The OPERA experiment was designed to perform the first detection of νμ → ντ neutrino oscillations in a direct appearance mode. We present the analysis results of the 2008–2009 statistics corresponding to 4.88 × 1019 p.o.t. In this sample, one ντ candidate event has been observed in the τ → h channel. The statistical significance of this observation is estimated to be 95%

Guler, A. M.

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DOI 10.1393/ncc/i2012-11349-7Colloquia: LaThuile12IL NUOVO CIMENTO Vol. 35 C, N. 6 Novembre-Dicembre 2012OPERA neutrino oscillation resultsA. M. Guler on behalf of the OPERA CollaborationMiddle East Technical University - 06800 Ankara, Turkeyricevuto il 7 Settembre 2012Summary. — The OPERA experiment was designed to perform the first detectionof νμ → ντ neutrino oscillations in a direct appearance mode. We present theanalysis results of the 2008–2009 statistics corresponding to 4.88 × 1019 p.o.t. Inthis sample, one ντ candidate event has been observed in the τ → h channel. Thestatistical significance of this observation is estimated to be 95%.PACS 14.60.Pq – Neutrino mass and mixing.PACS 25.30.Pt – Neutrino-induced reactions.1. – IntroductionNeutrino oscillations were first observed by the Super-Kamiokande experiment [1] in1998. In recent years, several other experiments [2] using atmospheric, solar, reactor andaccelerator neutrinos have confirmed the existence of neutrino oscillations and measuredthe mixing parameters. However, the direct observation of the appearance of ντ froman oscillated νμ is still missing. The observation of ντ appearance in an accelerator neu-trino experiment will unambiguously prove that νμ → ντ is the dominant channel for theneutrino atmospheric sector. This is the main goal of the OPERA experiment [3]. In par-ticular, the aim is to find the signal events coming from ντ charged-current interactions:ντN → τ−X,(1)followed by one of the following decay topologies:τ− → μ−ν¯μντ(2)→ e−ν¯eντ→ h−(nπ0)ντ→ h−h−h+(nπ0)ντ .The oscillation parameters and very short decay length (87μm) of the tau require i) along baseline, ii) high-energy neutrino beam and iii) a massive detector with a highc© Societa` Italiana di Fisica 5152 A. M. GULER on behalf of the OPERA COLLABORATIONspatial resolution. OPERA is exposed to the long-baseline CNGS νμ beam, 732 km awayfrom its neutrino source at CERN. The average neutrino energy is ∼ 17GeV well abovethe production threshold for the tau. The ν¯μ contamination in terms of interactions is2.1%, the νe and ν¯e contaminations are lower than 1% while the prompt ντ in the beamis negligible. The challenge of the OPERA experiment is to achieve the very high spatialaccuracy required for the detection of the tau inside a large-mass active target. Thetechnology chosen for this challenge are emulsion films interleaved with lead plates, his-torically called Emulsion Cloud Chamber (ECC). The submicrometer spatial resolutionof the nuclear emulsion allows a precise three-dimensional reconstruction of the neutrinovertex as well as of the decay vertex associated short-lived particles, including the tau.The large target mass given by the lead plates allows to collect enough statistics.2. – The detectorOPERA is a hybrid detector made of two identical Super Modules (SM) each consistsof a target section, of a scintillator tracker detector (TT) and a spectrometer. The totalmass is 1250 kTons. A target section is a succession of walls filled with elements calledbricks, interleaved with planes of scintillator strips composing the Target Tracker (TT)that provide real time detection of the outgoing charged particles.The target sections consist of about 150000 ECC bricks and each of them is made of 56lead plates and 57 emulsion films for a total weight of 8.3 kg. An OPERA emulsion filmhas two layers each 44μm on both sides of base. The total thickness is about 290μm. Thetransverse size is 12.5× 10.0 cm2. A pair of nuclear emulsion films are used as interfacebetween electronic detector and ECC brick. Tightly packed doublets of emulsion filmsare glued to the downstream face of each brick and can be removed without opening thebrick.The electronic detectors trigger the readout, identify and measure the trajectory ofcharged particles and locate the brick where the interaction occurred. The momentumof muons are measured by the spectrometers which consist of a dipolar magnet made oftwo iron arms. The trajectory of muons are traced back through the scintillator planesup to brick where the track originates. When no muons are observed, the scintillatorsignals produced by electrons or hadronic showers are used to predict the location of thebrick that contains the primary neutrino interaction vertex. A detailed description ofthe OPERA detector is given in [4].The scanning of the emulsion films is performed with two different types of automaticmicroscope, the European Scanning System (ESS) and Japanese S-UTS. The Europeanscanning system makes use of commercial subsystems in a software based framework.The horizontal stage movable in XY coordinates with a CMOS camera mounted onthe optical axis along which it can be moved to change the focal plane. The controlworkstation hosts a motion control unit that directs the stage to the area to be scannedand drives the camera along the Z-axis to produce optical tomographic image sequences.Then, the images are enhanced by means of a vision processing board in the controlworkstation. The reconstructed clusters in an emulsion layer is called micro-tracks. Thelinking of two matching micro-tracks produces the base-track. The system can work ata speed of 20 cm2/h/layer. The Japanese system has been developed in Nagoya and isbased on highly customized components. The feature of this system is removal of thestop and go process of the stage in the data taking stage. The optical system is movedby a piezo-electric device. The dedicated board make the track recognition, buildingmicro-tracks. The system can reach the speed of 72 cm2/h/layer.OPERA NEUTRINO OSCILLATION RESULTS 533. – Event location and decay searchDuring years 2008–2009, OPERA has collected 31576 triggers corresponding to 5.13×1019 protons on target. Among these events 5255 events reconstructed as occurring insidethe OPERA target.The first step of event location is the extraction of the brick from the target wall. Thenthe CS is detached and its films are searched for compatible with the electronic data toverify the brick selection. In the case this search is unsuccessful, the brick is equippedwith a fresh CS and inserted back into the target. All tracks measured in the CS aresearched in the most downstream films of the brick and followed back until they are notfound in three consecutive films. The stopping point is considered as the signature eitherfor a primary of a secondary vertex. The vertex is then confirmed by scanning a volumewith a transverse size of 1 cm2 on at least 6 films downstream and 2 films upstream ofthe stopping plate is scanned around each stopping point. The data are processed by anoffline program to reconstruct all tracks originating inside the volume. These tracks areinput for a vertex reconstruction algorithm which is tuned to find also decay topologies.The mean efficiency of event location is found to be 74 ± 2% and 48 ± 4% for νμcharged-current(CC) and neutral-current(NC) events, respectively. The expected num-ber of located events in the 2008–2009 data sample is 2978±75. But the result presentedin this paper comes from the decay search analysis of 2738 events corresponding to 92%of the located sample.When a secondary vertex is found the kinematical analysis of the whole event isdone using the ECC brick data. The momentum of charged particles are determined bymultiple coulomb scattering [5] measured in the ECC brick. The energy of γ-rays andelectrons is estimated by a Neural Network algorithm that uses the combination of thenumber of track segments in the emulsion films and the shape of the electromagneticshower, together with the multiple Coulomb scattering of the leading tracks.4. – The candidate eventBy applying decay search procedure, one ντ candidate was observed in the 2008–2009data sample. The candidate event has 7 prongs at primary vertex out of which 4 areidentified as originating from a hadron and 3 have a probability lower than 0.1% of beingcaused by a muon. The parent track exhibits a kink topology and the daughter track isidentified as produced by a hadron through its interaction. Its impact parameter withrespect to the primary is 55 ± 4μm, the impact parameter for other tracks is smallerthan 7μm. Two γ-rays point to the secondary vertex. The event passes all selectioncriteria described in [3] and summarized in table I. The invariant mass of two γ-rays is120±20(stat.)±35(syst.)MeV/c. If we assume the secondary hadron is π− the invariantmass becomes 640+125−80 (stat.)+100−90 (syst.) MeV/c2 the decay mode is compatible thereforewith being τ− → ρ−(777)ντ whose branching ratio is about 25%. A detailed descriptionof the candidate event can be found in [6].5. – Background estimationThe charmed particles have lifetimes similar to that of the tau and have similartopologies. The finding efficiency of the decay vertices is therefore also similar tothat of tau decays. Comparing the observed charm event sample with simulationwould be a test for corresponding efficiencies and backgrounds. Table II shows the54 A. M. GULER on behalf of the OPERA COLLABORATIONTable I. – Selection criteria for ντ candidate event.Varibale Cut-off Candidate EventMissing PT at Primary Vertex (GeV/c) < 1.0 0.57+0.32−0.17Angle between parent track and primaryhadronic shower in the transverse plane π23.01± 0.03Kink angle (mrad) > 20 41± 2Daughter momentum (GeV/c) > 2 12+6−3Daughter PT when γ-rayat the decay vertex (GeV/c) > 0.3 0.47+0.24−0.12Decay length (μm) < 2000 1335± 35Table II. – The observed and expected charm topologies in the 2008–2009 sample.Topology Observed charm events Expected charm events with backgroundC1 13 17.8V2 18 16.5C3 5 5.8V4 3 2.1Total 39 42.2± 8.3comparison between observed charm events and expected from simulation. There is agood agreement between them.The main background source to all τ decay channels is constituted by charmed par-ticle production in νμCC interactions where the primary lepton is not identified. Thecharm background was evaluated using charm cross-sections measured by the CHORUSCollaboration [7].Table III. – Expected number of signal and background events in the 2008–2009 sample.Decay channel Number of signal for Number of background for4.88× 1019 p.o.t 4.88× 1019 p.o.tτ → μ 0.39 0.02± 0.01τ → e 0.63 0.05± 0.01τ → h 0.49 0.05± 0.01τ → 3h 0.15 0.04± 0.01Total 1.65 0.16± 0.03OPERA NEUTRINO OSCILLATION RESULTS 55The second main source of background in τ → h decay channel comes from one-pronginelastic interactions of primary hadrons produced in νμNC interactions or in νμCCinteractions where the primary lepton is not identified and in which no nuclear fragmentscan be associated with secondary interaction. This background has been evaluated withMonte Carlo simulation based on FLUKA [8] and cross-checked with data.6. – ConclusionThe OPERA experiment, aiming at the first detection of neutrino oscillations in directappearance mode where the oscillated neutrino is identified. The analysis of the 2008–2009 data corresponding to 4.88×1019 p.o.t. intensity has been completed and a single ντcandidate event which is compatible with the expectation was observed. All backgroundsources and expected number of tau events are summarized in table III. The significanceof the observation of one decay in the τ → h channel is found to be 95%. Considering alldecay channels, the number of expected signal and background events are respectively1.65± 0.41 and 0.16± 0.03(syst.), the probability for the event to be background being15%.The analysis of 2010–2011 data samples is in progress.∗ ∗ ∗This work is supported by TUBITAK under Grant No. 108T324.REFERENCES[1] Fukuda Y. et al. (Super-Kamiokande Collaboration), Phys. Rev. Lett., 81 (1998)1562.[2] Ahn M. H. et al. (K2K Collaboration), Phys. Rev. D, 74 (2006) 072003; MichaelD. G. et al. (MINOS Collaboration), Phys. Rev. Lett., 97 (2006) 191801; MichaelD. G. et al. (CHOOZ Collaboration), Eur. Phys. J. C, 27 (2003) 331; Ahmad Q. R.et al. (SNO Collaboration), Phys. Rev. Lett., 89 (2002) 011301.[3] Guler M. et al. (OPERA Collaboration), CERN-SPSC-2000-028.[4] Acquafredda R. et al. (OPERA Collaboration), JINST, 4 (2009) P04018.[5] Agafonova N. et al. (OPERA Collaboration), New J. Phys., 14 (2012) 13026.[6] Agafonova N. et al. (OPERA Collaboration), Phys. Lett. B, 691 (2010) 138;Agafonova N. et al. (OPERA Collaboration), New J. Phys., 14 (2012) 33017.[7] Kayis-Topaksu A. et al. (CHORUS Collaboration), New J. Phys., 13 (2011) 093002.[8] FLUKA package, http://www.fluka.org/fluka.php.

OPERA neutrino oscillation results

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