrumand of the beam angular distribution. Calculations show that when theneutrino beam reaches Gran Sasso, 732 km from CERN, it will have a diameterof about two kilometers.The SPS proton intensity is one of the main ingredients needed toachieve the physics goal of the OPERA experiment. By assuming a 400GeV/c proton beam and 200 running days per year, the expected numberof proton on target 1 is 4.5 · 10 19 /year [47].1 An increase of the beam intensity by a factor of 1.5 is now under study.21


ν µ (m −2 /pot) 7.78 × 10 −9ν µ CC events/pot/kt 5.85 × 10 −17〈E〉 νµ (GeV) 17.7ν e /ν µ 0.8%¯ν µ /ν µ 2.1%¯ν e /ν µ 0.07%Table 2.1: Performances of the CNGS neutrino beam [48].∆m 2 ν τ CC interactions/kton/year1 × 10 −3 eV 2 2.533 × 10 −3 eV 2 22.55 × 10 −3 eV 2 60.5Table 2.2: Expected number of ν τ CC events at Gran Sasso per kton peryear. Results of simulations for different values of ∆m 2 and for sin 2 2θ =1 are given. These event numbers do not take detector efficiencies intoaccount.The CNGS beam features are summarized in Tab. 2.1. They indicatethe numbers of neutrino CC interactions including deep-inelastics (DIS)and quasi-elastics plus resonances (QE). The expected rate of ν τ CC interactionsfor sin 2 2θ = 1 and different values of ∆m 2 are given in Tab. 2.2.2.3 Detector structure and operationsThe OPERA detector is located in the Hall C of the Gran Sasso undergroundlaboratory. It is made of 2 Supermodules, each of them formedby a target section followed by a muon spectrometer. In front of the firstSupermodule there is a Veto plane made of Resistive Plate Chamber. Aschematic view of the OPERA detector is reported in fig. 2.2. The targetsection is made of 31 brick walls interleaved with 31 Target Tracker (TT)planes. Each wall consists of 3328 bricks removable by using an automaticsystem called Brick Manipulator System (BMS). The whole target sectionon each Supermodule contains 103168 bricks equivalent to about 900 tons.Each brick consists of a pile of 57 nuclear emulsion sheets interleavedwith 56 lead sheets. An additional emulsion sheet, called Changeable Sheet(CS) is independently packed and placed downstream of each brick, withthe purpose of early check of recorded tracks before opening the relatedbrick.22


Figure 2.3: Schematic structure of en ECC cell in the OPERA experiment.The ν τ interaction and the τ decay topology are reconstructed by usingtrack segments in the emulsion films.Each emulsion layer has transverse dimensions of 10 × 12.5 cm 2 . Thebrick has a total thickness of about 7.6 cm (10 X 0 ) and a weight of 8.3 kg.The dimensions of the bricks are determined by conflicting requirements:the mass of the brick selected and removed for analysis shouldrepresent a small fraction of the total target mass; on the other hand, thebrick transverse dimensions should be substantially larger than the uncertaintieson the interaction vertex position predicted by the electronictrackers.The brick thickness in units of radiation lengths is large enough toallow electron identification through their electromagnetic showering andmomentum measurement by multiple coulomb scattering following tracksin consecutive cells. An efficient electron identification requires about 3-4X 0 and the multiple scattering requires ∼ 5 X 0 . With a 10 X 0 brick thickness,for half of the events such measurements can be done within the samebrick where the interaction took place, without the need to follow tracksinto downstream bricks.2.3.2 Target trackersElectronic detectors placed downstream of each emulsion brick wall areused to select the brick where the neutrino interaction took place and toguide the scanning, by defining the region of the films to be scanned. The24


changeable sheet is used to confirm the brick choice, so a moderate spatialresolution can be tolerated, which allows to reduce the cost of the electronictrackers covering a large surface. For this purpose, plastic scintillator stripsread out by Wave Length Shifting (WLS) fibres have been chosen. Theywill be also used to sample the energy of hadronic showers and to contributeto the identification and reconstruction of penetrating tracks.Each brick wall is followed by two electronic tracker planes (with stripsoriented along the X and Y axis respectively). The planes are squares of∼ 6.7 m edge-to-edge and contain 256 scintillator strips. Each group of 64strips constitutes an independent unit, read out on each side by a 64-pixelphotodetector, such that 8 photodetectors are required for each trackerplane. No multiplexing scheme is foreseen.The scintillator strips Target Trackers are 2.6 cm wide and 1 cm thick.Simulations have shown that a transverse segmentation below this valuedoes not significantly improve the physics performances, in particular thebrick finding efficiency. Their energy resolution is what expected from acalorimetric sampling, about 0.65/ √ E(GeV) + 0.16; during the data taking,muons generated in the interaction of CNGS neutrinos in the cavernrock (rock muons), cosmic ray muons, radioactive sources and light injectionsystems will be used to calibrate the system.The selection of the brick containing the neutrino interaction vertex isperformed by combining different algorithms based on the observed transverseand longitudinal event profiles as well as on the presence of individualreconstructed tracks. As an illustration, fig. 2.4 shows a simulated ν τ eventwith a muonic decay for one of the two projections transverse to the beamdirection.2.3.3 Muon spectrometersThe main goals of the spectrometers are the muon momentum and chargemeasurements. The OPERA spectrometers consist of active detectors(RPC, XPC and Drift Tubes) and a warm dipolar magnet made of twoiron walls interleaved by pairs of high resolution trackers. Each wall ismade of 12 iron plates 5 cm thick. The iron is magnetised by a current ofabout 1600 A circulating in the top and bottom copper coils. The magneticflux density in the tracking region is 1.57 T with vertical field lines ofopposite directions in the two magnet walls. The transverse useful dimensionsof the magnet are 8.75 m (horizontal) and 8 m (vertical) providingadequate geometrical acceptance also for muon originating in the upstreamtarget volume.The high resolution trackers, denoted as Precision Trackers, consist ofvertical drift tube planes with an intrinsic resolution of 0.3 mm in thebending direction. Allowing for some misalignment, an overall resolution25


Figure 2.4: Display of a simulated τ → µ event in the OPERA target. Theneutrino beam comes from the left. The primary vertex occurs in the thirdbrick wall. The muon track corresponds to the longest track escaping onthe right.on each measured coordinate of 0.5 mm has been assumed in the following.The two tracker planes housed between the two magnet walls provide anangular measurement of the track with a 100 cm lever arm. The lever armfor the external trackers is > 50 cm. This design leads to a momentumresolution better than 30% in the relevant kinematical domain.The so-called Inner Trackers are inserted between the magnet ironplates. They are made of RPC detectors. On each face of the chambers,the induced pulses are collected by 3 cm wide pickup copper strips inthe horizontal and vertical directions.The Inner Trackers allow a coarse tracking inside the magnet to identifymuons and ease track matching between the Precision Trackers. They alsoprovide a measurement of the tail of the hadronic energy leaking from thetarget and of the range of muons which stop in the iron.2.4 Physics performances2.4.1 τ detectionThe signal of the occurrence of ν µ → ν τ oscillations is the charged currentinteraction of the ν τ ’s in the detector target (ν τ N → τ − X). The reactionis identified by the detection of the τ lepton in the final state through thedecay topology and its decay modes into an electron, a muon and a single26


or three charged hadrons:τ − → e − ν τ ¯ν eτ − → µ − ν τ ¯ν µτ − → (h − h + )h − ν τ (nπ 0 )Measurements of the branching ratio (BR) of the decay modes give17.8%, 17.7% and 64.7% for the electronic, muonic and hadronic channelrespectively. For the typical τ energies expected with the CNGS beam oneobtains the decay length distribution shown in fig. 2.5, with an averagedecay length of ∼ 450 µm.Figure 2.5: τ decay length distribution.Neutrino interactions will occur predominantly inside lead plates. Oncethe τ lepton is produced, it will decay either within the same plate (shortdecays) or further downstream (long decays). Short decays are detected bymeasuring the impact parameter (IP) of the daughter track with respect tothe tracks originating from the primary vertex, while for long decays theτ is detected by measuring the angle between the charged decay daughterand the parent direction (see fig. 2.6).The distribution of the τ decay kinkfor the electronic channel is showed in fig. 2.7.27


Figure 2.6: Schematic picture of the τ detection technique in the ECC cellfor long (top) and short (bottom) decays.Figure 2.7: τ kink angle distribution for the τ → e decay mode.28


The detection of the τ decay into an electron benefits from the densebrick structure given by the cell design, which allows the electron identificationthrough its showering in the downstream cells.For the muonic decay mode the presence of the penetrating (often isolated)muon track crossing the whole detector structure allows an easiervertex finding. The potential background from large angle scattering ofmuons produced in ν µ CC interactions can be reduced to a tolerable levelby applying cuts on the king angle and on the muon transverse momentumat the decay vertex.Hadronic decay modes have the largest branching ratio but are affectedby background due to hadron interaction. One of the primary hadrons, infact, can interact in the first lead plates and it may simulate the decay ofthe τ. Strong kinematical cuts can be used to reduce this background.An important tool for background rejection is the determination of thetransverse momentum of the daughter particle with respect to the directionof the τ track candidate. For electronic τ decays the ECC technique is wellsuited to identify electrons and to determine their energy by measuring thedensity of track segments associated to their showering in the brick. Forcharged hadrons and muons, the momentum is deduced from the measurementof the multiple scattering in the lead plates. The muon momentumis also measured by the electronic detectors in a large fraction of cases.The τ detection efficiency is estimated by evaluating the efficiency relatedto the various steps of data reconstruction:• Trigger and brick finding• Vertex finding• Decay detection• Kinematical analysisThe overall detection efficiency (including branching ratios) is reported intab. 2.3DIS long QE long DIS short Overallτ → e 2.7% 2.3% 1.3% 3.4%τ → µ 2.4% 2.5% 0.7% 2.8%τ → h 2.8% 3.5% − 2.9%Total 8.0% 8.3% 1.3% 9.1%Table 2.3: τ detection efficiencies (including branching ratios) for theOPERA experiment. Overall efficiencies are weighted sum on DIS andQE events.29


2.4.2 BackgroundThe possible background sources are:• prompt ν τ production in the primary proton target and in the beamdump;• decays of charmed particles produced at the primary interaction vertex;• large angle muon scattering;• hadronic reinteractions;The contribution of the above sources to the total background depends onthe actual decay channel.Prompt ν τ originate from the decay of τ’s produced in the CNGS targetby the decay of D s mesons. The rate of ν τ production from the interactionof 450 GeV/c protons in a Be target and in the downstream beam dumphas been evaluated in Ref. [49] [51] for the CERN Wide Band Beam. Theseresults have been scaled down according to the features of the CNGS beamand the distance of the experiment from the source. Using the methodillustrated in Ref. [49], we expect O(10 −6 )×N CC ν τ interactions, where N CCis the total number of ν µ CC events collected. If one also takes into accountthe detection efficiency and the fact that the experiment will integrateO(10 4 ) events, the contribution to the background is completely negligible.Charmed particles are produced in CC and NC neutrino interactionsthrough the reactions:ν µ N → cµXν µ N → c¯cµXν µ N → c¯cν µ XCharged mesons have masses and lifetimes similar to those of the τlepton. The above processes may thus constitute a background to the oscillationsignal if one fails to detect the primary muon in the first reaction,the charm partner in the third of both in the second. The most relevantsource is given by single charm production, i.e. the first reaction.In order to evaluate the expected charm background the uncertaintieson the charm production cross section and decay branching ratios have beentaken into account. The precision on the background evaluation benefitsfrom the charm studies recently performed by the CHORUS experiment[50]. The evaluated background from charm is 0.382 events.Muons produced in ν µ CC events and undergoing a scattering in thelead plate following the vertex plate could mimic a muonic τ decay. Thebackground is evaluated by using Monte Carlo simulations. Once the selectioncriteria on kink angle θ > 20 mrad and transverse momentum p t > 25030


τ → e τ → µ τ → h totalCharm background 0.210 0.010 0.162 0.382Large angle µ scattering − 0.116 − 0.116Hadronic background − 0.093 0.116 0.209total per channel 0.210 0.219 0.278 0.707Table 2.4: Expected number of background events (5 years run, nominalintensity) in each τ decay channel and the overall yield.MeV/c are applied, the total number of expected events becomes 0.116.The last source of background is due to reinteractions in the lead ofhadrons produced in ν µ CC and in ν µ NC interactions in which the primarymuon is not identified. An experimental evaluation of this backgroundsource has been performed by FLUKA [52] and validated by CHORUS allowingto quantify to total number of BG events for this topology, that is0.209.The expected numbers of background events for the OPERA experimentare reported in tab.2.4. The total number of events is less than one: thismakes OPERA an almost background free experiment. For this reason, theunambiguous evidence for ν τ appearance can be claimed even in presenceof a few τ candidates.2.4.3 Sensitivity to ν µ → ν τ oscillationsIn tab. 2.5, it is reported for different ∆m 2 values and for maximal mixingthe number of signal and background events expected after 5 years runs,assuming the nominal beam intensity of 4.5 × 10 19 pot/year.Given the total background of 0.707 events, 6 ν τ observed events wouldbe sufficient to claim for ν µ → ν τ oscillation discovery with a 4 σ significance.The sensitivity of the OPERA experiment to ν µ → ν τ oscillationis shown in fig. 2.8. The region allowed by the atmospheric neutrino pastexperiments is also shown: the OPERA sensitivity completely covers theallowed region.According to the effective ∆m 2 value, OPERA could be also able tostudy the energy dependency of the neutrino oscillation probability.31


Figure 2.8: The OPERA sensitivity to ν µ → ν τ oscillations. The parametersregion allowed by SuperKamiokande and K2K combined is alsoreported.∆m 2 (eV 2 ) 2.0 × 10 −3 2.5 × 10 −3 3.0 × 10 −3 BGevent num. 7.3 11.5 17.2 0.7Table 2.5: Expected number of signal and background events (5 years run,nominal intensity) for different ∆m 2 values.2.4.4 Search for ν µ → ν e appearanceSubdominant ν µ → ν e oscillations at the atmospheric scale are driven bythe mixing angle θ 13 . The angle is constrained by the CHOOZ experimentto be small (sin 2 2θ 13 < 0.1 [30]).Due to its electron identification capability exploited for the reconstructionof the τ → e decay, OPERA is able to perform also a ν µ → ν eoscillation search.In the CNGS beam the expected ν e contamination is relatively smallcompared to the dominant ν µ component (ν e /ν µ = 0.8%) and allows searchfor the oscillation ν µ → ν e seeking after an excess of ν e charged current32


events. The systematic error associated with the ν e contamination playsan important role for the oscillation search, the statistical fluctuation ofthis component being the irreducible limiting factor.The overall ν e rate is defined as:R(∆m 2 23, sin 2 2θ 23 , sin 2 2θ 13 ) = S +B (τ→e) +B (beam) +B (NC) +B (νµCC→νµNC)(2.1)where the signal S is the convolution of the ν µ flux (φ νµ (E)) with the ν echarged-current cross section ( σν CCe(E) ) ( , the ν µ → ν e oscillation probabilityPνµ →ν e(E) ) and the signal efficiency (ε signal ):∫S = Aφ νµ (E)P νµ→ν e(E)σ CCν e(E)ε signal dE (2.2)The normalization A takes into account the effective target mass.The background coming from the dominant ν µ → ν τ oscillation channelwhere the produced τ decays in one electron is:∫B (τ→e) = A φ νµ (E)P νµ→ντ (E)σν CCτ(E)ε (τ→e) dE (2.3)where ε (τ→e) is the efficiency and σν CCτ(E) is the ν τ charged-current crosssection. This background includes mainly τ’s decayed in the same leadplate as the primary vertex plus a small contribution (4%) from long decayswhere the kink has not been detected.The background coming from the ν e beam contamination is:∫B beam = A φ νµ (E)P νe→νe (E)σν CCe(E)ε (beam) dE (2.4)where ε (beam) is the efficiency for CC events from the ν e beam contamination.The background B (NC) , from the decay of neutral pions created byneutral-current interactions is:∫B (NC) = A φ νµ (E)σ ν (NC) (E)ε (NC) dE (2.5)where ε (NC) is the reduction efficiency.The background from ν µ events with the primary muon not identifiedis given by∫B (ν µCC→ν µ NC) = A φ νµ (E)P νµ→ν µ(E)σν CCµ(E)ε (ν µCC→ν µ NC) dE (2.6)where ε (νµCC→νµNC) is the probability for a ν µ CC to be identified as aν µ NC and with another track miming an electron.33


θ 13 sin 2 2θ 13 ν e CC signal τ → e ν µ NC ν e CC beam ν µ CC → ν µ NC9 o 0.095 9.3 4.58 o 0.076 7.4 4.57 o 0.058 5.8 4.6 5.2 18 1.05 o 0.030 3.0 4.63 o 0.011 1.2 4.7Table 2.6: Expected number of signal and background events in the ν µ → ν eoscillation search by assuming ∆m 2 23 = 2.5 × 10 −3 eV 2 and θ 23 = 45 o .The expected number of signal and background events for OPERAassuming 5 years data taking with the nominal CNGS beam is reported intab. 2.6 for different values of θ 13 . The 90% confidence level limit for theOPERA experiment is sin 2 2θ 13 < 0.06 which improves the CHOOZ valuesin 2 2θ 13 < 0.10.34


Chapter 3Nuclear emulsions3.1 IntroductionThe use of photographic emulsions to study nuclear particles began in 1896,when H. Becquerel for the first time observed a blackening of photoplatesin contact to salts of uranium.The development of the nuclear emulsion method for recording highenergycharged particles involved several physicists worldwide during the1930s: the work of the Viennese ladies Marietta Blau and Hertha Wambackeris well known [53].The real breakthrough was made in 1946, when an industrial chemist,C. Waller, produced the first ”concentrated” emulsions, with a halide/gelatineratio higher than previously used, sensitive enough to make the tracksof mesons visible for the first time: in 1947 the pion was discovered [54]by observing the π → µ → e decay chain in nuclear emulsions exposed tocosmic rays. In the following years, several particle such as K meson andhyperons were observed by using emulsion detectors.In the 1960s accelerators started to replace cosmic rays as source ofhigh-energy particles and fast-response detectors, such as counters andspark chambers, started to replace cloud chambers and nuclear emulsions.Anyway, nuclear emulsions were not abandoned, because of their uniquepeculiarities, being extremely sensitive with the ability to resolve particletracks to less than 1µm, and therefore the ideal device to detect short-livedparticles.The development of automatized scanning systems during the last twodecades has made possible the use of large nuclear emulsion detectors.Indeed, nuclear emulsions are still successfully used nowadays, especiallyin neutrino experiments: we mention experiments like WA17 at CERN[55], aiming at the search for charmed particles in neutrino charged currentinteractions, or E531 at Fermilab [56], aiming at the measurement35


of charmed particle lifetimes in neutrino interactions, or WA75 at CERN[57], searching for beauty particle production induced by a 350 GeV/c π −beam. Furthermore, the use of nuclear emulsion allowed the observationof τ neutrinos by DONUT collaboration [58].Neutrino oscillation experiments as CHORUS and OPERA use a largeamount of nuclear emulsion as target and tracking detector.3.2 Basic properties of emulsionsNuclear emulsions consist of small crystals of silver halide (usually bromideAgBr) with linear dimensions between 0.1 and 1 µm embedded in a gelatincomposed of organic materials. The passage of charged particles becomesvisible through a chemical amplification of the atomic-scale perturbationsinduced by energy losses of ionizing particles.Nuclear emulsions are similar to photographic emulsions, but with severalpeculiar features: silver halide crystals are very uniform in size andsensitivity; the silver to gelatin ratio is much higher than in a conventionalemulsion; furthermore, the film thickness is larger.3.2.1 The latent image formationA characteristic of silver halides is that, if they are opportunely sensibilizedby light or ionising radiation exposure, the energy provided to the crystalsproduces a latent image which is almost stable in time. The absorptionof a light quantum causes the ionization of a bromine ion, which is thustransformed into a bromine atom. The ejection of an electron leads to aregion of positive charge in the lattice which is referred to as a positive hole.The ejected electron is not free: it can be regarded as associated with thesilver lattice and it can move inside it neutralizing in turn one of the silverions. The effect of the absorption of the quantum is therefore to transformtwo ions into two atoms.Both electrons and positive holes can diffuse inside the crystal: theholes are trapped at lattice imperfections on the outer surface 1 while electronsare trapped by impurities used for sensibilization.It is important for latent image formation that a significant portion of1 This can be understood by considering the energy of the system: at an externallattice point an electron associated with a bromine atom has less positively chargedneighbours than one in the body of the crystal. The electron therefore leaves the externalpoint so that a bromine atom instead of an ion appears there. Alternatively it may besaid that the hole, which effectively carries a positive charge, is trapped by a bromineion at a kink site, because the electric potential of this site is less than that of a site inthe body of the crystal36


electrons and positive holes are trapped separately, otherwise they couldrecombine and regenerate halide ions. The silver halide crystal containsfree (interstitial) silver ions, which can move through the lattice. When aninterstitial silver ion encounters a trapped electron, the charges are neutralizedand an atom of metallic silver is formed. In this way, the absorption ofenergy in a sensitisied crystal of silver halide leads to a concentration of afew silver atoms into an aggregate which can act as a development center,i. e. a latent image.The formation and preservation of the latent image depends on externalconditions such as temperature, humidity and pressure. As temperatureand humidity increase, the sensitivity decreases and the latent image isless stable (fading). The fading can be artificially induced in order to erasethe image of unwanted tracks accumulated before the exposure (refresh).Moreover, in particular conditions, it’s possible to refresh emulsion withoutspoiling their sensitivity.3.2.2 Development and fixationA feature of fundamental importance with silver halide emulsions is thatwhen placed in a reducing solution, the developer, those grains with a suitabledevelopment center are transformed into metallic silver. The physicochemichalprocesses involved are exceedingly complex and will not be described.It suffices saying that a considerable number of competing mechanismsare involved all of which can be profoundly modified by small changesin the constitution of the developing solution. Furthermore, the developeris chosen with the aim of reducing completely those crystals containing alatent image center, while leaving unchanged those not containing a center.The development time used for processing should be sufficient for thosecrystals with a latent image center to be reduced completely, but not solong that unexposed crystals are developed. In practice, a certain numberof crystals will be developed even though they do not contain a developmentcenter. These grains, when developed, constitute what is known asfog or background.After the development a fixation procedure must be held, in order toremove all the residual silver halide. This, if otherwise left in the emulsion,would slowly induce the browning and degrading of the image. The fixingagents most widely used are sodium or ammonium thiosulphate, whichform thiosulphate complex with the silver halide. Silver thiosulphate issoluble in water and so may be removed from the emulsion by washing.Emulsions must be washed very thoroughly to remove all the silverthiosulphate complexes: if any remains, they will eventually break down,forming silver sulphide which is brown and will obscure the image.37


3.3 OPERA emulsion filmsThe total area of emulsion films in the OPERA detector is ∼ 150000 m 2 .This quantity is a few order of magnitude larger than the ones used byprevious experiments. This makes the emulsion pouring by hand impossible,unlike it was always done in the past. To overcome this problem,an automatic procedure for the OPERA nuclear emulsion production hasbeen performed by the Fuji Film company, in Japan.In fig. 3.1 a picture of the cross section of an OPERA emulsion filmis showed. Two emulsion layer of 42 µm are coated on both sides of a200 µm thick triacetate base. A thin (∼ 1 µm) protective film (gelatin)is placed over both emulsion layers in order to prevent the occurrence ofblack patterns, due to silver chemically deposited during the development.The removal of these stains had been the most time-consuming task in theemulsion preprocessing for the past experiments. By means of the protectivecoating, surface cleaning is not needed anymore and the preprocessingprocedure become compatible with the daily handing of thousands of emulsionfilms as needed for the OPERA experiment. In addition, the presenceof this protective layer allows direct contact with lead plates.Unlike hand-made films, the thickness of the emulsion layer can be preciselycontrolled as in the case of commercial color films.The crystal diameter distribution is rather uniform (fig. 3.2) around0.2 µm. The grain density is expected to be ∼ 35 grains/ 100 µm. Severalmeasurements performed with horizontal high-energy π beam have confirmedthis value. The fog due to accidental grains is randomly distributedin the emulsion volume and is kept at the level of ≤ 5 fog grains/1000µm 3 .This can be achieved by applying a moderate processing of the emulsionfilms, still keeping the desired sensitivity of 30-35 grains/100 µm.The intrinsic position resolution of the emulsion films can be investigatedby measuring the position residuals of the center of each grain withrespect to a fitted straight line. The result is shown in fig. 3.3. The measuredresolution of σ ∼ 0.06 µm can be compared with the expected valueof ∼ 0.2/ √ 12 µm, where 0.2 µm is the diameter of the original crystal.The physics properties of the emulsion layer are the following: densityρ = 2.4, average atomic number < A >= 18.2, average atomic charge = 8.9, radiation length X 0 = 5.5 cm, (dE/dx) mip = 1.55 MeV/g/cm 2or 37 keV/100 µm, nuclear collision length λ T = 33 cm and nuclear interactionlength λ I = 51 cm.38


Figure 3.1: Sketch of the cross section of a machine-coated OPERA emulsionfilm. Diluted emulsion layers of 42 µm thickness are coated on bothsides of a 200 µm thick triacetate base. A thin (∼ 1 µm) protective film(gelatin) is placed over the emulsion layer at the same time of coating.3.3.1 Production, refreshing and transportationOPERA emulsions are produced in Japan by Fuji Photo Film Company.After an R&D activity jointly conducted with Nagoya University the processof automatic machine coating of nuclear emulsion films was established.Emulsions are thus produced with the production line for commercial photographicfilms. The total amount of emulsion sheets needed for the experimentis about 12 millions. The production started on April 2003 and4.3 millions have already been produced.Due to their continuous sensitivity, the emulsion plates collect latenttrack images, mainly from cosmic-rays and environmental radioactivity,since the very beginning of their lifetime at the production firm Fuji andall along their way to the OPERA detector in the Gran Sasso cavern. In theevents induced by the CNGS beam, background tracks increase the scanningtime and may affect the patter recognition and may cause degradationof the measurements of the energy of electromagnetic showers. A remedywas proposed and successfully tested at Nagoya University: a treatment ofthe emulsion sheets at moderate temperature and very high humidity fora few days cancel out a significant fraction (about 96%) of the previouslystored latent track images. This is known as refreshing procedure.39


Figure 3.2: Crystal diameter of the Fuji emulsion produced for the OPERAexperiment.After production, the OPERA emulsions are stored in the Tono mine,in Japan, where a refreshing facility has been set up 2 . Emulsions are storedfor three days at 30 o C and 98% relative humidity. The characteristics ofthe refreshed samples are reported in tab. 3.1.Initial sensitivity before refreshing 36 grains/100 µmSurviving grain density of recorded 13 grains/100 µmtracks after refreshingSensitivity after refreshing 33 grains/100 µmFog density before refreshing 2.5 grains/1000 µm 3Fog density after refreshing 3.5 grains/1000 µm 3Table 3.1: Sensitivity and fog grain density for the OPERA emulsions, asmeasured before and after the refreshing procedure.From tab. 3.1 we can argue that there is a low reduction of the emulsionsensitivity (from 36 to 33 grains/100 µm) with a corresponding reasonableincrease of the fog density (from 2.5 to 3.5 grains/1000 µm 3 ) are observed.Moreover, the surviving grain density of recorded tracks is reduced to aboutone third of the original value which will produce the desired refreshing efficiencyas we will see.More then one million of nuclear emulsion have already been refreshed2 Being impossible to have such a facility in Gran Sasso underground laboratories40


140012002χ / ndf122.4 / 9Constant 1234 ±25.9Mean 0.006002 ±0.000842Sigma 0.05566 ±0.0008210008006004002000-0.25 -0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2 0.25y - ym (µm)Figure 3.3: Measurement of the OPERA emulsion position resolution obtainedwith a precision measurement tool.in the Tono mine.After refreshing treatment, emulsions are packed in brick-like boxes (withoutlead) and then shipped to Gran Sasso laboratory where the bricks areassembled. Since emulsions will keep the same disposition in the OPERAbricks, cosmic rays collected during transportation will be easily taggedafter the brick exposure and unpacking.The first delivery of emulsions at Gran Sasso is foreseen by the end of2004. Brick assembling will start as soon as the Brick Assembly Machinewill be ready for operations.3.3.2 DevelopmentOPERA is expected to take data for 5 years starting in 2006, for about200 days/year. About 46 neutrino events per running day are foreseen,each belonging to a different brick. The candidate bricks will be regularlyextracted for processing. The CS attached to them will be developedunderground, while the bricks will be routed to the development surfacelaboratory. Due to organizational and manpower reason, it is planned todistribute the bricks extracted in a week over 5 working days of development,thus the films to be developed per day will correspond to about 65bricks (65 CS films in the cavern, about 3800 films at surface).The development procedure is a many-step chemical treatment in adarkroom, lasting about 3 h. Given the above numbers, careful design ofparallel processing and suitable automation [60] are demanded in order to41


Step Time Chemical CompositionPresoak 10’ Na Sulphate (70 g/l)Development 25’ Developer + starter (by Fujifilm Co.)Stop 10’ Al Sulphate - n H 2 O (8.5 g/l)Acetic Acid (5 ml/l)Fixation 35’ Na Hyposulphite (300 g/l)Na Sulphite (12 g/l)Acetic Acid (9 ml/l)Al-K Sulphate (5 g/l)Washing 4 × 20’ WaterAlcohol 20’ Ethyl Alcohol (600 ml/l)Glycerine (200 ml/l)Dry well 1” Drywell (5 ml/l) by Fujifilm Co.Table 3.2: The full development procedure for the OPERA emulsion sheetscope with such a large scale and high-quality development task.A ”standard” development procedure at constant temperature (20 o C)consists of a sequence of the steps described in tab. 3.2 with the relatedchemical composition of the solutions (demineralized water as solvent).The developed plates are available for scanning after about half day ofdrying.Such an amount of nuclear emulsions to be developed needs automatedprocedure. As a first milestone of the project for automated development,a full scale prototype chain has been designed, constructed and assembledin a test hall laboratory made available for OPERA at the LNGS externallaboratory and replicated in a darkroom at CERN for the chain test duringa beam exposure in November 2004.A picture of the prototype chain is showed in fig. 3.4. The variouschemical treatments described in tab. 3.2 are held in consecutive tankswhich are disposed along the chain and a motorized mechanic arm movesthe emulsion plate holders from one tank to the next one. Six of thesechains simultaneously working are needed for the OPERA experiment andthe software to drive all them is under developement.3.3.3 Processed emulsionsThe possibility of obtaining sub-micrometric tracking precision is strictlylinked to the stability of grain relative positions inside the emulsion volumeduring and after the development. Two main deformation effects, theshrinkage and the distortion, have to be taken into account and minimized.42


Figure 3.4: The first prototype of automated scanning chain for OPERAemulsion development.ShrinkageThe silver halide crystals occupy a certain volume of the emulsion, but onlya tiny fraction of them contributes to the latent image. The remaining partis removed during the processing and this leads to a substantial reductionin the emulsion thickness. The shrinkage factor is defined as the emulsionthickness at the time of exposure divided by the thickness after the development.The typical value of shrinkage factors for nuclear emulsions usedin the past experiment was ∼ 2.The shrinkage factor must be taken into account by the tracking algorithm,because the measured track slopes must be multiplied by this factorto obtain the real value (see fig. 3.5).For the OPERA emulsions, the volume reduction is less crucial, becausea lower halide/gelatine ratio has been chosen. Furthermore, a glycerinetreatment of the emulsions after the development has been defined, andthis helps to obtain an emulsion swelling, to restore the original volume.DistortionsEmulsions are mechanically very strong but essentially soft and pliable,subject to mechanical alteration. The main source of distortion is therelease of the tension existing at the time of exposure. During the processing,the emulsion is swollen, to make the mobility of chemical agents easier.This causes the tensions built up at the time of pouring to relax, inducing43


Figure 3.5: The shrinkage effect: the measured track slope ∆z ′ /∆x does notcoincide with the real slope ∆z/∆x. The shrinkage correction is obtainedby multiplying the measured slope by the shrinkage factor ∆z/∆z ′ .distortion effects. As far as OPERA is concerned, the machine controlledemulsion pouring reduces the tension, leading to lower distortion effects.Distortion is a local effect: a typical distortion map measured in anOPERA emulsion is showed in fig. 3.6. The arrows indicate the distortiondirection. The absolute value of the distortion is indicated by the lengthof the arrow. The average value of measured distortions is ∼ 5 mrad.Y (mm)20181614121085 mrad64200 2 4 6 8 10 12 14 16 18 20X (mm)Figure 3.6: A typical distortion map of an OPERA nuclear emulsion.The use of double-sided emulsions coated on a plastic support plate44


improves the angular resolution at a level of 2 mrad, because the trackdirection can be defined by the two points near the support plate, beingpractically free of distortion.3.4 Scanning the OPERA emulsionsThe scanning procedure of the OPERA emulsions will consist of the followingsteps:• Changeable sheet analysisOnce the brick finding algorithm recognizes the brick where the neutrinointeraction took place, the related CS is extracted and analyzed.The scanning procedure is the so-called general scanning: all trackswith any angle are read out within the scanned area.Electronic detectors predict the position of the neutrino interactionvertex inside the brick with about 1 cm accuracy for CC events whilefor NC ones the hadron track accuracy does not allow to provide anyposition prediction within the brick. This implies that about 50 cm 2need to be scanned on average. Unlike hadron tracks, the muonangle can be reconstructed with an accuracy of a few tens of mradand therefore the muon angular information can be used to constraintrack candidates in the CS.The number of neutrino events foreseen during the OPERA datataking is about 50 per day. About half of them are foreseen to bescanned in the European laboratories. Since the efficiency for brickfinding is about 80% we have to consider cases where a second CSneeds to be analyzed in order to define the right brick. A total ofabout 30 CS per day in the European scanning laboratories is thenobtained, for a total of O(10 3 ) cm 2 .The OPERA emulsion sheets consist of two emulsion sides to be separatelyanalyzed. Hence with a scanning speed of 20 cm 2 /h, 10 cm 2 /hof CS can be scanned. Taking into account also dead times, the numberof microscopes needed for the CS analysis is of the order of 10.The CS scanning will be performed in a dedicated ”Scanning Station”located at the Gran Sasso laboratory.• Scan backIf the neutrino interaction is confirmed by the CS analysis, the brick isextracted by the brick manipulator system, exposed to cosmic rays forsheet to sheet alignment, developed and sent to the OPERA scanninglaboratories for the analysis. Starting from the most downstreamemulsion sheet (the nearest to the CS), all tracks found in the CS45


are followed up from sheet to sheet until the track disappears. Thelead plate downstream of the first plate where the track disappearedis defined as the vertex plate.The procedure of track following is called Scan Back. The scanningis performed in an area determined by the accuracy of the predictedposition. Track segments are searched for in an angular range limitedby the prediction angular accuracy and by the possible distortion ofthe emulsion layers. A 3 × 3 mm 2 area is needed for each track inthe first emulsion layer, as the mechanical alignment between the CSand the brick is of about 1 mm. In the following emulsion sheets,data taking of a single field of view of (∼ 360 × 280 µm 2 ) for eachtrack is sufficient, since the intercalibration done by means of passingthrough cosmic rays allows a precision of 10 − 20 µm.The number of plates to be analyzed in each event depends on theneutrino interaction position inside the brick. On average, about30 emulsion sheets will have to be analyzed. The time needed forthe scan back procedure is dominated by intercalibration and platechanging. The first operation requires a 1 cm 2 area scanned in generalscanning mode for each plate and therefore takes a few minutes.• Vertex reconstructionOnce the neutrino vertex position is found, 10 sheets are used (2 upstreamof the lead plate and 8 downstream) for vertex confirmationand study. A 5 × 5 mm 2 general scanning area is needed for thispurpose. One microscope is capable of performing the scan back andvertex reconstruction of two bricks per day. This implies the needfor about 15 microscopes. They will be dislocated in the Europeanlaboratories of the different institutes.The procedure described above is performed in a fully automatic way.Each microscope will be provided with a computer driven automatic platechanger. All the information on tracks and vertexes will be stored andsubsequently used by dedicated analysis tools, aiming at particle identification,momentum or energy measurement and shower reconstruction. Thefull kinematic analysis of candidate events will require precision measurementsperformed by dedicated microscopes.46


Chapter 4The automatic scanningsystem4.1 IntroductionThe feasibility of an experiment using a large mass emulsion target dependson the realization of very fast automatic scanning systems.Nuclear emulsion plates record tracks of charged particles with an accuracyof less than 1 µm. The emulsion analysis is done by microscopeswith a focal depth of a few microns: the three dimensional structure of thetracks is reconstructed by raising or lowering the focal plane in order tospan the whole emulsion thickness.The first fully automated system (Track Selector, TS) [61] was developedat the University of Nagoya in Japan in 1985. The basic idea wasto put together two front-end processors for automatic mechanical controland video image processing. A first prototype, called DOMS Interface, hadalready been developed in the ’70s for a semi-automatic emulsion analysissystem. It guided a dc-motor to drive the stage of the microscope dynamically,while reading out the position with a linear encoder with an accuracyof 1 µm in each direction. The video image processor (called ”Track Selector”)was equipped with 16 frames memories. The focal plane is movedalong the depth of the emulsion layer, which is sampled at 16 regular intervals.After binarization, the image of the in-focus grains at each depthis stored into each frame memory. A tomographic three dimensional imageis then constructed. To search for a track at given angle, the imagesare regularly shifted with respect to each other by an amount such thatthe track should appear as perpendicular to the emulsion sheet. A trackis then recognized by piling up the images and evaluating the coincidencepulse height.The basic TS tracking principle was used in its improved versions New47


Track Selector (NTS, developed in 1996) and Ultra Track Selector (UTS,developed in 1998) [62]. The UTS takes advantage of the implementationof several image processors working in parallel.The UTS has been used for the emulsion analysis of the CHORUS [59]and DONUT [58] experiments . It is able to scan emulsions with a maximumspeed of about 2 cm 2 /h.As discussed in the previous chapter, OPERA requires an improvementof about a factor of ten in speed, i.e. a speed of about 20 cm 2 /h. To achievethis goal, two different R&D programs are carried out.The Nagoya University group aims to a further improvement of the UTSsystem (the so-called Super-UTS). The key features of the S-UTS are thehigh speed camera with 3 kHz frame rate and a piezo-controlled movementof the objective-lens, synchronized to a continuous stage motion inorder to avoid go-stop of the microscope stage while taking images. Thesystem uses a Fast Programmable Gate Array (FPGA), a Fast Memoryand a grabber board connected to a CCD camera (512 × 512 pixel).A different system has been developed in a collaboration among Europeanlaboratories, working together to the realization of an EuropeanScanning System (ESS). The R&D was based on a previous work done bythe Salerno group with the SySal (Salerno System) track reconstructionsystem [63] for the CHORUS experiment and by the Naples group withthe microscopes’ mechanics, camera and frame grabbers [64].The ESS uses a different approach with respect to the TS. It is based onthe Multi-Track approach: all tracks in each field of view are reconstructedregardless of their slope. The raw data are a series of tomographic imagesof the emulsion taken at different depths. Information about cluster positionsand sizes are stored. The tracks are then reconstructed by combininggrains from different layers with a dedicated software program.The ESS is based on the use of the state of the art commercial hardwarecomponents rather than of dedicated hardware as for S-UTS. The softwareused for data taking and track reconstruction is conceived in a modularstructure, which provides the flexibility needed to upgrade the system followingthe technological progress.4.2 The ESS hardwareThe ESS design had to take into account the request of a high scanningspeed and of position and angular accuracies adequate for event analysis.The system was conceived with the following features:• high-performance mechanics with sub-micron position accuracy andvery small settling-time;48


• optics with a large field of view;• camera with mega-pixel resolution and high frame rate;• powerful image processors.The first prototype of the ESS was assembled in the Napoli laboratory inMarch 2001. Its scanning speed was about 1 cm 2 /h. Since then, it hasbeen upgraded several times, both for the hardware and software point ofview. The version of the ESS system set up in 2004 reaches the desiredscanning speed of 20 cm 2 /h.Figure 4.1: The ESS systemOne of the three ESS systems presently working in the Napoli laboratoryis shown in fig.4.1. A motor driven scanning table for horizontal(XY) movements and a granite arm are fixed to a high quality table, whichprovides a virtually rigid and vibration-free working surface, holding thecomponents in a fixed position. The light system is located below the microscopetable. Vertical movements (Z) are obtained by a motor drivenstage, which is fixed to the granite arm. The optics and the digital camerafor image grabbing are mounted on the vertical stage. The emulsionsupport is provided with a vacuum system to avoid unwanted movementsduring the data taking.49


4.2.1 Mechanical stagesThe scanning table and the vertical stage have been developed in collaborationwith the Micos company 1 . They are commercial products modifiedaccording to OPERA specifications.The stages are equipped with a stepping motor ”NanoStep RFK Series5-Phase Microstepping System” produced by the Japanese companyVexta. Stepping motors are excellent for precise position control. Typicaldisadvantages of stepping motors in comparison with servo motors do notoccur because of the use of microstepping units and the feedback of linearencoders (closed-loop).The stage controller is a FlexMotion board provided by National Instrumentsand is inserted into the host PC.A Micos MS-8 scan table with a 20.5cm range in both horizontal directionsis mounted on the tabletop. The stage horizontal coordinates areread out by two linear encoders with an accuracy of 0.1 µm. Limit switchesare integrated on each axes.The movement of the horizontal stage is one of the crucial points toobtain a suitable scanning speed. The speed, the acceleration and the timeprofile of the movement have to be chosen in order to minimize the timeneeded to change the field of view.The horizontal displacements when changing field of view are 360 µm(X coordinate) and 280 µm (Y coordinate). The shape of the camera sensoris in fact rectangular. The total time for the displacement is given bythe sum of the moving time and the settling time. The former is necessaryto reach the target point, the latter to wait for oscillations to be containedwithin 0.3 µm, which corresponds to the linear dimension of the emulsionarea grabbed by a single camera pixel. Of course, by increasing speed andacceleration, the moving time decreases and the settling time increases.After several tests the optimized working conditions were set as:speed :30 mm/sacceleration : 200 mm/s 2for both X and Y axes. The resulting total time needed is shown in fig. 4.2.The X axis needs about 90 ms, while the Y axis about 140 ms. The RMSof the two distributions is very different. The different behavior is due tothe scanning table design: Y movements carry the whole table, while Xmovements involve only a lighter part of the table. The scanning procedureminimizes the number of Y displacements.The vertical axis is provided by the Micos company (model MT-85).It is equipped with a linear encoder (0.1µm) and integrated limit switches.1 MICOS ITALIA GmbH, via S. Protaso, 39 I-20010 Bareggio MI.50


Figure 4.2: The total time needed for X (left) and Y (right) displacementsduring the ESS data taking operations.The lower switch was modified and substituted by an external one in orderto make easier its positioning: its task is very important, because it preventsthe objective to touch and scratch the emulsion.The positioning of the vertical stage and of the optic support to thegranite arm is crucial, in order to prevent any appreciable slant with respectto the vertical direction. Especially delicate is the X direction, because ofthe rotational degree of freedom at the assembling level. The operation isdone by using mechanical comparators, with a few milliradiants accuracy.The residual slant can be evaluated and corrected by comparing tracksacquired in two different data taking, using the same scanning area androtating the emulsion sheet by 180 o . If the sheet has been exposed to someparticle beam with well defined angles, both the angular projections arereversed after the sheet rotation while the microscope vertical slant is inone case added, in the other subtracted to the absolute value of the angularprojection. This makes possible the evaluation of the slant within a fewhundred microradiants, as shown in fig. 4.3.The two emulsion layers of the emulsion sheet are scanned separately,in order to avoid a repetitive time-consuming plastic base crossing. Thevertical stage reaches its starting point (the top edge of the emulsion layer)during the change of field of view. The starting points, called Relevant Z’s,are individuated by a dedicated procedure which periodically performs afull tomography of the emulsion sheet, starting about 50 µm above theemulsion layer. The layer edges are recognized by counting the grains insideeach frame. The Relevant surface is obtained by interpolating themeasured Relevant Z’s, in order to obtain the starting point for each fieldof view.During data taking, the vertical stage moves at constant speed, in orderto have equally spaced frames. There is no time to read out the vertical51


Figure 4.3: Evaluation of the ESS vertical slant. The same emulsion areais scanned twice (the second time after a 180 o rotation). By comparing theangular positions of particle beams a few hundred microradiant accuracycan be obtained on the slant evaluation.positions during the stage movement: the tracking algorithm assumes thatthe frames are equally spaced. The constant speed movement can be testedwith a dedicated tool which uses a buffer of the stage control board: it providesa few percent accuracy (about 0.1 − 0.2 µm) as shown in fig. 4.4.The vertical speed is calculates by taking into account the camera framerate (376 frames per second), the number of desired frames and the layerthickness (42 µm). The number of frames should be enough to allow a goodtrack recognition and their relative distances should be comparable withthe microscope effective (see section 4.3.2) focal depth of about 2.5 µm inorder to prevent sub- or over-samplings: 16 frames are then grabbed foreach view. A 987 µm/s movement speed is obtained. The time needed toscan an emulsion layer is 42.5 ms.For test purposes, a tool to simulate microscope movements duringdata taking was developed. Even if the read out of the coordinates is notpossible during data taking, a certain quantity of data can be stored in alocal buffer of the stage controller board. An internal clock with 3 millisecondsampling time allows a very precise description of the movements.Results are showed in fig. 4.4: the insertion of a dead time of a few millisecondsbefore and after the frame grabbing brings to a 150 ms cycletime. By taking into account the effective (see the next section) field ofview size (360 × 280 µm 2 ) we obtain the maximum scanning speed of theESS system, which is about 24 cm 2 /h. The above cited average speed of20 cm 2 /h includes also the time needed for the Relevant Z’s search.52


Figure 4.4: Simulation of the ESS movements cycle during data taking.4.2.2 The optical system and the image grabbingThe main components of the optical system are the objective and thetrinocular tube, both from the Nikon company.The optics is based on a Nikon infinity optical system: the objective isdesigned so that light emerging from the rear aperture is focused to infinity.A second lens hosted in the trinocular, known as tube lens, forms theimage at its focal plane.The ESS objective is requested to have a working distance (WD) higherthan 300 µm, in order to be able to scan both emulsion sides, and a NumericalAperture (NA) sufficiently large 2 . It is an oil immersion objective,since the ESS uses oil as intermediate medium between emulsion and theobjective front lens: in this way light rays passing through the emulsionsheet encounter an optically homogeneous medium because the refractiveindexes of the crossed media are very similar (n base = 1.48, n emulsion = 1.51- 1.52, n oil = 1.51).The ESS objective is produced by Nikon 3 and its characteristics fit theESS requests. Chromatic aberrations are completely corrected inside theobjective, while a flat-field correction yields flat images. The objective focaldepth is 0.54 µm.2 The Numerical Aperture is a critical value that indicates the light acceptance angle,which in turn determines the light gathering power, the resolving power, and the depthof field of the objective.3 Nikon CFI Plan Achromat 50x oil, NA 0.90, WD 0.4 mm.53


Images from the objective go across the trinocular and reach the camerasensor, which is placed at about 23 cm from the objective front lens.The ESS camera is the MC1310 from the Mikrotron 4 company. It isa high-speed megapixel CMOS camera with Full Camera Link interface 5 .Unlike high resolution CCDs, modern CMOS sensors offer high resolutionand extremely high data rates, according to the ESS requests. The CMOSsensor has 1280 × 1024 pixels, each having size of 12 µm (1.5 × 1.2 cm 2 isthe sensor area). The camera is able to work up to a rate of 500 framesper second (fps) at full resolution, which implies a maximum data rateof 660 MB/s. The ESS uses a camera configuration at 376 fps, which issatisfactory for the requested scanning speed.The selected exposure time for the ESS data taking is 1/6000 (about0.17 ms). The movement of the vertical stage during this time is ∼ 0.16 µm:this introduces an indetermination on the cluster Z positions which is comparablewith the one from the constant speed movement, as explainedabove. We will see in section 4.3.2 that the indetermination introduced bythe ESS effective focal depth is about one order of magnitude larger.The images are grabbed in a 256 level grey scale (the information storedby one pixel is converted in a 8-bit sequence) and sent to the frame grabberin the host PC.A typical emulsion image, as it appears in the ESS monitor, is showedin fig. 4.5, together with its pixel grey level distribution.Figure 4.5: The image of one ESS field of view (left) with its grey levelcontent distribution in logarithmic scale (right), used to make visible thepeak due to the grains.4 Mikrotron GmbH Landshuter Str.20-22 D-85716 Unterschleissheim Germany.5 Camera Link is a new digital standard that sends back serialized data instead ofparallel ones.54


The measured field of view (FOV) is 394.5 ± 0.1 × 315.6 ± 0.1 µm 2 .The above scanning speed calculations used the effective field of view dimensions(360 × 280 µm 2 ) because an overlap area between two adjacentfield of view is needed to avoid tracking inefficiencies in the edge. By choosinga ∼ 35 µm overlap area, the angular acceptance is assured up to 700mrad for any tracks. Multiple counts are recognized by the tracking routine.The procedure to evaluate the camera resolution is based on the grabbingof a pattern of clusters in different positions within the camera sensorreference system. The comparison between displacements in pixels and inmicrons (as read out by the scan table encoders) allows to determine thecamera resolution. The resolution of the sensor is:r = 0.3082 ± 0.0001 µm(4.1)pixelThe described procedure is also useful to evaluate the camera angularrotation with respect to the scanning table reference system, with an accuracybetter than 1 mrad.The frame grabber and the image processor are integrated in the sameboard: the Matrox Odyssey Xpro, the must recent vision processor producedby the canadian Matrox company. The Odyssey board is specificallydesigned for fast transfers and on-board processing. It bears a MotorolaG4 PowerPC microprocessor, a NOA unit (Neighborhood Operations Accelerator),designed to quickly perform convolution operations, and 1 GBDDR SDRAM memory. The internal bandwidth can manage over 4 GBper second, while the external I/O bandwidth up to 1 GB per second.4.2.3 The light systemThe light system is placed below the scanning table. It was developed byNikon-Italy after a joint R&D activity carried out in collaboration withthe Napoli and Bari groups.The light is supplied by a tungsten-halogen lamp that operates on adirect current and produces up to 100 watts power for illumination. The12 volts lamp voltage is controlled by a power supply which uses a serialconnection to the motion control board to be interfaced to the ESS software.The light from the lamp-house is directed into the microscope basethrough a collector lens, and then through a glass diffuser, before beingfocused on the aperture diaphragm of the substage condenser.In the so-called Köhler illumination, an image of the light source isfocused at the condenser aperture diaphragm to produce parallel (and unfocused)light through the plane of the specimen or object. The condenser55


gathers light and concentrates it into a cone of light that illuminates thespecimen with uniform intensity over the entire field of view. A wide coneof illumination is required for optimum resolution of the specimen. Thecondenser light must be properly adjusted to optimize the intensity andangle of the light entering the objective front lens. The size of the condenseraperture diaphragm can be used to control the numerical apertureof the light cone that illuminates the sample. A field diaphragm controlsthe quantity of light entering the substage condenser and, consequently,the rest of the microscope.4.3 The online DAQ4.3.1 The DAQ softwareThe online DAQ software for the emulsion automatic scanning and tracksegments reconstruction was developed on a Windows platform 6 in the.NET framework by using the object-oriented C++ and C# languages. Itis based on a modular structure where each object carries out a well definedtask.Each object has a corresponding parameter window for configurationsetting. In fig. 4.6 the program control panel and the parameter windowof the main object are shown.The list of all modules with their functionality is reported below:Objective stores the information related to the used objective and performsthe pixel to micron conversion.Odyssey2 drives the Odyssey board where the camera frame flux in handledand the clustering process is hold.FlexStage3 is interfaced to the stage controllers and set the movementsmodalities.SmartTracker7 is responsible for track pattern recognition, recognizingsequences of geometrically aligned clusters.SmartFitter performs the tracks fit.DataIO handles data Input/OutputSheetMap2 transforms coordinates and vectors from the current stagereference frame to the emulsion local reference system (which is defined bya grid of fiducial marks printed on the emulsions).ScanServer allows the remote PC to start and controls the data acquisitionsprocesses.6 The choice was forced, being the only one supported from both the motion controland frame grabbing boards56


Figure 4.6: Left: the control panel of the ESS online DAQ, where theobjects with their configuration files are represented. Right: the windowfor parameter setting of the main object configuration.VertigoScan3 is the main module, which uses all the other objects tomanage the scanning.The scanning output is a collection of raw data files (in binary format)which are temporarily written on the local disk. They contain informationrelated to detected microtracks plus some general information related to theacquisition process, such as the detected Relevant Z’s, useful to investigatethe flatness of the emulsion surface and its support and the number ofcluster contained in each grabbed frame, with which the thickness of theemulsion and the correct evaluation of the starting points can be checked(Fig. 4.7).An Oracle database, where the output of data taking will be stored, isunder development.4.3.2 Frame processing and clusteringThe grabbed images are sent to the Odyssey board where the cluster searchis performed. The first operation is the flat field subtraction. The flat fieldconsist of one image grabbed outside of the emulsion, which will be latersubtracted from the inside images. Its clusters are camera dark spots,essentially due to dust present on the camera sensor. They have to beremoved because otherwise they would lead to vertical fake tracks.The second step is the image filtering, aiming at the enhancement of57


5045403530252015105020181614121086420Relevant Z4 (µm)View Y Coordinate (mm)02 4 6 8 10 12 14 16 1820View X Coordinate (mm)Clusters3000250020001500100050000 10 20 30 40Z frame (µm)Figure 4.7: Left: Detected Relevant Z4 in a 2 × 2 cm 2 data acquisition.Few microns of Z variations are observed. Right: Number of clustersversus corresponding frame scatter plot. The central dip is due to a 1 µmplastic layer.the contrast between focussed and unfocussed grains (i.e. shadows of grainswhich lie out of the investigated frame). Filtering is obtained by applyinga convolution filter: each pixel output value is the result of a weighted sumincluding the neighborhood pixels input values. The weights are given bythe filter kernel. The ESS uses a 6 × 6 kernel:K =and the output pixel value is given by:p(i, j) =6∑6∑m R =1 m C =11 1 1 1 1 11 2 3 3 2 11 3 −13 −13 3 11 3 −13 −13 3 11 2 3 3 2 11 1 1 1 1 1k(m R , m C ) · g(i + m R + d R , j + m C + d C ) (4.2)where k is the kernel element, m R(C) is the row (column) index, g is thegrey level input value.The effect of the convolution extends the original 255-values grey levelscale to a wider one, making the background shape flat and therefore easierto be removed. After the convolution, a threshold is applied and the dataare binarized: pixels with a value that exceeds the threshold are classifiedas black, the remaining ones as white.It is important to stress that the chosen kernel and threshold define insome way the effective focal depth of the apparatus, being both crucial in58


the definition of the shadow acceptance. The typical threshold values usedfor the ESS operations (700-1000) and the above described kernel bring toan effective focal depth of ∼ 2.5 µm: this value is obtained by a dedicatedtool which follows clusters until they disappear in frames grabbed veryclose each other.In order to have an homogeneous cluster distribution inside the grabbedframes, an equalization procedure is performed. It consists in definingseveral cells, scanning an emulsion surface and counting the total numberof detected clusters in each cell with different applied thresholds. In orderto keep constant the total number of clusters in each cell, a threshold maphas to be used instead of a unique threshold value.Figure 4.8: A typical ESS equalization threshold map.A typical threshold map is shown in fig. 4.8. The uniformity of the mapreflects the uniformity of the illumination of the field of view and of thecamera sensor response. The dark corner indicates the need of a thresholdlowering in the corresponding area (at a level of 70% of the central value).Fig. 4.9 shows the distribution of clusters of an ESS data acquisition inpixel coordinates.The last step is the cluster recognition. The algorithm looks for blacksequences inside each row and joins them to assemble clusters. A cut onthe cluster area helps to discard background due to the noise in the camerasignal. Finally, the cluster barycenter is calculated, and its position andarea are stored in the output file. The resolution on the X,Y barycenterpositions is about 0.15 µm (about one half of the camera resolution).59


1400120010008006004002001000 0Y pixel800600400200002004006008001000 1200X pixelFigure 4.9: Bidimensional cluster distribution of an ESS data acquisitionin pixel coordinates. The distribution is uniform, as a result of opticalparameter tuning.Figure 4.10: The clustering process: the first image shows the grabbedimage, the second one is the result of filter application, the last one is afterbinarization and minimum area cut.4.3.3 TrackingThe ESS tracking is performed by the two processors in the host PC. Asshown in fig. 4.4, the ESS cycle is divided in two phases: image grabbing(about 50 ms) and stage displacement to the next field of view (about100 ms). The image clustering starts as soon as the first frame has beengrabbed and terminates some tens of milliseconds after the end of the grabbing.The remaining time (∼ 60−70 ms) would not be enough for tracking.For this reason an asynchronous scanning mode has been developed: thetracking is performed during the whole cycle time (∼ 150 ms) using theframes grabbed in the previous cycle.60


The tracking consists of two main steps: pattern recognition and trackfitting. Pattern recognition combines clusters from different layers to recognizegeometrical alignments. Track fitting performs a linear fit of theclusters and evaluates track slopes.In order to reduce the CPU time, the field of view is divided in cells,whose linear dimension is about 25 µm.The first operation is the trigger search. The layers are numbered (1-16)and several layer sequences are defined. A sequence is formed by one layerbelonging to the top sector of the emulsion, another to the bottom sectorand some central layers. For a given sequence, all the cluster combinationsbetween the corresponding cells of the top and the bottom layers are considered:the line joining the two clusters is defined and an acceptance roadaround it is defined. Central layers are then considered and other clustersare searched for: if at least one cluster is found in one of the central layers,a trigger is produced.The number and the layer content of the sequences is optimized byMonteCarlo simulations. A maximum number of trials is defined in orderto limit the processing time.The second operation is the track following. The same acceptance volumeis defined for each trigger and all layers are investigated. For trackfollowing the neighboring cells are also considered, in order to increase theangular acceptance.In order to define the cluster acceptance area, two new variables, calledlongitudinal and transverse distances, are introduced. These variables correspondto the cartesian coordinate of the cluster in a new reference frame,which is defined in the focal plane where the cluster lies. The referenceframe is defined in the following way: the axes origin is the point wherethe line temporary assumed as microtrack intersects the plane. The lineprojection on the plane defines the longitudinal axis, while the transverseaxis is the perpendicular one. The transverse distance is the cluster coordinatewith respect to the transverse axis. Its distribution is gaussian andindependent on the track angle, as the transverse distance mainly dependson camera resolution. On the other hand, the longitudinal distance has twocontributions, one is gaussian as for the transverse distance, the other isdue to focal depth and is uniformly distributed: this introduces an angulardependance, as we will see in the next section.In each frame the acceptance area is an ellipse, whose minor semi-axisis 0.4 µm (about 3 σ of transverse resolution), and the major semi-axisis 0.4 + F/2 · θ, where F is the effective focal depth (2.5 µm) and θ themicrotrack angle. All clusters found inside the volume are assigned to thesame microtrack. If the number of clusters exceeds a defined threshold, thecluster sequence is transferred to the fitting routine.Once all clusters belonging to each microtrack have been found, a bidi-61


mensional linear fit is performed. The microtrack slope with the coordinatesof the base intersecting point, the number of cluster and the microtrackσ mtrk (the average transverse distance of clusters from the fitting line)are stored in the raw data output file.Figure 4.11: A schematic view of the ESS tracking. Horizontal grey stripsrepresent the focal planes, while horizontal black lines the correspondingZ coordinates. The crosses show the cluster positions. The shadowed areaincluding the fitted track shows the allowance volume.4.3.4 Microtrack characteristicsIn the ESS system errors on cluster X,Y position are mainly dependenton camera resolution, while the main error source for Z coordinate is theeffective focal depth (F ≃ 2.5 µm).62


As previously seen, we have σ x,y ∼ 0.14 µm andσ z =F √12≃ 0.7 µm (4.3)since the cluster Z coordinate is uniformly distributed.In order to evaluate the microtrack angular resolution, a basetrack hasto be reconstructed. The basetrack is defined by joining the microtrackpoints closest to the plastic base. Since that points lie in region unaffectedby distortion effects, the basetrack has angular resolution approximativelyone order of magnitude better than the microtracks. Thus the angulardifference between microtrack and basetrack will provide an estimation ofthe microtrack angular resolution.Before showing the achieved resolutions, we present some considerationsto explain their dependance on the angle.In a two-dimensional reference frame (i.e. considering a microtrack inthe X-Z plane) and for a fixed ∆Z = Z true − Z frame , if we assume that theZ coordinate is unaffected by errors (i.e. considering Z as the independentvariable during the tracking fit) the induced error on X position is proportionalto the tangent of the microtrack angle (fig. 4.12). This implies thatthe angular resolution increases with increasing angle of the microtracks.Figure 4.12: The tracking algorithm assigns to each cluster the Z coordinateof its parent frame and use it as independent variable for the tracking fit.The error on Z position, which depends on the effective focal depth, isin this way transferred to the horizontal coordinate, the last one beingproportional to the tangent of the microtrack angle.Fig. 4.13 shows the resolutions for angles θ x = 0 and θ x = 0.4 rad.The microtrack angular resolutions are respectively about 10 and 26 mrad.63


3503002502χ / ndf19.08 / 23Constant 361.9 ±8.4Mean7.582e-05 ±1.936e-04Sigma 0.01047 ±0.00015100802χ / ndf74.46 / 64Constant 91.83 ±2.76Mean 0.0006888 ±0.0006084Sigma 0.02617 ±0.00050200601504010050200-0.1 -0.05 0 0.05 0.1∆ θ (rad)0-0.1 -0.05 0 0.05 0.1∆ θ (rad)Figure 4.13: Microtracks angular resolution for vertical tracks (left) andfor tracks with 0.4 rad inclination angle.With a good approximation, for both X and Y projections the angularresolution is expressed by:σ(θ) = σ(0)(1 + 4 · θ) (4.4)The error ∆X in the fig. 4.12 corresponds to the longitudinal distance.If we define σ mtrk as the average of transverse distances of all clustersbelonging to a given microtrack, the average σ mtrk is about 0.14 µm and isindependent on the microtrack angle (fig. 4.14).900080007000600050004000300020001000Mean 0.1436RMS 0.05336σ mtrk0.40.350.30.250.20.150.10.0500 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4σmtrk00 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1Microtrack angle (rad)Figure 4.14: Left: σ mtrk distribution of an ESS acquisition: the mean valueis ∼ 0.14µ. Right: Scatter plot of σ mtrk versus the microtrack angle.The number of clusters in microtrack depends on the emulsion sensitivity.As seen in the previous chapter, the OPERA emulsion sensitivity is 33grain/100 µm: a vertical track crossing one emulsion layer should have on64


average 42 × (33/100) = 13.9 grains. The measured value is 13.7, as shownin fig. 4.15, is in a good agreement with the prediction. This means thatthe sampling is correct and no sub- or over-sampling is induced.700600500400300200100Mean 13.74RMS 2.01350 Mean 12.83RMS 2.0493002502001501005006 8 10 12 14 16Cluster number06 8 10 12 14 16Cluster numberFigure 4.15: Cluster number distributions for vertical tracks (left) and fortracks with 0.4 rad inclination angle (right).A different result is reported for tilted tracks. For an angle of 0.4 radwe would expect 14.9 average grains while only 12.8 are found. This behavioris explained as follows: grains belonging to vertical tracks are piledup along a vertical line which is parallel to the light propagation direction.Grain shadows are arranged along the same line and determine an increaseof the grey level values of their adjacent grains. This does not happenfor tilted tracks, because grain shadows remain vertical. Therefore thereis a dependence on the angle of grains grey level content. If the same filterand threshold is used we obtain a different effective focal depth. Thetilted tracks are slightly sub-sampled and this explains the results abovereported.The criterium applied for microtrack selection requires a minimum of 7clusters for each microtrack. This criterium slightly effects the microtrackfinding efficiency: in fact the cluster number distributions follow the Poissondistribution and the probability to observe 6 or less for distributionswith mean value of 13.8 (vertical microtracks) or 12.9 (0.4 mrad inclination)is 1.6% and 2.7% respectively. On the other hand, the measured backgrounddue to random alignment of fog grains is ∼ 200 microtracks/view(or 1500 microtrack/mm 2 ), an acceptable value as we will see in the nextchapter.65


4.4 Track reconstructionThe online DAQ is aimed at the microtrack reconstruction, as described inthe previous section. In the general scanning mode all microtracks withina given angular range are searched for, while in the scan back mode, welldefined position and angular area are investigated.The track reconstruction is performed by an offline analysis tool inseveral steps:• microtrack linking (or basetrack reconstruction);• emulsion sheets intercalibration;• track reconstruction.The offline reconstruction tool used for the following analysis is FE-DRA 7 (Framework for Emulsion Data Reconstruction and Analysis), anobject-oriented tool based on C++ language and developed in Root framework.4.4.1 Basetrack reconstructionThe basetrack reconstruction is performed by projecting microtrack pairsacross the plastic base and searching for an agreement within given slopeand position tolerances (fig. 4.16).In order the reduce the CPU time, the two emulsion sides are subdividedin cells. The cell dimension defines the angular acceptance.The microtrack slopes are used only to define the angular agreement,while, as previously seen, the basetrack is defined by joining the microtrackpoints closest to the plastic base.The basetracks are selected on the base of a χ 2 cut. The χ 2 is definedas:χ 2 = 1 [(θXt − θ XB ) 2+ (θ Xb − θ XB ) 2+ (θ Y t − θ Y B ) 2+ (θ ]Y b − θ Y B ) 24 σX2 σX2 σY2 σY2 (4.5)where θ X(Y )b(t) are the microtrack X(Y) angular projections of the bottom(top) side, θ X(Y )B the X(Y) basetrack X(Y) angular projections and σ X,Ythe microtrack angular resolution.A typical χ 2 distribution is shown in fig. 4.17. A χ 2 < 6 cut wasapplied. The requirement of χ 2 < 6 is aimed to minimize the signal loss.We will see in the next chapter that a good signal to background ratio witha small loss of the tracking efficiency is obtained with a cut defined in the7 http://ntslab01.na.infn.it/fedra/66


Figure 4.16: The principle of basetrack reconstruction: the microtracksmatching is obtained when an acceptable agreement in slope and positionis found. The basetrack is formed by joining the two points closer to thebase.1000Entries 30538800signal600400200background00 1 2 3 4 526χFigure 4.17: Basetrack χ 2 distribution. The background is due to randomfog grain alignments.(χ 2 , cluster number) plane.The basetrack angular resolution is obtained by studying the angularresiduals with respect to fitted tracks. The obtained resolution is shown infig. 4.18.The basetrack angular resolution is well approximated by the expression:σ B (θ x,y ) = σ B (0)(1 + 4 · θ x,y ) σ B (0) = 1.4 mrad (4.6)67


(rad)θ B-θ T0.030.020.010-0.01-0.02-0.030 0.1 0.2 0.3 0.4 0.5θ x (rad)Figure 4.18: Scatter plot of the basetracks angular resolution versus thebasetrack angle.which is the analogous of the expression 4.4 for microtracks.4.4.2 Emulsion plate intercalibrationIn order to define a local reference frame, a grid is printed on each emulsionsheet. Track positions are measured with respect to this grid and thismakes any subsequent repositioning easier.The Brick Assembly Machine is designed to build the OPERA bricksin an automated way: the emulsions sheets relative positions should be definedwith a few hundreds µm precision. The manual packing and griddingis even worse, bringing to misalignments up to 1 mm.In the OPERA standard operation, the bricks extracted from the detectorare exposed to cosmic rays for the alignment before the unpacking.For beam tests, if bricks are exposed to high density beams no cosmic raysexposure is needed, as the beam particles themselves can be used for thealignment.The emulsion plate intercalibration is done by subdividing plates in severalcells and applying some relative displacements. The right translationwill maximize the number of track coincidences. The alignment betweentwo plates is described by an affine transformation, defined as:( ) ( ) ( x′ a11 ay ′ =12 xa 21 a 22 y68)+(b1b 2)(4.7)


The use of a large quantity of passing through tracks allows a submicrometricdetermination of the relative displacements. Fig. 4.19 showsthe distribution of the differences of track positions in adjacent emulsionsheets. The average of the distribution corresponds to the error on thesheet intercalibration. The measured value is about 0.1 µm.500χ 2/ ndf164.4 / 70Constant 498 ±7.2Mean 0.0956 ±0.0087Sigma 0.7607 ±0.00684003002001000-6 -4 -2 0 2 4 6- X 2X 1(µm)Figure 4.19:sheets.The difference of track position in two adjacent emulsion4.4.3 Track reconstructionThe algorithm used for track reconstruction is essentially based on findingand fitting a sequence (chain) of basetracks. The basic principle consistsin building basetrack pairs, trying to extend the pairs in both directionsinside the brick and building basetrack chains. The chain is interruptedwhen a decay or an interaction vertex is found, or by the inefficiencies ofthe basetrack finding algorithm. A merging procedure of tracks recoverssuch inefficiencies. The track fitting is based on Kalman filtering algorithm[65].69


Chapter 5Measurement of the ESSperformances5.1 IntroductionThe performances of a scanning system are mainly specified by the scanningspeed, the tracking efficiency and the tracking purity. In this chapterwe describe the measurement of the ESS performances.The larger the efficiency, the lower the purity, therefore the optimizationof these two physical quantities must be done at the same time keepingin mind the requirements of the experiment. Measurements of tracking efficiencyand of instrumental background have been performed all along thework for the development of the microscopes. They have been instrumentalin understanding the ways to reach the required scanning speed withadequate performances.The first ESS prototype was assembled in the Napoli laboratory in 2001.Its scanning speed was about 1 cm 2 /h. The scanning of such thin emulsionsrevealed new difficulties. The attempt to adapt previously developedsoftware was unfruitful. In the first prototype, the tracking efficiency andthe background density were far from the desired values.The optimization of the scanning system, both from the hardware andsoftware point of view, took about two years. A new tracking algorithmwas defined, and a long parameter tuning was performed. On the otherhand, several hardware components were tried until the most suitable configurationwas defined.A large effort was made to improve the microscope stability. As describedin the following, the signal selection and the background rejectionare based on the basetrack χ 2 analysis. If the microtrack angular resolutionis improved, the χ 2 rejecting power improves accordingly, since the χ 2 ofthe signal stays the same while the χ 2 of fake tracks increases.71


A procedure for the efficiency evaluation was defined and some referenceemulsion sheets exposed to particle beams were periodically scannedto test the improvements. Simultaneously, unexposed emulsion sheets wereused to evaluate the instrumental background.Satisfactory results were reached in 2003, with a scanning speed ofabout 5 cm 2 /h. Two major changes were then performed to reach the desiredscanning speed. One consisted in the use of a faster camera and anew image processing board, the other one in speeding up of the trackingalgorithm to handle with the increased data flow.The time evolution of the ESS scanning speed is shown in fig. 5.1.The system reached its final configuration in summer 2004 and becameavailable for physics measurements. This chapter ends with some preliminaryresults obtained for the momentum measurement by the MultipleCoulomb Scattering. The agreement with previous measurements constitutesa further validation of the performances of the new system.Figure 5.1: Time evolution of the ESS scanning speed.5.2 OPERA test beams at CERNIn order to tune the ESS system and to measure its tracking efficiency,emulsions are exposed to particle beams.The evaluation of the tracking efficiency is based on the reconstructionof tracks passing through an OPERA-like brick: the fraction of found holesduring the track reconstruction gives an estimate of tracking inefficiency.Several tests have been performed at the CERN PS, where high intensitypion beams are available. The PS is particulary suitable for theOPERA test beam. High momentum particles (up to 10 GeV/c) can be72


provided, in order to minimize their Multiple Coulomb Scattering insidethe OPERA brick. Since emulsions record all charged particles, a suitabletrack to track distance has to be ensured while performing the exposure.Therefore, there is the requirement for a low intensity flux, of the order offew particles per mm 2 . This flux can be easily achieved by tuning collimatorsand targets in the PS control area.The tracking efficiency is expected to depend on the track angle. Thisimplies the need of rotating the brick with respect to the beam directionduring the exposure. To simplify this task, a rotating brick support providedwith an angular scale was designed.The last test exposure, aimed at collecting a common sample of referenceemulsion sheets to be shared among the European laboratories, wasperformed on June 2004 with Fuji emulsions belonging to the OPERA finalproduction.The electronic setup for the beam monitoring is shown in fig.5.2. Onthe beam line there is a couple of scintillators, which trigger the passageof a beam particle, and two multi-wire chambers for beam position monitoring.Figure 5.2: Electronic set up of the June 2004 OPERA test beam at CERNPS.After three days of standard refreshing procedure, three bricks weremanually assembled and exposed to a 10 GeV π − particle beam. One brick73


and tracks reconstructed. Then the emulsion sheet adjacent to the previousones is scanned. The extrapolation of previously found tracks on the latestsheet is produced: the basetrack efficiency is the percentage of basetrackswithin an angular and position agreement with this extrapolation betterthan a given value.The efficiency is independent on the number of emulsion sheets usedfor track sample definition, provided that this number is large enough toavoid fake tracks due to random coincidences.By using 8 consecutive emulsion sheets from the above mentioned June2004 test beam (exposure without lead), and by defining the track samplewith 4 sheets, 8 independent measurements of the efficiency are possible.The obtained efficiencies and their errors are reported in Tab. 5.1 andshown in fig. 5.4 as a function of the spatial basetrack angle θ.θ x (rad) θ y (rad) ε (%)0.010 0.015 93.7 ± 1.40.093 0.014 92.5 ± 1.30.211 0.017 89.5 ± 1.70.294 0.012 86.4 ± 2.30.405 0.019 85.2 ± 2.30.492 0.011 87.2 ± 2.00.017 0.184 88.1 ± 2.00.088 0.184 87.3 ± 2.70.220 0.183 85.7 ± 1.40.291 0.185 86.4 ± 2.30.423 0.181 87.2 ± 1.60.495 0.187 88.7 ± 2.0Table 5.1: The measured basetrack finding efficiency, evaluated with anOPERA brick from the June 2004 test beam exposure.The efficiency distribution is uniform over the field of view. The efficiencyvalues appear to be lower than expected at a first sight. As shownin the previous chapter, the selection criterium on the number of grainsper microtracks induces a ∼ 1-3% microtracks loss, which in turn meansa ∼ 2-6% basetrack inefficiency, while the χ 2 cut (χ 2 < 6) should give anegligible contribution to the inefficiency, since the probability to obtain aχ 2 value greater than 6 when the number of degree of freedoms is 4 is verysmall.The understanding of the source of inefficiencies comes from visual inspectionof the missed tracks. Two main reasons were identified of comparableentity and both dependent on the emulsion properties. The first is75


ε (%)100959085800 0.1 0.2 0.3 0.4 0.5(rad)θ (rad)Figure 5.4: Basetracks finding efficiency as a function of the basetrack anglefor the ESS.due to distortion effects, which sometimes significatively modify the microtracksangles and prevent their matching for basetrack linking. The secondis due to local emulsion defects, which can reduce the number of visiblegrains, thus having the microtrack rejection by the tracking algorithm. Infig. 5.5 such inefficiency effects are showed.Figure 5.5: Main sources of tracking inefficiencies: the left picture showsthe occurrence of a misalignment due to the emulsion distortion, while theright picture illustrates the possibility of microtrack missing because localemulsion defects reduce the grain visibility.76


5.3.1 ν τ detecting efficiency in the CSAs shown above, the basetrack finding efficiency is 85 to 94%, according toits angle. Since this number has been obtained in general scanning mode,it corresponds to the tracking efficiency of the CS scanning.In order to check the implications of this efficiency on the ν τ detection,we have generated ν τ CC MC events for different τ decay topologies.The MC samples were generated with the standard OPERA MC which webriefly describe.Neutrino-nucleon interactions are generated through the JETTA [66]and RESQUE [67] packages, developed in the framework of the CHORUScollaboration. The JETTA package is derived from JETSET 7.4 [68], combinedwith LEPTO 6.1 [69], and deals with deep-inelastic neutrino interactions.The Fermi motion of the nucleons is treated by using a simple Fermigas model in which nucleons are considered as free fermions in a finitevolume. Hadronic fragmentation at the primary vertex is accomplishedaccording to the LUND [70] string model, with some tuning in order toreproduce experimental neutrino data. The RESQUE package simulatesquasi-elastic interactions. In order to take into account the reinteractionsof hadrons while crossing the nucleus a dedicated tool [71]developed by theNOMAD collaboration was implemented.The detector simulation was performed with GEANT 3 [72] package.It takes into account the detector geometry and the materials. All theprimary tracks from the neutrino interaction in the target are generatedaccording to the event generators and traced through the entire apparatustaking into account all relevant physics processes. Secondaries down to 1MeV kinetic energy for electrons and γ’s down to 10 MeV for all other particlesare also traced. Information about each particle crossing the activedetectors is recorded, to provide data input for the event reconstructionprograms. A detailed simulation of the brick has been performed to studythe event topology in the vertex region and to estimate the physics background.In this study we have used a simplified version of the generator whichdeals with stand-alone bricks. In doing so, we do not consider the brickfinding algorithm.After the event generation and reconstruction the tracks in the CS wereconsidered. The distribution of track multiplicity in the CS for τ → µ isshowed in fig. 5.6.In tab. 5.2 we report the probability of finding at least one track inthe CS with the current tracking efficiencies. The overall probability, evaluatedby taking into account the relative cross sections for QE and DIS ν τinteractions, and the τ decays BR, is (97.8 ± 1.4)%.77


200 Entries 1000180Mean 6.89516014012010080604020800 Entries 1000700Mean 1.07460050040030020010000 5 10 15 20 25 30 35 40N tracks in the CS00 1 2 3 4 5N tracks in the CSFigure 5.6: Track multiplicity in the CS for MC-simulated ν µ τ DIS (left)and QE (right) events with τ decaying in the muonic channel. The bin atzero is due to geometrical acceptance in the stand-alone brick configuration.Decay channel Detecting probability (%)τ → µ (DIS) 98.7 ± 0.4τ → e (DIS) 99.1 ± 0.4τ → h (DIS) 99.4 ± 0.2τ → µ (QE) 92.6 ± 0.9τ → e (QE) 98.1 ± 0.4τ → h (QE) 95.9 ± 0.6Overall 97.8 ± 1.4Table 5.2: Detection probability for the different decay channels.5.4 BackgroundTwo different background sources need to be accounted for. The instrumentalbackground is due to random coincidences of two microtracks generatedby the random combination of fog grains. The physical backgroundcomes from any physical source different from the neutrino interaction (i.e.cosmic rays not completely erased after refreshing, lead or environmentalradioactivity).The evaluation of the background has to be done by means of the visualinspection, since only the human eye check can confirm whether the foundtrack is due to instrumental background rather than to a physical source.5.4.1 Instrumental backgroundThe instrumental background measurement is performed by using emulsionsdeveloped immediately after the end of their refreshing.78


The χ 2 cut used for basetrack linking brings to a (160 ± 10)/cm 2 backgrounddensity. This value is too high for the scan back available time, sothat a more stringent selection criterium is needed. The number of clusters(n c ) associated to each basetrack allows to define this criterium: the newcut is defined in the (χ 2 , n c ) plane.The criterium must affect marginally the tracking efficiency while suppresslargely the background. In fig. 5.7 we report the distribution of theχ 2 as a function of n c for the the signal (selected by looking at the verynarrow angular peaks of a pion exposure) and for tracks found in a sheetdeveloped without any exposure (instrumental background plus a smallcontribution from cosmic rays).2χ6 Entries 223452χ6 Entries 29915544332211014 16 18 20 22 24 26 28 30 32Cluster number014 16 18 20 22 24 26 28 30 32Cluster numberFigure 5.7: The χ 2 as a function of the cluster number for the signal(left) and for the tracks found in an emulsion sheet developed soon after arefreshing procedure.The selection criterium is defined asχ 2 < 1.2 · n c − 25.2 (5.1)which bring to a background of 2.0 ± 0.9 basetracks/cm 2 .The new criterium implies a reduction of the tracking efficiency in therange of 1 - 3%, as shown in fig. 5.8. The overall probability of finding atleast one track in the CS is reduced at (97.5 ± 1.4)%.The measured instrumental background is compatible with the foreseenavailable scanning time for the scan back procedure.5.4.2 Physical background in the CSThe CS plays an important role in the OPERA event analysis. The CSis scanned according to the prediction of the electronic detectors. The de-79


ε (%)100959085800 0.1 0.2 0.3 0.4 0.5(rad)θ (rad)Figure 5.8: Basetracks finding efficiency after the application of the combinedcut showed in fig. 5.7.cision whether the corresponding brick has to be removed or not dependson the results of the CS scanning. To be a valid trigger device, in the CSthe number of tracks different from the ones originating from the neutrinointeraction inside the brick should be negligible.The refreshing of the CS has been tested both at the Gran Sasso laboratoryand at CERN. Nevertheless, the measurement of the refreshingefficiency at surface is biased, since the emulsions accumulate cosmic raysduring their refreshing. The refreshing efficiency is therefore evaluated bycomparing the cosmic rays density inside two emulsion sheets, one refreshedat the Gran Sasso and the other not.The measured density in the emulsion non-refreshed is 30−35 tracks/mm 2 .This value is not a real estimation of the CS physical background for tworeasons: the first one is that they were not refreshed in Tono mine aftertheir production, as it happens for the OPERA CS’s, the second one is thatemulsions from Japan to CERN travelled by plane, while the emulsions forthe OPERA detector will travel by ship.The refreshing efficiency has been evaluated at a level of about 96%.The residual number of tracks after the refreshing is evaluated at a levelof ∼ 1 − 2 tracks/mm 2 . These tracks should be completely erased by theemulsion self-refreshing effect, based on the past experimental knowledgeof the fading properties of machine made emulsion films. The CS has tobe packed with a relative humidity from 85 to 90 %, higher than usual,assuming that the operational temperature in the underground hall will be80


about 20 o C. The self-refreshing will also erase tracks produced by lead orenvironmental radioactivity.The fading life-time in these conditions is under test and is expected tobe of the order of one month. The combined effect of the initial refreshingand the emulsion self-refreshing is expected to reduce the unwanted tracksat a level of ≤ 1 in the whole CS after a few months.A complete test of refreshing and self-refreshing at Gran Sasso is ongoing.In case the self-refreshing will not fulfill the track erasing task at thedesired level the possibility to use a couple of CS is under investigation.5.5 Momentum measurementsThe momentum measurement of charged particles in emulsion through thedetection of multiple Coulomb scattering is a technique that has been usedsince the early days of the emulsion technique [73].A charged particle traversing a thickness will experience both a deviationand a displacement due to the combined effect of a large numberof small-angle scatters. At least for small angles, the deviation is roughlygaussian: the scattering angle distribution is peaked at zero with a standarddeviation given by [14]:√13.6 xϑ 0 (mrad) =(5.2)βp(Gev/c) X 0where p is the particle momentum and X 0 is the radiation length of thetraversed material of thickness x. In OPERA, the scattering is dominatedby the lead since the radiation length of emulsion layers and plastic base islarger by more than one order of magnitude than that of lead. Moreover,what matters is the traversed material before and after the measurementand therefore we may assume X 0 ≃ 5.6 mm (lead).There are essentially two measurement methods, the position methodand the angle method. They measure the deviations of the trajectory froma straight line on the basis of position or angle measurements, respectively.The use of one method rather the other depends on the accuracy neededand on the spatial and angular resolution achievable. In our case, the ECCcell allows a precise measurement of the angle (the base angles are measuredwith errors of 2 mrad) which is does not depend on the precise knowledgeof the relative positions of the different emulsion sheets. Therefore theangular method is preferred.The first automated momentum measurement in an ECC was done in2002 by using the NTS system [74]. A 3 cm 2 scanning area were consideredin 19 emulsion sheets exposed to 2, 3 and 4 GeV/c π − beams. A pionmomentum resolution of 28 %, 35 % and 36 % was achieved respectively.81


To test the performance of the ESS a momentum measurement by themultiple scattering technique has been performed by using the emulsionsheets of an OPERA-like brick, exposed to a 2 GeV/c π − beam on July2003 at the CERN PS. An area of 6 × 6 cm 2 was scanned in 30 plates 1 .Let ϑ i the angle of a given track in the ith emulsion sheet, with i =1, . . . , 30, and∆ϑ ik = ϑ i+k − ϑ i (5.3)the scattering angle after crossing k cells. The observed RMS of the scatteringangle distribution for a given cell depth k in the brick is defined asthe RMS of all N scattering angles:< ϑ 2 obs > k = ∑ iThe observed RMS can be rewritten as:(∆ϑ ik ) 2N(5.4)< ϑ 2 obs >=< ϑ 2 sc > +σ 2 (5.5)where ϑ sc is the scattering angle and σ is the measurement error: the fit ofthe observed RMS scattering angle as a√( ) 2 13.6( x)f(x) = σ 2 +p 5.6(5.6)function (p is expressed in GeV/c, x in mm and σ in mrad) of the numberof cells traversed for all tracks gives both a statistical measurement of theaverage beam momentum p and the angular accuracy of the basetracks.Results are showed in fig. 5.9. The measured beam momentum is 1.986 ±0.014 GeV/c and the value obtained for σ is 3.17 ± 0.05 mrad, which issignificatively worse of the above cited angular resolution. The reason isthat emulsions were stored in non-optimal conditions for a period, bringingto a substantial increasing of the distortion effects.A sample of 661 tracks were followed up along 30 cells. By insertingthe measured σ value in eq. 5.2 it’s possible to perform a fit track by track,in order to evaluate the associated momentum. Results are showed in fig.5.10: the scattering-angle distribution is scaled to give 1/p, whose gaussianfit gives:1p = 0.49 ± 0.11 c · GeV−1 (5.7)1 We consider only half a brick: indeed the position of the primary interaction vertexis uniformly distributed inside the brick.82


Figure 5.9: The RMS of the scattering angle as a function of the numberof traversed cells. The result of the fit is given by the function f(x) =√3.166 2 + ( 13.61.986) 2(x/5.6)The momentum distribution (see eq. 5.2) can be parameterized as:f(x) = a 0exp [−((1/x) − (1/a 1 )) 2 1/a 2 2]x 2 (5.8)where a i are the parameters of the function, which do not correspond directlyto the momentum resolution. By taking into account the obtainedresult about the inverse momentum (5.7), the momentum is evaluated as:p = 2.0 +0.6−0.3 GeV/c (5.9)In this preliminary analysis, the angular deflection used does not containthe subtraction of reference track trajectory angle and therefore thesystematic uncertainty is not minimize. The achieved resolution of 22%improves the previously reported values although with a larger materialdepth and is therefore an encouraging result for the physics performancesof the ESS.83


10080Entries 6612χ / ndf69.46 / 15Constant 104.1 ±6.0Mean 0.4896 ±0.0064Sigma 0.1137 ±0.004710080Entries 6612χ / ndf52.35 / 16p0 396.5 ±23.0p1 2.055 ±0.025p2 -0.1772 ±0.008060604040202000 0.2 0.4 0.6 0.8 1 1.2-1inverse momentum (cGeV )00 1 2 3 4 5 6 7momentum (GeV/c)Figure 5.10: Left: Measured 1/p distribution. The fitting distributionfunction is Gaussian. Right: The corresponding p distribution: the fittingdistribution is given in 5.8.84


ConclusionsThe OPERA experiment aims at the confirmation of ν µ → ν τ oscillationsthrough the direct observation of τ neutrinos in an initially pure ν µ beam.The OPERA experiment will use nuclear emulsions as high precisiontrackers. In order to analyze the required amount of emulsions, an R&Dproject has been carried out within the OPERA collaboration to developautomatic scanning systems with the required accuracy and scanning speed.Two different automatic systems have been developed. One, calledSuper-UTS, in the Nagoya University, the other, the European ScanningSystem (ESS), is a joint project of several European groups.The thesis work has contributed to the realization of the ESS and to theoptimization of its performances. To reach the required scanning speed andmeasurement accuracies, the ESS design is characterized by a mechanicswith sub-micron positioning accuracy and a very short settling-time, anoptics with large field of view, a camera with mega-pixel resolution andhigh frame rate and powerful image processors.The R&D for the ESS development took almost three years. In thisperiod, the ESS design underwent continuous upgrades, profiting of thefast evolution of electronic devices, such as digital cameras and image processors.At the same time, the track reconstruction software was developed andcontinuously improved thanks to the feedback from the emulsion analysiswith a prototype system. The work described in this thesis largely consistedof providing feedback and hints for the improvement of the reconstructionprogram.In 2004, the goal of a 20 cm 2 /h scanning speed was achieved. Thetracking efficiency was measured to be above 90% for minimum ionizingparticles with an incident angle up to 200 mrad. This gives a probabilityof about 98% to find in the so-called Changeable Sheet at least one trackemerging from ν τ interactions inside the brick.The measured instrumental background, due to random alignment offog grains, was reduced to about 2 tracks per cm 2 . This value is well belowthe level of physical background from cosmic rays.Finally, after the optimization of the scanning performances and as85


a first measurement of a physical parameter performed with the ESS, apreliminary measurement of the momentum of 2 GeV negative pions wasperformed through the detection of their multiple scattering while traversinglead and emulsion plates.86


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