aa r X i v : . [ h e p - e x ] M a y NuPhys2016-Patrizii
Results from the OPERA Experiment
Laura Patrizii
INFN Bologna, Italy on behalf of the OPERA Collaboration The OPERA experiment reached its main goal by proving the appear-ance of ν τ in the CNGS ν µ beam. A sample of five ν τ candidates wascollected allowing to reject the null hypothesis at 5 . σ . The estimation of∆ m in “appearance mode” has been obtained. Updates on the searchfor ν µ → ν e oscillations and on the search for sterile neutrino mixing inthe ν µ → ν e and ν µ → ν τ channels are also reported.PRESENTED AT NuPhys2016, Prospects in Neutrino PhysicsBarbican Centre, London, UK, December 12 – 14, 2016 the OPERA Collaboration is listed in the last page Introduction
The OPERA experiment at the Gran Sasso Lab was exposed to the CNGS ν µ beam,730 km away from the beam source.The CNGS was a conventional neutrino beam optimised for ν τ appearance search.Unlike other neutrino beams designed to measure ν µ disappearance at the atmosphericsquared-mass splitting scale, the mean energy (17 GeV) of the CNGS was not tunedat the oscillation maximum which for L = 730 km is at E ν ∼ . τ production threshold. The prompt ν τ contamination was negligible, O (10 − );the ν e component was relatively small: in terms of CC interactions, the ν e and ¯ ν e contaminations were together < . × protons on target, PoT)resulted in 19 505 neutrino interactions in the OPERA target fiducial volume.The OPERA detector was made of two identical super modules (SMs) each con-sisting of a target section made of lead/emulsion-film modules, of a scintillator trackerdetector, needed to pre-localize neutrino interactions within the target, and of a muonspectrometer. The topology of neutrino interactions were recorded in emulsion cloudchamber detectors (ECC bricks) with submicrometric spatial resolution. Each brickwas a stack of 56 1 mm thick lead plates, and 57 nuclear emulsion films with a12.7 × cross section, a thickness of ∼
10 X and a mass of 8.3 kg. In thebricks, the momenta of charged particles were measured by their multiple Coulombscattering in the lead plates. A changeable sheet (CS) doublet consisting of a pair ofemulsion films was attached to the downstream face of each brick. The full OPERAtarget was segmented in about 150 000 bricks, arranged in each SM in 31 walls. Down-stream of each target wall two orthogonal planes of electronic target trackers (TTs),made of 2.6 cm wide scintillator strips, recorded the position and deposited energyof charged particles. A spectrometer, consisting of iron core magnets instrumentedwith resistive plate chambers and drift tubes was mounted downstream of each targetmodule. The spectrometers are used to identify muons, determine their charge, andmeasure their momentum with an accuracy of about 20%. A detailed description ofthe OPERA detector can be found in Ref. [1]. Neutrino events were classified either as 1 µ , i.e. events with at least one track taggedas a muon, or as 0 µ [2]. A dedicated program reconstructs tracks in the electronicdetectors and builds a 3D probability map for bricks to contain the neutrino vertex.The CS films of the brick with the highest probability are developed and analysedwith high-speed automatic optical microscopes [2], searching for tracks compatiblewith the TT prediction. The tracks found in the CS doublet are extrapolated to1he most downstream film of the brick and then followed upstream in the brick untilthe stopping point (primary vertex). A procedure is then applied to detect chargedand neutral decay topologies, secondary interactions or photon conversions in theneighborhood of the primary vertex. If a secondary vertex is found a full kinematicalanalysis is performed extending the scanned volume and following the tracks also inthe downstream bricks. This analysis integrates the complementary information pro-vided by emulsions and electronic detectors, making use of the angles measured in theemulsion films, the momenta determined by multiple Coulomb scattering measuredin the brick, the momenta measured by the magnetic spectrometers, and the totalenergy deposited in the instrumented target acting as a calorimeter [2]. The energy ofphotons and electrons is also estimated using calorimetric techniques [3]. The detailsof the event analysis procedure are described in Ref. [4]. ν µ → ν τ Five events out of all 0 µ events and 1 µ events with p µ <
15 GeV/c fulfill the topolog-ical and kinematical cuts required for ν τ candidates [5]. In one of them the τ leptonundergoes a muonic decay [6], one event is a τ → h decay [7], and three events are τ → h decays [8, 9].The numbers of expected signal and background events are estimated from thesimulated CNGS flux [10]. The expected detectable signal events in the 0 µ events and1 µ samples are obtained using the reconstruction efficiencies and the ν τ event rate inthe flux normalised to the detected ν µ interactions. A similar normalisation procedureis also used in the background expectation. The details of the signal and backgroundestimation are described in Ref. [7]. The expected numbers of ν τ events for each decaychannel are computed assuming ∆ m = 2 . × − eV [11] and maximal mixing(see Table 1). The total expected signal amounts to 2 . ± .
53 events. The totalsystematic uncertainty on the expected signal is then set to 20% [5].The main sources of background in the search for ν τ appearance are charmed par-ticle decays, hadronic interactions and large-angle muon scattering (LAS). The uncer-tainties on the charm and hadronic backgrounds are 20% [4] and 30% [5], respectively.A recent re-evaluation of the LAS background led to a significant reduction of its con-tribution [12]. From this study it follows that the number of LAS background eventsthat satisfy the selection criteria amounts to [1 . ± . ± . × − /ν CCµ interactions. The estimated background events for the analysed data set with thecorresponding uncertainties are listed in Table 1. The total expected backgroundamounts to 0 . ± .
05 events.The significance of the observed ν τ candidates is evaluated as the probabilitythat the background can produce a fluctuation greater than or equal to the observed2 hannel Exp. Background Exp. Signal ObservedCharm Hadronic re-int LAS Total τ → h ± ± ± ± τ → h ± ± ± ± τ → µ ± ± ± ± τ → e ± ± ± ± ± ± ± ± Table 1:
Expected signal and background events in the analysed data set [5]. number of events. Two test statistics are used, one based on the Fisher’s method, theother one based on the profile likelihood ratio. Both methods exclude the background-only hypothesis with a significance of 5.1 σ [5]. The observed number of ν τ candidatesis also compatible with the expectations in the three neutrino oscillation framework.Based on the number of observed signal candidates ∆ m has been evaluated in“appearance mode” for the first time. Assuming full mixing the 90% C.L. intervalfor ∆ m is [2 . , . × − eV [5]. ν µ → ν e The possibility to efficiently disentangle electrons from photon conversion in the ECCbricks bases the search for oscillations in the ν µ → ν e channel. A dedicated procedureis applied to 0 µ events aiming at identifying “shower hints” from track multiplicity inthe changeable sheet doublets. An additional scanning of volume extending from themost downstream film up to the interaction vertex is performed in order to reconstructelectromagnetic showers. Events with a shower initiated by a single track emergingfrom the primary vertex are classified as ν e candidates. A first result correspondingto 5 . × pot was published in Ref. [3]. The search has been extended to the wholedata set yielding 34 ν e candidates. The expected number of ν e CC interactions due tothe intrinsic beam contamination is 37 ±
5. Background events amount to 1 . ± . π in ν µ interactions without a reconstructed muonand ν τ CC interactions with τ decaying into an electron. In the whole energy range2 . ± . ν e CC events are expected assuming sin θ = 0.098, sin θ = 1,∆ m = 2.44 × − eV , δ CP = 0, and neglecting matter effects. In conclusion, thenumber of observed events is compatible with the 3-flavour oscillation model. The results on ν τ appearance have been interpreted in the context of the 3+1 neutrinomodel deriving limits on oscillations induced by a massive sterile neutrino. Exclusionregions are obtained in the (∆ m , sin θ µτ ) parameter space. The limits on ∆ m are extended up to 10 − eV for relatively large mixing, sin θ µτ > .
5. At largevalues of ∆ m ( > ), marginalising over the CP -violating phase, values of the3ffective mixing parameter sin θ µτ > .
119 are excluded at 90% C.L. [13].In Ref. [3] the number of ν e candidates was compared to the expectation froman approximated two-state model parametrised in terms of two effective parameters,∆ m new and θ new . The approximation is valid assuming CP conservation, neglectingstandard oscillations, treated as a background, and for large values of ∆ m new ( > eV ). To optimise the sensitivity only events below 30 GeV were considered. Sixevents were observed to be compared to an expectation of 9 . ± . m new values the 90% C.L. upper limit on sin θ new is at 7.2 × − . This analysisis being updated in the 3+1 neutrino model using the whole ν e data sample. The OPERA experiment has discovered ν τ appearance with a significance of 5 . σ observing 5 ν τ candidates with a background of 0.25 events.The results on ν µ → ν τ search, compatible with the standard 3 ν model, have beenused to constrain the parameter space of oscillations induced by a massive sterileneutrino. Limits on the sterile neutrino mixing have also been derived in the ν µ → ν e appearance channel.In order to estimate oscillation parameters with reduced statistical uncertaintyan analysis using a selection with released cuts and multivariate techniques is beingperformed. The unique feature of the OPERA experiment to identify all neutrinoflavours will allow a joint fit of all oscillation data. References [1] R. Acquafredda et al. (OPERA Collaboration), JINST , P04018 (2009)[2] N. Agafonova et al. (OPERA Collaboration), New J. Phys. , 053051 (2011)[3] N. Agafonova et al. (OPERA Collaboration), JHEP , 004 (2013)[4] N. Agafonova et al. (OPERA Collaboration), Eur. Phys. J. C , 2986 (2014)[5] N. Agafonova et al. (OPERA Collaboration), Phys. Rev. Lett. , 121802 (2015)[6] N. Agafonova et al. (OPERA Collaboration), Phys. Rev. D , 051102 (2014)[7] N. Agafonova et al. (OPERA Collaboration), JHEP , 036 (2013)[8] N. Agafonova et al. (OPERA Collaboration), Phys. Lett. B , 138 (2010)[9] N. Agafonova et al. (OPERA Collaboration), PTEP ∼ psala/Icarus/cngs.html[11] K. A. Olive et al. , [Particle Data Group], Chin. Phys. C et al. , IEEE Trans. Nucl. Sc. , 2216 (2015)413] N. Agafonova et al. (OPERA Collaboration), JHEP , 069 (2015) The OPERA Collaboration:
N. Agafonova a , A. Aleksandrov b , A. Anokhina c , S. Aoki d , A. Ariga e ,T. Ariga e,e , D. Bender f , A. Bertolin g , I. Bodnarchuk h , C. Bozza i , R. Brugnera g,j , A. Buonaura b,k ,S. Buontempo b , B. B¨uttner l , M. Chernyavskiy m , A. Chukanov h , L. Consiglio b , N. D’Ambrosio n ,G. De Lellis b,k , M. De Serio o,p , P. Del Amo Sanchez q , A. Di Crescenzo b , D. Di Ferdinando r ,N. Di Marco n , S. Dmitrievski h , M. Dracos s , D. Duchesneau q , S. Dusini g , T. Dzhatdoev c , J. Ebert l ,A. Ereditato e , R. A. Fini p , F. Fornari r,u , T. Fukuda t , G. Galati b,k , A. Garfagnini g, j , J. Goldberg v ,Y. Gornushkin h , G. Grella i , A.M. Guler f , C. Gustavino w , C. Hagner l , T. Hara d , H. Hayakawa x ,A. Hollnagel l , B. Hosseini b, , K. Ishiguro x , K. Jakovcic y , C. Jollet s , C. Kamiscioglu f,f , M. Kamis-cioglu f , S. H. Kim z , N. Kitagawa x , B. Klicek y , K. Kodama aa , M. Komatsu x , U. Kose g, , I. Kreslo e ,F. Laudisio g,j , A. Lauria b,k , A. Ljubicic y , A. Longhin g , P. F. Loverre w , M. Malenica y , A. Malgin a ,G. Mandrioli r , T. Matsuo t , V. Matveev a , N. Mauri r,u , E. Medinaceli g,j, , A. Meregaglia s , S. Mikado ad ,M. Miyanishi x , F. Mizutani d , P. Monacelli w , M. C. Montesi b,k , K. Morishima x , M. T. Muciaccia o,p ,N. Naganawa x , T. Naka x , M. Nakamura x , T. Nakano x , K. Niwa x , S. Ogawa t , T. Omura x , K. Osaki d ,A. Paoloni ab , L. Paparella o,p , B. D. Park z , L. Pasqualini r,u , A. Pastore o,p , L. Patrizii r , H. Pessard q ,D. Podgrudkov c , N. Polukhina m , M. Pozzato r , F. Pupilli g , M. Roda g,j, , T. Roganova c , H. Rokujo x ,G. Rosa w , O. Ryazhskaya a , O. Sato x , A. Schembri n , I. Shakirianova a , T. Shchedrina b , A. Sheshukov h ,E. Shibayama d , H. Shibuya t , T. Shiraishi x , G. Shoziyoev c , S. Simone o,p , C. Sirignano g,j , G. Sirri r ,A. Sotnikov h , M. Spinetti ab , L. Stanco g , N. Starkov m ,S. M. Stellacci i , M. Stipcevic y , P. Strolin b,k ,S. Takahashi d , M. Tenti r , F. Terranova ab,ae , V. Tioukov b , S. Vasina h , P. Vilain af , E. Voevodina b ,L. Votano ab , J. L. Vuilleumier e , G. Wilquet af , C. S. Yoon z . a INR - Institute for Nuclear Research of the Russian Academy of Sciences, RUS-117312 Moscow, Russia; b INFNSezione di Napoli, 80125 Napoli, Italy; c SINP MSU - Skobeltsyn Institute of Nuclear Physics, Lomonosov MoscowState University, RUS-119991 Moscow, Russia; d Kobe University, J-657-8501 Kobe, Japan; e Albert Einstein Centerfor Fundamental Physics, Laboratory for High Energy Physics (LHEP), University of Bern, CH-3012 Bern, Switzer-land; e Faculty of Arts and Science, Kyushu University, J-819-0395 Fukuoka, Japan; f METU - Middle East TechnicalUniversity, TR-06800 Ankara, Turkey; f Ankara University, TR-06560 Ankara, Turkey; g INFN Sezione di Padova,I-35131 Padova, Italy; h JINR - Joint Institute for Nuclear Research, RUS-141980 Dubna, Russia; i Dipartimento diFisica dell’Universit`a di Salerno and “Gruppo Collegato” INFN, I-84084 Fisciano (Salerno), Italy; j Dipartimento diFisica e Astronomia dell’Universit`a di Padova, I-35131 Padova, Italy; k Dipartimento di Fisica dell’Universit`a FedericoII di Napoli, I-80125 Napoli, Italy; l Hamburg University, D-22761 Hamburg, Germany; m LPI - Lebedev Physical Insti-tute of the Russian Academy of Sciences, RUS-119991 Moscow, Russia; n INFN - Laboratori Nazionali del Gran Sasso,I-67010 Assergi (L’Aquila), Italy; o Dipartimento di Fisica dell’Universit`a di Bari, I-70126 Bari, Italy; p INFN Sezionedi Bari, I-70126 Bari, Italy; q LAPP, Universit´e Savoie Mont Blanc, CNRS/IN2P3, F-74941 Annecy-le-Vieux, France; r INFN Sezione di Bologna, I-40127 Bologna, Italy; s IPHC, Universit´e de Strasbourg, CNRS/IN2P3, F-67037 Stras-bourg, France; t Toho University, J-274-8510 Funabashi, Japan; u Dipartimento di Fisica e Astronomia dell’Universit`adi Bologna, I-40127 Bologna, Italy; v Department of Physics, Technion, IL-32000 Haifa, Israel; w INFN Sezione diRoma, I-00185 Roma, Italy; x Nagoya University, J-464-8602 Nagoya, Japan; y Center of Excellence for AdvancedMaterials and Sensing Devices, Rudjer Boˇskovi´c Institute, HR-10002 Zagreb, Croatia; y Rudjer Boˇskovi´c Institute,HR-10002 Zagreb, Croatia; z Gyeongsang National University, 900 Gazwa-dong, Jinju 660-701, Korea; aa Aichi Uni-versity of Education, J-448-8542 Kariya (Aichi-Ken), Japan; ab INFN - Laboratori Nazionali di Frascati dell’INFN,I-00044 Frascati (Roma), Italy; ac Dipartimento di Fisica dell’Universit`a di Roma “La Sapienza”, I-00185 Roma, Italy; ad Nihon University, J-275-8576 Narashino, Chiba, Japan; ae Dipartimento di Fisica dell’Universit`a di Milano-Bicocca,I-20126 Milano, Italy; af IIHE, Universit´e Libre de Bruxelles, B-1050 Brussels, Belgium. now at Imperial College, London, UK; now at CERN, Geneva, Switzerland; now at Osservatorio Astronomico diPadova, Italy; now at University of Liverpool, UK.now at University of Liverpool, UK.