Recent results from the OPERA experiment - Infn

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Recent results from the OPERA experiment - Infn

Nuclear Physics B (Proc. Suppl.) 188 (2009) 185–187www.elsevierphysics.comRecent results from the OPERA experimentL. Scotto Lavina for the OPERA Collaboration a ∗a INFN Sezione di Napoli, Complesso Universitario Monte Sant’Angelo Ed. G, Via Cintia, 80126 Napoli,ItalyOPERA is a long baseline neutrino oscillation experiment designed to observe ν μ → ν τ oscillations by lookingat the appearance of ν τ in an almost pure ν μ beam. The beam is produced at CERN and sent towards the GranSasso INFN Laboratories where the experiment is running. OPERA started its second year of data taking inJune 2008. This paper reviews the status of the experiment.1. IntroductionOPERA [1] is a long baseline neutrino experimentlocated in the Gran Sasso underground laboratory(LNGS) in Italy. The experiment is amassive hybrid detector with nuclear emulsionsused as very precise tracking devices and electronicdetectors to locate the neutrino interactionevents in the emulsions. It is designed to primarilysearch for ν τ appearance in the CERN high energyν μ beam CNGS [2] at 730 km from the neutrinosource, in order to establish unambiguouslythe origin of the neutrino oscillations observed atthe atmospheric scale. The ν τ direct appearancesearch is based on the observation of events producedby charged current interaction with a subsequentτ decay. The detector concept which isdescribed in the next sections combines micrometertracking resolution, large target mass togetherwith good lepton identification. In 5 years of datataking, OPERA should be able to observe 10 to15 ν τ events after oscillation at full mixing in therange 2.5 × 10 −3 < Δm 2 < 3 × 10 −3 eV 2 ,withatotal background of 0.76 events.to optimize the number of ν τ charged current interactionsin the OPERA detector. The neutrinobeam mean energy is 〈E〉=17 GeV with a smallcontamination of ¯ν μ (4.1%) and of ν e ,¯ν e (less than1%). The average L/E ratio is 43 km/GeV, farfrom the oscillation maximum, but dictated bythe high energy needed for τ appearance.The beam has been designed to provide 4.5 ×10 19 pot/year with a running time of 200 daysper year.3. Detector overviewThe OPERA detector is installed in the Hall Cof the Gran Sasso underground laboratory. Figure1 shows a recent picture of the detector whichis 20 m long with a cross section of about 8×9 m 2and composed of two super modules, each of themhaving a target section and a muon spectrometer.2. The CNGS neutrino beamThe Cern Neutrino to Gran Sasso (CNGS) facilityis a wide-band neutrino beam which providesan almost pure ν μ source traveling 730 kmunder the Earth crust from CERN to Gran Sasso.The beam parameters have been designed in order∗ scotto@na.infn.itFigure 1. View of the OPERA detector.0920-5632/$ – see front matter © 2009 Elsevier B.V. All rights reserved.doi:10.1016/j.nuclphysbps.2009.02.042


186L. Scotto Lavina / Nuclear Physics B (Proc. Suppl.) 188 (2009) 185–187The spectrometer allows a determination of thecharge and momentum of muons going throughby measuring their curvature in a dipolar magnetmade of 990 tons of iron, and providing 1.53Tesla transverse to the neutrino beam axis. Eachspectrometer is equipped with six vertical planesof drift tubes as precision tracker together with22 planes (8 × 8m 2 ) of RPC bakelite chambersreaching a spatial resolution of ∼1 cmandanefficiency of 96%. The final resolution of this detectoris better than 500 μm. The physics performanceof the complete spectrometer should reducethe charge confusion to less than 0.3% andgives a momentum resolution better than 20% formomentum less than 50 GeV. The muon identificationefficiency reaches 95% adding the targettracker information for the cases where the muonsstop inside the target.The target section is composed of 31 verticallight supporting steel structures, called walls, interleavedwith double layered planes of 6.6 m longscintillator strips in the two transverse directions.Their main goals are to provide a trigger for theneutrino interactions, an efficient event patternrecognition together with the magnetic spectrometerallowing a clear classification of the ν interactionsand a precise localisation of the event. Theelectronic target tracker spatial resolution reaches∼0.8 cm and has an efficiency of 99%.The walls contain the basic target detectorunits, called ECC brick, sketched in Fig. 2 whichare obtained by stacking 56 lead plates with 57emulsion films. This structure provides many advantageslike a massive target coupled to a veryprecise tracker, as well as a stand-alone detectorto measure electromagnetic showers and chargedparticle momentum using the multiple Coulombscattering in the lead.Downstream of each brick, an emulsion filmdoublet, called Changeable Sheet (CS) is attachedin a separate envelope.The bricks were assembled underground by adedicated fully automated Brick Assembly Machineand were installed in the walls by using twoautomated manipulator systems (BMS) runningon each side of the detector. The brick fillingis essentially finished in July 2008, with a totalof 146621 bricks (1.25 kton of target). 5000 ad-Figure 2. Picture of an assembled brick. Eachbrick weights about 8.6 kg and has a thickness of10 radiation lengths (X 0 ).ditional brick will be added once the additionallead will be delivered during the next winter shutdown.When the brick containing a neutrino interactionhas been located by the electronic detectors,it is removed using the BMS and the changeablesheets are detached and developed. The two filmsare then analysed by automated optical microscopesin one of the two Scanning Stations (in Europe,at the LNGS laboratory, and in Japan, eachone using a different technology [4,5]) to searchfor the tracks originating from the neutrino interaction.If none are found, then the brick is leftuntouched and another one is removed. When aneutrino event is confirmed, the brick is exposedto cosmic rays to collect enough alignment tracksbefore going to the development. The emulsionsare sent to the scanning laboratories in Europeand Japan.The laboratories start a detailed analysis consistingof locating the neutrino interaction vertexand acquiring data on all the tracks inside a volumearound the vertex region. The collected dataare stored in central and synchronized databasesand will be analysed searching for a decay topology.4. The first runsThe first CNGS technical run occurred in August2006 with a delivered luminosity of 0.76 ×10 18 pot. The OPERA target was empty at thattime but the electronic detectors were taking dataand commissioned [3].


L. Scotto Lavina / Nuclear Physics B (Proc. Suppl.) 188 (2009) 185–187 187The first CNGS physics run took place in October2007, when OPERA had 40% of the targetmass installed. The beam experienced faults andonly 0.82 × 10 18 pot were delivered.A total of 38 events were recorded and reconstructedinside the OPERA target for 32 ± 6expected. Among them, 29 were classified asCharged Current (CC) and 9 as Neutral Current(NC) in agreement with expectation. Among the19 events shared in the European laboratories,18 were confirmed by CS (the rejected one is anevent with the muon passing between two bricks),15 of them being successfully located inside thebrick. Two of the three not located events havethe neutrino interacting in dead material. Thelast event occurs in a brick with a too high levelof instrumental background (nuclear fog). Thisset of bricks will be replaced before the 2009 Run.A major revision of the CNGS project has beentaken in the beginning of 2008 in order to improvethe radiation shielding of the electronics and reducethe beam losses.5. Status of the 2008 RunA new physics run started in June 2008, withan expected number of integrated pot of 2.28 ×10 19 in 123 days of running assuming a nominalintensity of 2 × 10 13 pot/extraction. This intensity,when reached, should lead to about 2200neutrino interactions per day in the target andeventually the observation of the first τ event candidate.At the date of August 24 th (61 st day of the run)CNGS delivered 0.45 × 10 19 pot, instead of theexpected value (1.13 × 10 19 ) due to a slow startand various incidents. The expected number ofneutrino interacting in the bricks target is 434 ±21.In the same date, OPERA recorded 2558 eventson-time. Among these, 399 are observed interactingin the bricks, consistent within 2σ withthe expected value. The bricks confirmed bythe two Scanning Stations are weekly sent to theother scanning laboratories, for the location ofthe event inside the brick.6. ConclusionsAt moment of writing this contribution theOPERA 2008 Run is completed. It ended November3 rd with 1.78 × 10 19 pot delivered by CNGS(78% of the number expected for this run).OPERA detected 10058 on-time events and 1690neutrinos interacting in the bricks, consistentlywith the values expected from the CNGS integratedintensity. Presently, 245 interactions havebeen located in the bricks: 47 NC and 198 CC interactions.The Scanning Stations and the scanninglaboratories of the Collaboration are completingthe analysis of the bricks, locating theevents which will be finally analysed for the searchof the ν τ candidates.REFERENCES1. OPERA Collaboration, M. Guler et al .,CERN-SPSC-2000-028 and LNGS P25/2000(2000); CERN-SPSC-2001-025 and LNGS-EXP 30/2001 (2001).2. CNGS project: G. Acquispace at al., CERN-98-02 (1998); R. Bailey et al., CERN-SL/99-034 (1999); A. E. Ball et al., CERN-SL/Note-2000-063 (2000)3. OPERA Collaboration, R. Acquafredda etal., New Journal of Physics 8 (2006) 303.4. S. Aoki et al., Nucl. Instrum. Meth. B 51(1990) 466; T. Nakano, Proceedings of InternationalEurophysics Conference HEP20015. N. Armenise et al., Nucl. Instrum. Meth.A551 (2005) 261.

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