International audienceThe OPERA experiment has been designed to perform the first detection of neutrino oscillations in the Nu_mu-->Nu_tau channel, in direct appearance mode through the event by event detection of the tau lepton produced in Nu_tau Charged Current interactions. OPERA is a hybrid detector, made of emulsion/lead target elements and of electronic detectors, placed in the CNGS muon neutrino beam from CERN to Gran Sasso, 730 km away from the source. Neutrino interactions from the CNGS neutrino runs have been recorded from 2008 until the end of 2012. We shall report on the data sample analysed so far and give the latest OPERA results on Nu_mu-->Nu_tau and Nu_mu-->Nu_e oscillation searches

Del Amo Sanchez, P.

The OPERA neutrino experiment recently nished data-taking, its recorded sample compris- ing 18 : 0 10 19 POT delivered by the CERN CNGS beam from 2008 to 2012. The goal of the OPERA experiment is to establish ! oscillations in appearance mode by observing the leptons produced in Charged Current interactions. Here we report on the status of the data analysis, and describe, in particular, two ! candidate events. Results on ! e oscillations are also presented

del Amo Sanchez, P

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Recent results from the OPERA experiment

del Amo Sanchez, P.

Hal - Université Grenoble Alpes

HAL Université de Savoie

Recent results from the OPERA experimentP. Del Amo SanchezTo cite this version:P. Del Amo Sanchez. Recent results from the OPERA experiment. Auge´ E., DumarchezJ., Tran Thanh Van J. XLVIIIth Rencontres de Moriond - Electroweak Interactions and Uni-fied Theories, Mar 2013, La Thuile, Italy. The GIOI publishers, pp.163-168, 2013. <in2p3-00815880>HAL Id: in2p3-00815880http://hal.in2p3.fr/in2p3-00815880Submitted on 21 Nov 2013HAL is a multi-disciplinary open accessarchive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come fromteaching and research institutions in France orabroad, or from public or private research centers.L’archive ouverte pluridisciplinaire HAL, estdestine´e au de´poˆt et a` la diffusion de documentsscientifiques de niveau recherche, publie´s ou non,e´manant des e´tablissements d’enseignement et derecherche franc¸ais ou e´trangers, des laboratoirespublics ou prive´s.RECENT RESULTS FROM THE OPERA EXPERIMENTP. DEL AMO SANCHEZ on behalf of the OPERA CollaborationLAPP, 9, Chemin du Bellevue, BP 11074941 Annecy-le-Vieux, FRANCEThe OPERA neutrino experiment recently finished data-taking, its recorded sample compris-ing 18.0× 1019 POT delivered by the CERN CNGS beam from 2008 to 2012. The goal of theOPERA experiment is to establish νµ → ντ oscillations in appearance mode by observing theτ leptons produced in ντ Charged Current interactions. Here we report on the status of thedata analysis, and describe, in particular, two νµ → ντ candidate events. Results on νµ → νeoscillations are also presented.1 IntroductionNeutrino oscillations have been well established experimentally by neutrino disappearance re-sults1. In addition to neutrino disappearance, the oscillation framework also predicts the appear-ance of neutrinos of different flavours. Conclusive observation of such phenomenon is lackingin the atmospheric sector. In particular, the explanation of the atmospheric νµ disappearanceobserved by SuperKamiokande 2, namely that the atmospheric νµ oscillate into ντ , since noνµ → νe excess is seen, is worth testing. OPERA addresses both questions by aiming to observeντ appearance in a nearly pure νµ beam.2 The OPERA experiment2.1 The CNGS beam and the LNGS laboratoryOPERA is located in the INFN Gran Sasso underground laboratory (LNGS), where it is shieldedfrom cosmic rays by 3800 m of water equivalent of rock overburden. It has been exposed to theCERN Neutrino to Gran Sasso (CNGS) beam. This gives a baseline of 730 km. The meanneutrino energy, 17 GeV, is a compromise between the increase of the ντ Charged Current (CC)interaction cross section at higher energies and the νµ → ντ oscillation probability, which, forthe considered baseline favours lower energies. The two contradicting requirements imply amean oscillation probability of only P (νµ → ντ ) ≈ 1.7 %, a consequence of OPERA not beingon an oscillation maximum. The beam is an almost pure νµ beam, the sum of the ν¯µ, νe and ν¯einteractions in the detector amounting to less than 3 % of the total. The prompt ντ componentof the beam is negligible.Between 2008 and 2012, the CNGS beam system delivered 18.0 × 1019 protons on target(POT).2.2 Detection principleThe appearance of ντ is revealed by the production of τ leptons, which happens by means of CCinteractions during the scattering of the neutrinos off the OPERA target nuclei. Several hadronictracks are often also produced in primary interactions. Due to the high mean neutrino energyand the τ lifetime, the τ lepton travels about 1 mm before decaying. Together with the fact thatit decays nearly 85 % of the times to 1-prong final states, the typical ντ experimental signatureconsists of a track with a kink coming out of the interaction vertex with several hadronic tracksattached. In contrast, the dominant νµ interactions in the target, whether they are Charged orNeutral Current interactions, often produce a vertex of hadronic tracks, sometimes partnered(CC events) by a penetrating track (the muon).Efficient detection of τ leptons and good separation of ντ events from νµ events require anexcellent spatial resolution. Furthermore, the need of a large target mass to compensate for thesmall neutrino interaction cross section leads to the so-called Emulsion Cloud Chamber (ECC)concept. In OPERA, it has been implemented in the form of 57 nuclear emulsion films interleavedwith 56 1-mm thick Pb plates, the nuclear emulsion films providing the submicrometric resolutionneeded. Such ensemble constitutes an OPERA “brick”. In addition to the lead-emulsion target,electronic detectors are used to trigger, locate the brick containing the interaction vertex, andmeasure the properties of penetrating tracks.2.3 The OPERA detectorAn OPERA brick weighs 8.3 kg and corresponds to about 10 radiation lengths. The momentaof the tracks below 8 GeV/c a are measured thanks to their Multiple Coulomb Scattering in thelead 3. The excellent angular resolution achieved by the nuclear emulsions (∼ 2 mrad) allowsto measure the dispersion of the angle of the track in each emulsion with respect to the globaldirection of the track in the brick. This dispersion is closely related to the momentum of theparticle 1.The OPERA detector also has some particle ID capabilities. Nuclear fragments are readilyidentified by the higher grain density along their tracks in the emulsion, a quantity relatedto dE/dx 4. Electron tracks are identified thanks to the showers they originate. Muons areselected with a 95 % efficiency by choosing appropriately long tracks that stretch over the wholedetector 5. When muon identification is essential to validate oscillation candidate events, tracksthat fail the previous requirement are followed through the downstream bricks till they eitherexit the detector or interact. The study of their end-point allows to recover 30 % of the muonsmissed by the first algorithm 4.The OPERA target, with a total mass of 1.25 kton, is composed of about 150000 bricks,arranged in walls interleaved with plastic scintillator strips. The latter constitute the TargetTracker (TT) and locate the brick where the interaction has taken place. An equally importantfunction of the TT is muon identification, related to the tracking capabilities it provides for themost penetrating tracks. Last but not least, the TT also triggers the events.Downstream of the target section, that comprises the brick walls and the TT, there is a muonspectrometer, composed of a dipole magnet generating a field of 1.53 T, and instrumented byRPCs and drift tubes. It measures the charge of the penetrating tracks with a misidentficationprobability of 0.3 %, and the momentum of those tracks with a precision better than 20 %.Two additional removable emulsion films, called Changeable Sheets (CS), are attached onthe downstream face of each brick. When a brick is located by the TT, and extracted by theBrick Manipulating System (BMS) automaton, tracks compatible with the TT hits are searchedfor in the CS. The spatial resolution of the TT predictions is about 1 cm, whereas the angularresolution is around 25 mrad. If tracks corresponding to the TT predictions are found in theCS, the rest of the emulsion films of the brick are developed and sent to one of the scanninglaboratories in Europe or Japan, where high speed automated microscopes will scan them. Animportant step of the data acquisition is done at this stage, when the tracks and vertices recordedin the emulsion are digitally reconstructed.aLarger momenta can be measured in a similar fashion, but depend strongly on the local quality of themeasurements (angular resolution, density of emulsion black grains unrelated to tracks).Table 1: Status of data analysis.Years POT (×1019) Status Number of analysed events2008 & 2009 5.27 Completed 27832010, 2011 & 2012 12.7 In progress (∼ 26%) 1722A more complete description of the OPERA detector can be found elsewhere 6.2.4 BackgroundsA charm quark is produced in about 4% of the νµ CC interactions. Since charmed particles havelifetimes that are very similar to that of τ leptons, whenever the primary muon is not detectedor misidentified, the one-prong decays of charm mimic the sought τ decay kink topology, con-stituting a background to the ντ oscillation signal in all the channels. A particular backgroundto the τ− → µ− channel is the large angle Coulomb scattering of muons coming from νµ CCinteractions. Finally, a source of background affecting the τ− → h− and τ− → h−h+h− chan-nels, is the hadronic re-interaction background. Hadrons coming from νµ interactions scatterhadronically off nuclei (for NC events b, it happens 0.2% of the times), and sometimes result ina kink-like topology.Some of these backgrounds are suppressed by specific instruments of the detector. Forinstance, the charm contribution to the τ− → µ− channel consists of wrong-sign muons, whichare readily discarded by the muon spectrometer. Most backgrounds, however, are fought usingkinematic variables, such as the flight length of the τ candidate, the missing pT at the primaryvertex or the φ angle. The latter, a powerful discriminating variable, is defined as the anglein the transverse plane between the τ candidate and the hadronic shower. A true τ will beback-to-back with respect to the hadronic shower, whereas the angle will be small for fake τcandidates coming from charm or hadronic reinteractions.3 Charm sampleThe decay topologies and lifetimes of charm events, similar to those of the τ signal, make theman ideal control sample. The agreement of the most important variables between Monte Carlosimulations (MC) and the available data (see Table 1) has been checked, and found to be good(see Figure 1). This proves that no major problems exist in the reconstruction of the relevantvariables for OPERA within their ranges of interest, the examples given being the flight length,the φ angle and the impact parameter of tracks with respect to their vertices of interaction.A comparison by topology of the expected numbers of events from MC and the observednumbers of events in data also shows a good agreement (see Table 2).4 νµ → ντ oscillationsWe report here the current results of the νµ → ντ oscillation search. The number of expectedντ candidates on the data sample analysed so far (see Table 1) is 1.91 signal events with abackground of 0.18 events.A first ντ candidate was reported in 20107. The ντ candidate is consistent with the ap-pearance of a τ− lepton in the OPERA target, that decays through the channel τ− → ρ−ντwith ρ− → pi−pi0 and pi0 → γγ. The tau candidate track exhibits a kink with an angle ofbThe CC contribution to the hadronic background is less important since the probability of not detecting themuon is small (∼ 5%).Table 2: Comparison by topology of numbers of charm events observed in data and MC expectations.Topology Expected from MC Observed in data1-prong 28.5 192-prong 22.1 223-prong 8.2 54-prong 1.2 4Total 60± 8 50Figure 1: Comparison of charm data (dots with error bars) and MC distributions (yellow histograms) of theimpact parameter (left), the decay length (centre) and the φ angle (right).41±2 mrad. The kink is interpreted as arising from a τ lepton decaying into a 1-prong channel.Its decay length is 1.335± 0.035 mm. The daughter track is identified as a hadron thanks to itsinteraction 7 walls downstream of the τ decay point. The two observed photons are consistentwith pointing back to the kink, and their invariant mass is compatible that of a neutral pion,mγγ = 120 ± 20(stat) ± 35(syst). A dedicated scan of the emulsions around the kink did notdetect the nuclear fragments that often partner nuclear interactions. The measured momenta ofall the tracks lead to a value of φ = (173± 3) ◦, in very good agreement with the expectations,as are the rest of the discriminating variables.A second ντ candidate was announced at the Neutrino 2012 conference8, compatible witha τ decaying into 3 charged hadrons. This decay channel is not as clean as a typical τ → 1prong topology, with the signal yield being 3 times smaller than in the τ− → h− channel andthe backgrounds being just as high. The fact that the decay of the τ candidate happens inthe plastic base of the emulsion film, just between the two emulsion layers, helps reducing theprobability of the event being a hadronic reinteraction. Indeed, no highly ionizing tracks emittedin nuclear processes (see above) have been found in the emulsions close to the decay vertex. Theranges of the three decay daughters are consistent with the hadron hypothesis. The primarytrack stops after only two brick walls, also in a fashion compatible with that of a hadron (amuon of the same momentum would most likely cross more than 26 brick walls). Finally, themeasured decay length is 1.54 mm, and the observed momenta yield φ = (167.8± 1.1) ◦ as wellas values of other kinematic variables in line with the expectations for a τ decay.With more than half of the data still to be analysed, there is a high probability of findingadditional ντ candidates in the future.5 νµ → νe oscillationsA search for νµ → νe oscillations was performed on the 2008 and 2009 data sample, correspondingto 5.25×1019 POT. The νe energy is estimated from the energy deposited in the Target Trackerplanes, in a similar way as it is done for CC and NC νµ interactions5.Figure 2: Longitudinal view of the first ντ candidate. The τ− candidate is the short red track attached to thevertex, whereas the τ daughter is the long green track attached to it. The kink is clearly visible. The two γ’s aredrawn in yellow and red.Only 1.4 events are expected from the standard 3-flavour νµ → νe oscillation for sin2(2θ13) =0.098 1. In contrast, the main background, which arises from beam νe contamination, amountsto 19.3 ± 2.8 events. A further 0.5 ± 0.2 events come from misidentified τ → e and pi0 decays.Whereas the contributions of the first two backgrounds are entirely estimated from simulation,for the latter it is data-driven. The goal is to estimate the probability of a γ converting betweenthe interaction vertex and the first emulsion film and mimicking the electron of a νe candidate. Tothat end, the νe selection criteria have been applied on a large sample of γ’s from data convertingseveral lead plates downstream of the primary interaction. The result is then extrapolated toconversions before the first downstream emulsion film.A total of 19 events were observed (see Figure 3), in agreement with the expectations, whichtranslates into a rather loose upper limit on θ13: sin2(2θ13) < 0.44 at 90% C.L. As expected,OPERA is not able to compete in this matter with other experiments dedicated to this search,and which, contrary to OPERA, are located on an oscillation maximum. However, due to thedifferent L/E, OPERA do constrain a different region of the sin2(2θ)−∆m2 parameter space.The interest in such constraints has been spurred lately by LSND 9 and MiniBooNE 10 results.These experiments have reported possible non-standard νµ → νe oscillations. These oscillationscan be parametrised by an additional contribution given by the standard 2-flavour oscillationformula:Pνµ→νe = sin2(2θnew) sin2(1.27∆m2newL/E)(1)where L is expressed in km and E in GeV. The effects of additional neutrinos are more complexthan just adding an extra oscillation term 11, though, and care must be taken when comparingexperiments at different L/E.Thus, considering the standard 3-flavour νµ → νe oscillation as a background, OPERAexcludes the appearance of extra νe for certain values of sin2(2θnew) and ∆m2new (see Figure 3).In particular, we find sin2(2θnew) < 7.2× 10−3 at 90% C.L. for high values of ∆m2new.Figure 3: Left, reconstructed νe energy spectrum (data points) and expected distributions of the various sources(filled histograms). Right, exclusion plot for the parameters of the non-standard νµ → νe oscillation; the OPERA90% exclusion plot is the continuous blue line.6 ConclusionsThe OPERA experiment has successfully recorded events from 2008 to 2012 exploiting the18.0×1019 POT delivered by the CNGS beam. The analysis of the data sample is ongoing, about50% of it is completed. The search for νµ → ντ oscillations has so far identified two ντ candidates,in good agreement with the expectation of 1.9 signal and 0.2 background events. Concerningνµ → νe appearance, OPERA cannot compete with other experiments due to the fact thatits L/E does not correspond to an oscillation maximum. However, OPERA data do constraininteresting parts of the ∆m2new−sin2(2θnew) plane, relevant to the possible non-standard νµ → νeappearance suggested by the LSND and MiniBooNE experiments. In particular, an upper limitsin2(2θnew) < 7.2× 10−3 at 90% C.L. has been found for high values of ∆m2new.References1. J. Beringer et al. [Particle Data Group], Phys. Rev. D 86, 010001 (2012)2. Y. Fukuda et al. [Super-Kamiokande Collaboration], Phys. Rev. Lett. 81, 1562 (1998)3. N. Agafonova et al. [OPERA Collaboration], New J. Phys. 14, 013026 (2012)4. N. Agafonova et al. [OPERA Collaboration], New J. Phys. 14, 033017 (2012)5. N. Agafonova et al. [OPERA Collaboration], New J. Phys. 13, 053051 (2011)6. R. Acquafredda et al. [OPERA Collaboration], JINST 4, P04018 (2009)7. N. Agafonova et al. [OPERA Collaboration], Phys. Lett. B 691, 138 (2010)8. M. Nakamura on behalf of the OPERA collaboration, Results from OPERA, Talk givenat the Neutrino 2012 Conference, Kyoto, Japan (2012).9. A. Aguilar-Arevalo et al. [LSND Collaboration], Phys. Rev. D 64, 112007 (2001)10. A. A. Aguilar-Arevalo et al. [MiniBooNE Collaboration], arXiv:1207.4809 [hep-ex].11. B. Bhattacharya, A. M. Thalapillil and C. E. M. Wagner, Phys. Rev. D 85, 073004 (2012)

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