Nuclear Physics B (Proc. Suppl.) 169 (2007) 321–325www.elsevierphysics.comTau neutrino appearance in the CNGS muon neutrino beam: theOPERA experimentM. De Serio a for the OPERA Collaborationa Dip. Interateneo di Fisica Università degli Studi di Bari, via Amendola 173, 70126 Bari, ItalyThe detector, the experimental strategy and the physics potential of OPERA are reviewed. Results from thefirst CNGS neutrino beam test run, successfully carried out in August 2006, are also reported.1. IntroductionBased on the strong experimental evidence infavour of the existence of an oscillation mechanismin neutrino propagation obtained in recentyears [1–5], the OPERA experiment  has beendesigned to conclusively test the hypothesis ofν μ → ν τ conversion in the atmospheric sector(Δm 2 ∼ 2.5 × 10 −3 eV 2 , maximal mixing) usingthe CERN to Gran Sasso (CNGS) ν μ beam, optimisedfor ν τ appearance search. The detector,installed in the hall C of the Gran Sasso NationalLaboratory (LNGS, Italy), 732 km away from theneutrino source, combines visual and electronicdetection techniques to select, among all neutrinoflavour events, ν τ charged current interactionsin a massive lead - nuclear emulsion target bydirectly observing the outcoming tau leptons inemulsion, thus ensuring high background rejectionpower.2. The CNGS beamThe CNGS project  was conceived to designan almost pure ν μ beam satisfying the requirementsfor ν τ appearance experiments: the averageneutrino energy (≃ 17 GeV) is well above theτ production threshold; the prompt ν τ contaminationis negligible, the main beam contaminationcomponents being due to ν μ ’s (∼ 4%) andν e , ν e ’s (< 1%); the ν μ energy spectrum has beenspecifically optimised to maximise the ν τ interactionrate at LNGS.A 400 GeV proton beam, accelerated by theCERN SPS and extracted in 2 short pulses of10.5 μs duration every 6 s (2.4 × 10 13 p.o.t. perpulse), hits a segmented graphite target; positivelycharged secondary mesons are focussed bya system of two coaxial magnetic lenses into a1 km long decay tunnel, where the ν μ beam isproduced from the decay in flight of pions andkaons. A massive iron dump at the end of thetunnel stops residual mesons and protons. Twostations of arrays of ionisation chambers, installeddownstream of the hadron stopper, are used tomonitor the beam intensity and profile.Assuming the design intensity of the CNGSbeam (4.5 × 10 19 p.o.t. / year), about 6000 νevents (CC and NC interactions) per year are expectedin the OPERA target; the number of ν τCC interactions varies from about 20 to 40 forΔm 2 =2÷ 3 × 10 −3 eV 2 .The commissioning of the CNGS beam startedin July 2006 and was successfully completed onAugust 18th, 2006 when the first neutrino testrun took place (see Section 6).3. OPERA detectorThe appearance of ν τ ’s from ν μ oscillations willbe investigated by looking for ν τ charged currentinteractions in a 1.8 kt target through thedetection of the tau lepton decaying in one prong(muon, electron and single hadron channels) or inthree prongs.The observation of the decay kink and the shorttau decay length (of the order of 1 mm) requirea high-granularity detector with excellent spatialresolution. Moreover, the low ν interaction cross0920-5632/$ – see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.nuclphysbps.2007.03.019
322M. De Serio / Nuclear Physics B (Proc. Suppl.) 169 (2007) 321–325Figure 1. The OPERA detector installed in the Hall C of LNGS (left); detail of the detector (right).section demands a large target mass (O(kt)). Inorder to satisfy these requirements, a modulartarget section, segmented into basic units calledbricks, was designed.A brick consists of 57 thin nuclear emulsionfilms, acting as sub-micrometric resolution trackers,interleaved with 56 1 mm-thick lead plates,used as passive material. Each emulsion film ismade of two 44 μm sensitivelayersgluedontoa205 μm plastic base. The brick transverse dimensionsare 12.7 × 10.2cm 2 ; the thickness is about7.5 cm, corresponding to ∼ 10 radiation lengths.Bricks are arranged in planar structures, calledwalls, with transverse dimensions ∼ 6.7 × 6.7m 2 .Each wall, hosting 52 × 64 bricks, is equippedwith horizontal trays designed to slide bricks inand out of their rows during insertion and extractionphases. Walls are coupled to pairs of trackerplanes (TT) providing bi-dimensional track information.They are made of plastic scintillatorstrips of 1 cm thickness, 2.6cm width and 6.9mlength, arranged in modules and readout by WLSfibers and multi-anode 64-pixel PMT’s at bothends. Each tracker plane consists of 4 modules ofhorizontal or vertical strips.The main task of the TT is to provide the eventtrigger signal with high efficiency and identify inreal-time the brick where the ν interaction occurred.A sequence of 31 walls and related TT planesplus a downstream muon spectrometer form asuper-module. The OPERA detector consists oftwo super-modules.Each spectrometer is an instrumented dipolarmagnet (∼ 8.75 × 8m 2 ) made of two magnetizediron walls producing a field of 1.52 T inthe tracking region with vertical lines of oppositedirections in the two walls. In between the 12iron slabs of each wall, planes of bakelite RPC’s(2.9×1.1m 2 ) are inserted to measure the range ofstopping particles and track penetrating muons.The muon charge and momentum are measuredby 6 planes of drift tubes with 38 mm diameterand 8 m length, placed in front, behind and inbetween the magnet walls. The spatial resolution(∼ 300 μm) and high efficiency (> 99%) resultin a very low probability of wrong charge signassignment (< 0.3%), relevant for charm backgroundrejection, and high accuracy (better than25%) in the determination of the muon momentumbelow 25 GeV/c.In order to solve ambiguities in the reconstructionof multi-track events, two planes of RPC’swith crossed readout strips (±45 ◦ ) are placed upstreamof each dipolar magnet to complement theinformation provided by the drift tube planes.
M. De Serio / Nuclear Physics B (Proc. Suppl.) 169 (2007) 321–325 323The muon identification efficiency, obtained bya combined analysis of spectrometer and scintillatortracker data, is greater than 95%, thus allowinga clean CC event identification.Since the mass of each magnet is ∼ 1 kt, henceequivalent to that of each target section, thespectrometers will be also used to monitor theneutrino flux, measure the beam spectrum and,thanks to the sampling structure of the magnets,reconstruct the energy released by hadrons, relevantto complete the kinematics of tau candidateevents.In front of the first super-module, a veto system,consisting of planes of glass RPC’s, allowsto tag the interactions occurring in the upstreamrock.The installation of the electronic detectors atLNGS started in May 2003 and was completed inJune 2006 (Figure 1).Due to the large number of bricks (∼ 200 k) tobe used, a Brick Assembly Machine (BAM) anda Brick Manipulator System (BMS) have beendesigned to produce the lead - emulsion targetunits and insert (and remove) them in (from) thewall structures. The brick production has recentlystarted and is expected to be completedby summer 2007.4. Experiment strategyThe combined analysis of electronic data allowsto determine in real-time the brick(s) where the νinteraction is likely to have occurred. A 3D probabilitymap is computed and the brick with thehighest probability is promptly removed from thetarget during the run. In order to confirm theevent signal prior to brick disassembly, a doubletof emulsion films, called Changeable Sheets (CS),attached on the downstream side of each brickand acting as interfaces with the TT planes, aredeveloped and an area of 25 ÷ 100 cm 2 aroundthe TT reconstructed vertex point, depending onthe interaction type, is measured to locate eventrelated tracks. Only in case one or more tracksare found in both CS’s, is the brick exposed toa controlled flux of cosmic rays for film intercalibration and then unpacked; the emulsionfilms are developed and tracks measured in theCS’s are extrapolated to the most downstreamfilm of the brick and followed back to the interactionpoint. Once the vertex has been located, theevent is reconstructed and, if an interesting topologyis detected, it is fully analysed: thanks tothe dense structure of the brick and the excellentspatial resolution (< 1 μm) of nuclear emulsion,momentum measurements by multiple Coulombscattering , electron - pion identification andenergy reconstruction , pion - muon separationby energy loss close to range end can be performed.About 30 ν interactions per day, including allflavours, are expected to occur in the OPERAtarget at nominal CNGS intensity; correspondingly,a total emulsion surface of the order ofa few thousands of cm 2 will be daily measured.This represents a challenge for OPERA with respectto past experiments and required severalyears of R&D studies to develop fast automaticmicroscopes [11, 12] for the scanning of nuclearemulsions at a speed of ∼ 20 cm 2 / h.5. Physics potentialTable 1 shows the numbers of expected ν τevents to be collected in 5 years of data-takingassuming nominal CNGS integrated intensity(2.25 × 10 20 p.o.t.) for two different values ofΔm 2 and maximal mixing. For each τ decaychannel included in the analysis, the total numberof expected background events, mainly dueto charm production and decay with unidentifiedprimary muon, hadron re-interactions and largeanglemuon scattering in lead, is also reported.An average target mass of 1.6 kt accounting forbrick removal during the run was assumed in thecalculation.The experiment sensitivity and probability toobserve a number of signal events larger than a4 σ fluctuation of the background are shown inFigure 2. The curves correspond to different scenariosincluding possible CNGS beam intensityincrease and further OPERA background reduction(under study). The 90% C.L. allowed regionfrom SK L/E analysis is also shown.The calorimetric structure of the brick can alsobe exploited to search for sub-leading ν μ → ν e os-
324M. De Serio / Nuclear Physics B (Proc. Suppl.) 169 (2007) 321–325Table 1Expected numbers of signal and background events in 5 years of run at nominal CNGS intensity.τ decay channel Δm 2 =2.4 × 10 −3 eV 2 Δm 2 =3.0 × 10 −3 eV 2 Bkgτ → μ 3.6 5.6 0.23τ → e 4.3 6.7 0.23τ → h 3.8 5.9 0.32τ → 3 h 1.1 1.7 0.22ALL 12.8 19.9 1.0Figure 2. OPERA sensitivity and 4σ discoveryprobability in 5 years of run.cillations. By a simultaneous fit of visible energy,electron energy and missing transverse momentumdistributions, the resulting 90% C.L. upperlimit on sin 2 (2θ 13 ) after 5 years of run is 0.06 .6. First CNGS neutrino runThe first CNGS neutrino test run took placefrom August 18th to 30th, 2006. The intensitywas gradually increased up to 1.7×10 13 p.o.t. perpulse, corresponding to about 70% of the nominalvalue. The total integrated intensity was7.6 × 10 17 p.o.t. in 121 hours of run.The correlation between the CNGS beamdatabase at CERN and OPERA electronic detectorsat LNGS is based on GPS timing information.In order to check the time synchronisation,the event selection was performed within awindow of ∼ 2ms after the spill extraction time:a narrow peak with 10.5 μs width, comparableto the spill duration, and very low background(< 0.1%) was observed, as expected.The OPERA electronic detectors worked successfullytaking data over the whole run periodwith a live time greater than 95%.About 320 events correlated with the CNGSbeam were collected in total. Examples of reconstructedevents are shown in Figure 3.No bricks were inserted in the target walls forthis run. Nevertheless, in order to test the connectionbetween TT planes and bricks, 15×20 CSdoublets were used: muon tracks predicted by theelectronic detectors were searchedforinthetwoemulsion films and clear coincidences were foundfor a few events, thus assessing the validity of theprocedures.7. ConclusionsThe OPERA experiment, designed to searchfor ν τ appearance in the CNGS ν μ beam, has recentlyentered the startup phase.The first test run was successfully carried out inAugust 2006 and the first CNGS neutrino eventswere recorded in the OPERA electronic detectors.The observation of the first ν events in theOPERA bricks represents the next milestone forthe experiment. A test run is foreseen in October- November 2006.OPERA will start collecting data in 2007.
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