Physical Retardance of Gelatin from an Aspect of Growth ... - opera

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Physical Retardance of Gelatin from an Aspect of Growth ... - opera

The OPERA neutrino oscillation experimentPaolo StrolinIstituto Nazionale di Fisica Nucleare, Sezione di Napoliand Università di Napoli Federico IIComplesso di Monte S.Angelo, Via Cintia 19, 80126 Napoli (Italy)- 1 -


AbstractThe Kamiokande and Super-Kamiokande experiments, and independently the MACROexperiment, have observed a deficit, or “disappearance”, of muon neutrinos produced bycosmic rays which interact in the atmosphere. This effect is interpreted as the result of an“oscillation” of muon neutrinos into tau neutrinos. This interpretation must be confirmed bythe direct observation of the “appearance” of tau neutrinos. The interaction of a tau neutrinowith matter produces a short lived particle, the tau lepton, which can be taken as the signatureof the phenomenon.The discovery of the π meson and the first observation of a “charmed” particle, indicatethat nuclear emulsions provide a powerful tool to detect short lived particles. The OPERAexperiment make use of a massive Emulsion Cloud Chamber to search for the appearance oftau neutrinos in the CNGS beam from CERN to Gran Sasso, nominally consisting only ofmuon neutrino.The OPERA experiment implies the application of the emulsion technique at a verylarge scale. It has required new developments for the industrial production of nuclearemulsions, for their treatment to induce a fading of background tracks and for automatedemulsion scanning at very high speed. The experiment will start running for physics in 2008.An overview is given in this report.Keywords: nuclear emulsions, neutrino oscillation, atmospheric neutrinos, tau neutrino- 2 -


1. IntroductionThe so-called Standard Model of the elementary particles assumes neutrinos to bemassless, although there is no basic theory behind this assumption as instead it is the case forthe null photon mass on the basis of Gauge Theories. The discovery of a non-zero neutrinomass and its measurement represent therefore fundamental steps beyond the presenttheoretical framework of elementary particle physics.Neutrinos are commonly identified in terms of their flavour eigenstates ν e , ν μ and ν τ , soidentified because in their interaction with matter (e.g. in a detector) they produce electrons,muons and tau leptons, respectively. Neutrinos are in flavour eigenstates also at theirproduction, where they are associated with the above particles. These are the neutrinos towhich currently we refer: they are what is “seen” through the interactions.In free propagation, the flavour is however irrelevant and what matters is the mass,determining the energy-momentum relation and hence, according to wave-quantummechanics, the associated wavelength. Thus what matters are the mass eigenstates ν 1 , ν 2 andν 3 , invisible to interactions. In this picture, the flavour eigestates are a mixture of the masseigenstates, governed by the so-called Pontecorvo-Maki-Nakagawa-Sakata (PMNS) mixingmatrix.- 3 -


Neutrino oscillation 1,2,3)is a quantum mechanical interference phenomenon whichoccurs if neutrinos have non-zero mass and the mass eigenstates differ from the flavoureigenstates. In free propagation with increasing distance from the source, a phase shiftdevelops among the mass eigenstates: their mixture is thus changed, reflecting in modificationof the (visible) flavour eigenstates: a disappearance of the original (at production) flavourmay be seen (through an interaction), or a neutrino of a different flavour may appear.In general, interference phenomena allow to reach very high sensitivities. In particular,neutrino oscillation offers a very sensitive tool to investigate neutrino masses and neutrinomixing, by observing the apparent metamorphosis of neutrino flavours which results from theneutrino oscillation. Actually, the interference mechanism allows the measurement ofdifferences of square masses of the mass eigenstates (Δm 2 ), rather than of their absolutemasses.The first indication for neutrino oscillations came from the observation of a flux ofneutrinos from the Sun lower than expected. Here, we focus the attention on the observationsdone with neutrinos produced by cosmic rays interacting in the Earth’s atmosphere, theso-called atmospheric neutrinos. The KamiokaNDE and Super-KamiokaNDE experiments inthe Kamioka mine in Japan have observed a flux of atmospheric muon neutrinos lower than- 4 -


expected, i.e. a disappearance of muon neutrinos. No such effect is observed for electronneutrinos. A disappearance of atmospheric muon neutrinos has independently been observedalso by the MACRO experiment in the Gran Sasso Laboratory in Italy. A disappearance ofmuon neutrinos has later also been observed with neutrino beams produced by accelerators,by the long baseline (i.e. with detector placed far from the neutrino source) experiments K2Kin Japan and MINOS in the USA.The above observations of a disappearance are currently interpreted in terms of adominant oscillation of muon neutrinos in (experimentally unobserved) tau neutrinos. Themain aim of the OPERA experiment is to provide the definitive proof of such an interpretation,by performing the very difficult observation of the appearance of tau neutrinos in anoriginally pure muon neutrino beam, over a long baseline.The interaction of a tau neutrino with matter produces a short lived particle, the taulepton, the observation of which can be taken as the signature of the oscillation phenomenon.The clearest signal for tau lepton production is provided by the direct observation of its decay.The task is, however, very difficult: the flight length of the tau lepton before decay is veryshort (on average 0.5 mm at the beam energies required for its production to be abovethreshold) and the very sophisticated detector required to this purpose must be massive,- 5 -


ecause the process is rare.The discovery of the π meson 4) and the first observation of a charmed particle 5) , as wellas the first observation of production and decay of a beauty particle 6) , indicate that nuclearemulsions provide a powerful tool to detect short lived particles. The OPERA experiment 7,8)makes use of a massive Emulsion Cloud Chamber 9) (ECC) to search for the appearance of tauneutrinos in the CNGS neutrino beam from CERN to Gran Sasso 10) , nominally consisting ofpurely muon neutrinos. In the ECC emulsion films act as very high precision trackers andalternate with plates of passive material. In recent years the ECC technique has been used, onthe 100 ton mass scale, for the first observation of the ν τ in the DONUT experiment 11) atFermilab.The OPERA experiment implies the application of the emulsion technique at a verylarge scale: due to the low neutrino cross-sections and to the low neutrino oscillationprobability, an active neutrino mass of the order of one kton is required. The ECC techniqueoffers the possibility to reach such a high target mass. The experiment has required newdevelopments for the industrial production of nuclear emulsions 12) , for their treatment toinduce a fading of background tracks 12) and for automated emulsion scanning at very highspeed 13,14) . The OPERA ECC is complemented by electronic detectors to localise events in the- 6 -


emulsion and provide information for the event reconstruction.The OPERA ECC has also the capability of identifying electrons. In parallel to theν μ → ν τ oscillation search, the experiment will search for ν μ → ν e oscillation to investigatethe open question of the so-called θ 13 mixing angle, on which only an upper limit exists atpresent.The experiment has already operated in short technical runs 8) in 2006 and 2007 and willstart running for physics in 2008. It is a very complex and large experiment run by aCollaboration involving about 160 physicists from 36 Institutions in Europe and Japan. Thisreport provides a description of the experimental apparatus, of its operation and of theanalysis methods. Finally, its present status is reported. The report is aimed at giving anoverview of the experiment. For more detailed information, see the quoted papers andreferences therein.2. The experimental apparatusThe CNGS neutrino beam is produced by the CERN Super Proton Synchrotron and isdirected towards the Gran Sasso Laboratory in Italy, at a distance (baseline) of 730 km fromCERN. The average neutrino energy is 17 GeV, high enough to be over threshold for- 7 -


production of the heavy tau lepton. The υe+ υ e contamination is at the 0.9% level and theν τ contamination is negligible. At the nominal proton beam intensity, in one year (200 days ofrun) of operation the SPS can provide 4.5 x 10 19 protons on target (pot), resulting in 6200 ν μinteractions in the 1.3 kton target mass presently foreseen for the OPERA ECC. With thepresently known oscillation parameters (Δm 2 = 2.4 x 10 -3 eV 2 and full mixing) the oscillatedν τ rate should be ~25 per year. Of these only a fraction (see Section 3) can be observed dueto experimental inefficiencies.The challenge of the OPERA neutrino tau detector is to comply with two strongrequirements, hard to reconcile: to provide a large neutrino target mass (of the order of onekton, to cope with the low oscillation probability and interaction cross-section) and to havethe very high granularity and resolution required for the direct observation of the tau leptondecay (average decay length of about 0.5 mm). In addition, together with the electronicdetectors, it must allow the event identification and the reconstruction of its kinematics. TheECC is unique in complying with these requirements, thanks to the μm granularity andsub-μm resolution of the nuclear emulsion. An angle resolution of the order of 1 mrad can beachieved.The basic element of the OPERA ECC is the so-called brick. The brick consists of 57- 8 -


emulsion films interleaved with 56 lead plates, 1mm thick. The use of lead as the passivematerial in the ECC allows electron identification through the features of showers and particlemomentum measurement by multiple Coulomb scattering. The bricks are arranged in brickwalls. A doublet of emulsion films, the so-called Changeable Sheets (CS), is attached to eachbrick downstream of it. The use of a doublet allows to reduce the accidental background byrequiring a coincidence of tracks in the two films. The CS act as an interface between thebrick and the electronic Target Tracker (TT) plastic scintillator detectors 15) which are situatedimmediately downstream of each brick wall, as shown in Figure 1. The CS and the geometryof the emulsion films are shown in Figure 2. The emulsion films have an area of 12.5 x 9.9cm 2 . They consist of 44 μm emulsion layers deposited on both sides of a 205 μm thicktriacetylcellulose base.Emulsion films and lead plates are placed transversally with respect to the beam.Longitudinally with respect to the beam, the brick is about 10 radiation lengths thick. Itweighs 8.3 kg.OPERA requires a very large area of nuclear emulsion, about 100,000 m 2 for 1.3 ktontarget mass. For comparison, 300 m 2 were used in the CHORUS experiment 16) . For OPERA, apouring of the emulsion gel by hand, as traditionally done, is impossible. After an R&D- 9 -


carried out by the Nagoya University with the Fujifilm Company, an industrial massproduction was setup at Fujifilm with the recent commercial photographic technology 12) .Nuclear emulsions integrate all the background tracks (cosmic rays, environmentalradiation) from the production until the development. The large detector area to be coveredmakes it impossible the use of high precision electronic detectors (in CHORUS scintillatingfibre arrays were used, having a resolution of the order of 0.25 mm) to restrict the emulsionarea to be scanned. Hence to operate with acceptable scanning times, even accounting for thevery high speed of the newly developed automated microscopes 13,14) , the density ofbackground tracks has to be reduced. This has motivated the development of the so-calledemulsion refreshing technique and the setting up and operation of a large refreshing facility atTono mine in Japan 12) . To the same sake of keeping a low level of background tracks, the leadwith the lowest radioactivity available on the market (at an affordable cost and in the requiredamount) has been chosen and machined with great care at the JL Goslar Company inGermany.A schematic view of the full detector is shown in Figure 3. It consists by two identicalsupermodules, complemented by a veto system. Each supermodule consists of a neutrinotarget (ECC and TT) and of a muon spectrometer.- 10 -


A target mass of 1.3 kton in total for the two supermodules is obtained by piling up155,000 bricks in brick walls having an area of 6.7 x 7.3 m 2 . The lightweight wall supportstructure is shown in Figure 4. It is a matrix capable of containing up to 1664 bricks. Itconsists of superimposed horizontal trays, where bricks are inserted from the sides and fromwhere the hit bricks are extracted after that a neutrino interaction has been recorded by theelectronic detectors. Given the large number of bricks, automated machines are used for brickassembly (Brick Assembly Machine, in short BAM), for brick insertion and extraction (BeamManipulator System, in short BMS) and for other tasks.The main task of the TT 15) is to locate the brick where the neutrino interaction tookplace and to give calorimetric information on the event. A TT module consists of two planesof 2.63 cm wide plastic scintillator strips, one plane with horizontal and the other withvertical strips. The signal is read out on both sides of the strips using wavelength shiftingoptical fibres, as shown in Figure 5, and multianode photomultipliers.Background reduction requires the measurement of the sign of the muon charge. Eachsupermodule is thus equipped with a muon spectrometer. The bending field is provided by themagnetized iron of a large dipole magnet 17,18) , shown in Figure 6. The magnet yokes are madeof 5 cm thick iron slabs, with 2 cm gaps in between where planes of Resistive Plate- 11 -


Chambers (RPC) are inserted for calorimetry and tracking 19) . For better tracking (as requiredby the muon charge measurement) than it is provided by the RPC system, the spectrometersare equipped with Precision Trackers (PT), made of drift tube planes 20) triggered by the RPCsystem.The Veto system acts against neutrino interactions in the rock and in the materialsurrounding the OPERA detector. It is made of plane of glass RPC 21) .3. Data taking procedures and event analysisOnce the neutrino interaction has occurred in the detector, the hit brick is selected on thebasis of TT information and is extracted from its tray by the BMS. The analysis of theextracted bricks must be conducted quasi-online and represents a very demanding task: about30 bricks will be extracted daily with an emulsion area to be scanned amounting in average toas much as about 200 cm 2 for each event.Automated emulsion scanning has been pioneered by the Nagoya University, where thefirst automated microscope was equipped by the so-called Track Selector 22) . Impressiveimprovements have been realized in speed and in analysis methods in connection with theDONUT 11) and CHORUS 16) experiments. The speed aimed at for OPERA is 20 cm 2 / hour, one- 12 -


order of magnitude larger than previously achieved. This aim has motivated the newdevelopments reported at this Workshop 13,14) .A new system, named S-UTS (Super Ultra Track Selector) has been developed inJapan 13) . To avoid mechanical vibrations, the horizontal movement of the stage and thevertical movement of the objective are not any more in steps but continuous and mutuallysynchronized with great precision. As in the original Track Selector, the track reconstructionalgorithms are hardware coded.In Europe, the joint effort of several laboratories has lead to the development 14) of theESS (European Scanning System). It is based on the use of customized commercial optics andmechanics. The data acquisition software is asynchronous.The first step in the analysis of an OPERA event is the scanning of the CS, searching fortracks corresponding to the prediction by the TT. Given the coarse prediction provided by the2.3 cm wide strips of the TT, the CS have to be scanned over a large area. The area to bescanned amounts to 25 cm 2 for events with a muon in the final state (giving a well measuredtrack) and to the area of the full emulsion film area for muonless events. Hence, to avoidsspurious findings in the CS all efforts are made to keep a low density of background tracks inthe CS. In particular, they undergo a second refreshing at Gran Sasso before installation and- 13 -


they are developed in a special underground facility. Before being detached from the brickand developed, the CS and the most downstream emulsion film in the brick are provided ofreference marks for their relative alignment. These marks are spots created by X-rays, whichare absorbed by the first lead plate encountered. The result of the CS scanning is validated bya visual inspection to eliminate the remaining background due to chance coincidences in thetwo films of the doublet.In case the CS analysis gives a negative result, a new CS is attached to the brick tomake it ready for reuse in the detector.The bricks with a neutrino interaction confirmed by the CS undergo a second X-rayexposure, which produces lateral marks for film to film alignment. These bricks are thenexposed to cosmic rays for about 24 hours, in view of using cosmic ray tracks for a finerelative alignment of the emulsion films. The precise alignment is important for the physicsanalysis, like the detection of the apparent small kink in the trajectory (daughter neutrinos areinvisible) characterising one-prong tau lepton decays or the momentum measurement bymultiple Coulomb scattering. The development of the emulsion films of the bricks proceeds ina highly automated facility specially set up in the surface laboratory at Gran Sasso.In the first step of the brick analysis, all track candidates seen in the CS are extrapolated- 14 -


ack (by 4.5 mm) to the most downstream emulsion film in the brick and searched for aroundthe CS prediction. The X-ray spots for the relative alignment. The validated tracks are thenfollowed back until they disappear, with a procedure called scan back. In this procedure, thetrack angle prediction obtained from the downstream films is used to limit the angular rangein the track search and to make the scanning faster. The accuracy of the prediction improvesas the scan back proceeds. The lateral X-ray marks are used for the relative alignment.Apart from inefficiencies, a track stop is due either to a primary or to a secondary vertex.In order to study what happens at the stopping point, a volume around it is scanned accordingto the procedure called general scanning. This volume is given by of 1 cm 2 around along thedirection of the trajectory on 5 films downstream and 5 films upstream of the stopping point.In the general scanning, no a priori angle information is used and the track search proceeds ina wide angular range, typically ±400 mrad around the direction perpendicular to the emulsionfilm. The data are processed by an off-line program to reconstruct all tracks stopping withinthe volume: these tracks are input for a vertex reconstruction algorithm which is tuned to findthe primary interaction vertex and decay topologies of secondary particles.All the tracks attached to the primary vertex are followed downstream of it with aprocedure (scan-forth) analogous to the scan-back but in the opposite direction, to check- 15 -


whether they reach the end of the brick and to measure their momentum. If necessary, tracksare followed in downstream bricks, by the so called brick-to-brick connection.4. Status and outlookThe electronic detectors are fully installed and operating. The detector construction willterminate in July 2008, when the brick production will be completed.During the first beam tests in summer 2006, the first neutrino interactions were seen bythe electronic detectors 8) . A very short run with emulsions took place in fall 2007, with anintegrated proton flux of 0.08 x 10 19 pot in total, corresponding to 3.6 effective days atnominal proton beam intensity. This run has allowed to initiate tuning the event reconstructionby the electronic detectors and by the ECC. The number of neutrino interactions observed inthe ECC is 38 (compatible with 31.5 expected). Of these, 29 are interactions with a muon inthe final state and 9 muonless interactions, compatible with the relative cross-sections ofcharged and neutral current ν μ interactions. Figure 7 shows a high multiplicity event with acharmed meson hadronic decay in an ECC brick. Two electromagnetic showers are alsovisible and can be ascribed to an electron-positron pair. Figure 8 shows the same event in theelectronic detectors. The first physics run will take place in 2008.- 16 -


Table 1 give the expected number of signal and background events in a 5 year run withan integrated proton flux on the target of 4.5 x 10 19 pot / year , assuming 200 days of run peryear and nominal beam intensity. The events are given in total and for the various decaychannels. Full neutrino mixing is assumed and two values of Δm 2 are taken: 2.5 x 10 -3 eV 2(the present best fit) and 3.0 x 10 -3 eV 2 . The dominant backgrounds are charm production anddecay, hadron reinteractions and large-angle muon scattering in lead.Figure 9a shows the probability of observing ν μ → ν τ oscillation as a function of Δm 2 .Figure 9b shows the upper limit which can be reached in an exclusion plot in the oscillationparameter plane (mixing angle θ and Δm 2 ), in case no signal would be seen.In conclusion, the OPERA experiment is awaiting for the first physics run to observe forthe first time the ν τ appearance following ν μ → ν τ oscillation.- 17 -


References1) B. Pontecorvo, Sov. Phys. JETP, 7, 172 (1958).2) Z. Maki, M. Nakagawa and S. Sakata, Progr. Theor. Phys., 28, 870 (1962).3) B. Pontecorvo and V. Gribov, Phys. Lett., B 28, 493 (1969).4) C.M.G. Lattes, H. Muirhead, G.P.S. Occhialini, C.F. Powell, Nature, 159, 694 (1947).5) K. Niu, E. Mikumo and Y. Maeda, Progr. Theor. Phys., 46, 1644 (1971).6) J.P. Albanese et al., Phys. Lett., B158, 186 (1985).7) A. Ereditato, K. Niwa, P. Strolin, Nucl. Phys. Proc. Suppl., 66, 423 (1998) 423.8) R. Acquafredda et al., OPERA Collaboration, New J. Phys., 8, 303 (2006).9) R.M. Kaplon, B. Peters and D.M. Ritson, Phys. Rev., 85, 900 (1952).10) R. Acquistapace et al., “The CERN neutrino beam to Gran Sasso: conceptual technicaldesign”, CERN Yellow Report 98-02 (1998).11) K. Kodama et al., DONUT Collaboration, Phys. Lett., B504, 28 (2001).12) T. Nakamura et al., Nucl. Instr. Meth., A556, 80 (2006).13) T. Nakano, contribution to this Workshop.14) N. Armenise et al., Nucl. Instr. Meth., A551, 261 (2005).C. Bozza, contribution to this Workshop.15) T. Adam et al., Nucl. Instr. Meth., A577, 523 (2007).16) E. Eskut et al., CHORUS Collaboration, Nucl. Instr. Meth., A401, 7 (1997).17) M.Ambrosio et al., IEEE Trans. Nucl. Sci., 51, 975 (2004).18) A. Cazes et al., JINST 2, T03001 (2007).19) A. Bergnoli et al., IEEE Trans. Nucl. Sci., 52, 2963 (2005).20) R. Zimmermann et al., Nucl. Instr. Meth., A555, 435 (2005).21) C. Gustavino et al., Nucl. Instr. Meth., A527, 471 (2004).22) S. Aoki et al., Nucl. Instr. Meth., B51, 466 (1990).- 18 -


Table 1. Expected number of events for ν μ → ν τ oscillation, in a 5 year run at the nominalintegrated flux of 4.5 x 10 19 protons on target per year.τ decay channels Signal BackgroundΔm 2 = 2.5 x 10 -3 eV 2 3.0 x 10 -3 eV 2τ → μ 2.9 4.2 0.17τ → e 3.5 5.0 0.17τ → hadron 3.1 4.4 0.24τ → 3 hadrons 0.9 1.3 0.17Total 10.4 15.0 0.76- 19 -


Figure CaptionsFig. 1 Schematic view of the brick with the Changeable Sheets and the Target Trackerdownstream of it.Fig. 2 Schematic view of the emulsion films and of a CS doublet.Fig. 3 Schematic view of the OPERA detector.Fig. 4 Brick wall support structure during installation.Fig. 5 Schematics of the plastic scintillator strip readout by wavelength shifting optical fibres.Fig. 6 The magnetized iron dipole magnet equipping each muon spectrometer. Units are inmm.Fig. 7 (a) A ν μ charged current event with charmed meson production and hadronic decay inthe ECC, observed in the 2007 run.(b) A zoom on the primary vertex and on the decay region.Fig. 8 The charmed meson event of Figure 7, as seen by the electronic detectors.Fig. 9 (a) Discovery probability as function of Δm 2 .(b) Exclusion plot at 90% CL.- 20 -


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SM 1SM 2νECCμspectr.Fig. 3


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Fig. 5


Fig. 6


Top ViewSide ViewFront ViewDraw DetectorRotateOpenGLX3DNeighParmsTrackParms.ROOT- OPERA -FEDRA.0.5 mm(a)PickZoomUnZoom10 mmTop ViewSide ViewFront ViewDraw DetectorRotateOpenGLX3DNeighParmsTrackParms0.2 mm2.8 mm.ROOT- OPERA -FEDRA.(b)PickZoomUnZoomFig. 7


Fig. 8


(a)(b)Fig. 9

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