Status and results from the OPERA experimentTomoko Arigaon behalf of the OPERA collaborationA. Einstein Center for Fundamental Physics, LHEP, University of BernAbstract. The OPERA experiment is aiming at the first direct detection of neutrino oscillations in appearance mode throughthe study of the ν μ ν τ channel. The OPERA detector is placed in the CNGS long baseline ν μ beam 730 km away from theneutrino source. The analysis of a sub-sample of the data taken in the 2008-2009 runs was completed. After a brief descriptionof the beam and the experimental setup, we report on event analysis and on a first candidate event, its background estimationand statistical significance.Keywords: Neutrino; Oscillation; Appearance; Tau; EmulsionPACS: 14.60.Lm; 29.40.RgINTRODUCTIONThe atmospheric neutrino oscillations have been studiedby many experiments   . However, the direct observationof flavour transition in appearance mode hasnever been done. OPERA is aiming at the first direct detectionof ν μ ν τ oscillations in appearance mode. Leadplates and emulsion films are used as target and detector,satisfying the requirements for the ν τ appearance: a largetarget mass and a very high spatial accuracy to detect theshort-lived τ lepton decay (cτ =87μm), The observationof neutrino interactions in the OPERA detector have alreadybeen presented . In this document, we report onevent analysis and on a first candidate event, its backgroundestimation and statistical significance.THE CNGS BEAM AND THE OPERADETECTOROPERA is exposed to the CNGS long baseline ν μ beamfrom CERN to LNGS, 730 km away. The average ν μenergy is 17 GeV in order to maximise the numberof ν τ CC interactions. In terms of interactons, the ν¯μcontamination is 2.1% and the ν e and ν¯e contaminationsare lower than 1%, while the number of prompt ν τ isnegligible. The expected interactions for 225 ¢ 10 19 pot(nominal pot in 5 years) are about 24000 events. About10 τ decays are expected to be observed.OPERA is a hybrid detector made of two identicalsuper modules consisting of a target section made ofECC bricks, of a scintillator tracker detector, and of amuon spectrometer. An ECC brick consists of 57 emulsionfilms interleaved with 1 mm thick lead plates. AnOPERA film has 2 emulsion layers of 44 μm thicknesspoured on a 205 μm plastic base. In order to reducethe emulsion scanning load, removable pairs of emulsionfilms called Changeable Sheets (CS) have been used asinterfaces between the scintillator tracker and the ECCbricks.EVENT ANALYSISBricks predicted to contain a neutrino interaction by theelectronic detectors are removed from the target wall andthe CS is detached from the brick and developed underground,then analyzed to validate the brick. After abrick has been validated, its emulsion films are developedand dispatched to the various scanning laboratories.All tracks measured in the CS are followed backuntil they are not found. The stopping point is consideredas either a primary or a secondary vertex, then thevertex is confirmed by scanning a volume with 1 cm 2 ¢15 films, 5 upstream and 10 downstream of the stoppingpoint. A further analysis called decay search procedure isapplied to located vertices to detect possible decay or interactiontopologies. When secondary vertices are foundin the event, a kinematical analysis is performed usingtrack angles and momenta.Momenta of charged particles can be measured inECC using the angular deviations of tracks by MultipleCoulomb Scattering (MCS) in lead . This methodgives a momentum resolution better than 22% for particleswith momenta lower than 6 GeV/c, passing througha brick. For higher momentum, the position deviationsare used for the measurement. The resolution is betterthan 33% for particles with momenta lower than12 GeV/c, passing through a brick. Momenta of muonsreaching the spectrometer are measured with a resolutionof 20% up to 30 GeV/c.The analysis of a sub-sample of 1088 events of the
neutrino data taken in the 2008-2009 runs was completed,corresponding to 189 ¢ 10 19 pot.Charmed particles have similar lifetimes and decaytopologies if charged, so the detection of charm decays isused to check the τ efficiency. In the sample of ν μ CC interactions,20 charm decays have been observed that survivedall the cuts, in agreement with expectations froma MC study, 160 ¦ 29. Out of them 3 have a 1-prongtopology where 08 ¦ 02 are expected. The backgroundfor the total charm sample is about 2 events.Several ν e -induced events have also been observed.THE FIRST ν τ CANDIDATE EVENTThe decay search procedure yielded one candidate eventsatisfying the selection criteria for the ν τ interactionsearch (). The cuts are the same as those defined in theexperimental proposal (,). The event is displayed inFigures 1 and 2.The selection criteria are that there is no tracks compatiblewith a muon or an electron, the secondary vertexand the primary vertex survive the cuts shown in Table 1.TABLE 1. Kinematical variablesVariable Measured Selection criteriaKink angle 42 ¦ 2 >20(mrad)Decay length 1335 ¦ 35 Within 2 plates(μm)P daughter 12·6 3 >2(GeV/c)P T daughter 470·230120 >300 (γ attached)(MeV/c)Missing P T(MeV/c)570·320170 1000Angle Φ 173 ¦ 2 >90(deg)The primary vertex consists of seven tracks of whichone track has a kink. Two electromagnetic showerscaused by γ rays have been detected and they are compatiblewith pointing to the secondary vertex.The invariant mass of the two detected γ rays yieldsa mass consistent with the π 0 mass, ´120 ¦ 20´statµ ¦35´systµµMeVc 2 . Then, the invariant mass of the π γγsystem has a value compatible with that of the ρ (770),´640·12580 ´statµ·10090 ´systµµMeVc2 . The ρ appears inabout 25% of the τ decays.BACKGROUND ESTIMATION ANDSTATISTICAL SIGNIFICANCEThe secondary vertex is compatible with the decay ofτ h ´nπ 0 µν τ . The main background sources to thischannel are• the decays of charmed particles produced in ν μCC interactions where the primary muon is notidentified.• the 1-prong interactions of primary hadrons producedin ν μ CC interactions where the primarymuon is not identified or in ν μ NC interactions.The charm background produced in the ν μ interactionsin the analysed sample is 0.007¦0.004 events, that producedin the ν e interactions is less than 10 3 events.The background from hadron interactions has beenevaluated with a FLUKA based MC code, updated withrespect to the proposal simulations. The kink probabilityto occur in 2 mm lead integrated over the ν μ NC hadronicspectrum yield a background probability of ´19¦01µ¢10 4 NC. This probability decreases to ´38 ¦ 02µ ¢10 5 NC taking into account the cuts on the eventglobal kinematics. This leads to a total of 0.011¦0.006events when misclassified CC events are included.The cross-checks of the 1-prong hadron backgroundestimation have been performed. The tracks of hadronsfrom a sub-sample of neutrino interactions have beenfollowed far from the primary vertex to search decay-likeinteractions. A total length of 9 m has been measured andno event has been found in the signal region (Figure 3).This corresponds to a probability over 2 mm lead smallerthan 154 ¢ 10 3 at 90% C.L. Within the low statistics,the P T distribution agrees with the simulation.We observed 1 event in the 1-prong hadron τ decaychannel, with a background expectation 0.018¦0.007(syst.). The probability to observe 1 event due to a backgroundfluctuation is 1.8% (2.36σ). As all τ decay modeswere included in the search (1-prong μ, 1-prong e, 1-prong hadron, 3-prong hadron), the total background becomes0.045¦0.023 (syst.), the probability to observe1 event due to a background fluctuation becomes 4.5%(2.01σ).CONCLUSIONSData taking in the CNGS beam is going smoothly. Theanalysis of a sub-sample of the neutrino data taken inthe 2008-2009 runs was completed, corresponding to189¢ 10 19 pot out of 225 ¢ 10 19 proposed pot. Decaytopologies due to charmed particles have been observedin good agreement with expectations, as well as severalevents induced by ν e present as a contamination in theν μ beam. One muonless event showing a τ to 1-pronghadronic decay topology has been detected. It is thefirst ν τ candidate event in OPERA, with a statisticalsignificance of 2.36σ (1-prong hadronic decay mode)and 2.01σ (all decay modes). Analysis of the 2008+2009
500 μmFIGURE 1.Display of the ν τ candidate event.1000 μm2γ21537P T (GeV/c)Hadronic interaction, 1-prong10.90.80.70.126.96.36.199FIGURE 2.γ168daughterDisplay of the ν τ candidate event (beam view).0.20.100 1 2 3 4 5 6 7 8 9 10Daughter P (GeV/c)FIGURE 3. P-P T plot of the hadronic interactions found inthe reference sample. No event found in the signal region (blueregion).full sample will be completed early next year. Analysisof 2010 events is being performed in parallel.REFERENCES1. SUPER-KAMIOKANDE Collaboration, Y. Fukuda et al.,Phys. Rev. Lett. 81 (1998) 1562;SUPER-KAMIOKANDE Collaboration, K. Abe et al.,Phys. Rev. Lett. 97 (2006) 1718012. K2K Collaboration, M.H. Ahn et al., Phys. Rev. D 74(2006) 0720033. MINOS Collaboration, D.G. Michael et al., Phys. Rev.Lett. 97 (2006) 191801;MINOS Collaboration, P. Adamson et al., Phys. Rev. Lett.101 (2008) 2218044. OPERA Collaboration, N. Agafonova et al., JINST 4(2009) P060205. M. De Serio et al., Nucl. Instrum. Meth. 512 (2003) 539;M. Besnier, PhD thesis (2008) Universit de Savoie,LAPP-T-2008-026. OPERA Collaboration, N. Agafonova et al., Phys. Lett. B691 (2010) 138-1457. OPERA Collaboration, M. Guler et al., An appearanceexperiment to search for ν μ ν τ oscillations in theCNGS beam, Experimental proposal, CERN/SPSC2000-028.8. OPERA Collaboration, M. Guler et al., Status report onthe OPERA experiment, CERN/SPSC 2001-025.