Final results of the search for νμ → νe oscillations with the OPERA detector in the CNGS beam

Journal of High Energy Physics, Jun 2018

Abstract The OPERA experiment has discovered the tau neutrino appearance in the CNGS muon neutrino beam, in agreement with the 3 neutrino flavour oscillation hypothesis. The OPERA neutrino interaction target, made of Emulsion Cloud Chambers, was particularly efficient in the reconstruction of electromagnetic showers. Moreover, thanks to the very high granularity of the emulsion films, showers induced by electrons can be distinguished from those induced by π0s, thus allowing the detection of charged current interactions of electron neutrinos. In this paper the results of the search for electron neutrino events using the full dataset are reported. An improved method for the electron neutrino energy estimation is exploited. Data are compatible with the 3 neutrino flavour mixing model expectations and are used to set limits on the oscillation parameters of the 3+1 neutrino mixing model, in which an additional mass eigenstate m4 is introduced. At high Δm 41 2 (≳0.1 eV2), an upper limit on sin2 2θμe is set to 0.021 at 90% C.L. and Δm 41 2  ≳ 4 × 10− 3 eV2 is excluded for maximal mixing in appearance mode. Open image in new window

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Final results of the search for νμ → νe oscillations with the OPERA detector in the CNGS beam

JHE Final results of the search for The OPERA collaboration 0 1 2 3 6 7 9 0 M. Chernyavskiy 1 A. Meregaglia 2 N. Agafonova 3 G. Mandrioli 4 N. D'Ambrosio 5 T. Shchedrina 6 M.C. Montesi 7 T. Dzhatdoev 8 Now at University of Liverpool 9 RUS-117312 Moscow , Russia 1Now at INFN Sezione di Cagliari. 2Now at CERN. 4Now at Osservatorio Astronomico di Padova. - e oscillations aINR | Institute for Nuclear Research of the Russian Academy of Sciences, bINFN | Sezione di Napoli, I-80125 Napoli, Italy cSINP MSU | Skobeltsyn Institute of Nuclear Physics, Lomonosov Moscow State University, RUS-119991 Moscow, Russia dKobe University, J-657-8501 Kobe, Japan eAlbert Einstein Center for Fundamental Physics, Laboratory for High Energy Physics (LHEP), University of Bern, CH-3012 Bern, Switzerland f Faculty of Arts and Science, Kyushu University, J-819-0395 Fukuoka, Japan gINFN | Sezione di Padova, I-35131 Padova, Italy hDipartimento di Fisica dell'Universita di Salerno and \Gruppo Collegato" INFN, I-84084 Fisciano (Salerno), Italy iDipartimento di Fisica e Astronomia dell'Universita di Padova, I-35131 Padova, Italy j Dipartimento di Fisica dell'Universita Federico II di Napoli, I-80125 Napoli, Italy kLPI | Lebedev Physical Institute of the Russian Academy of Sciences, RUS-119991 Moscow, Russia lJINR | Joint Institute for Nuclear Research, RUS-141980 Dubna, Russia mINFN | Laboratori Nazionali del Gran Sasso, I-67010 Assergi, L'Aquila, Italy nDipartimento di Fisica dell'Universita di Bari, I-70126 Bari, Italy oINFN | Sezione di Bari, I-70126 Bari, Italy pLAPP, Universite Savoie Mont Blanc, CNRS/IN2P3, F-74941 Annecy-le-Vieux, France qINFN | Sezione di Bologna, I-40127 Bologna, Italy rIPHC, Universite de Strasbourg, CNRS/IN2P3, F-67037 Strasbourg, France sHamburg University, D-22761 Hamburg, Germany tDipartimento di Fisica e Astronomia dell'Universita di Bologna, I-40127 Bologna, Italy uNagoya University, J-464-8602 Nagoya, Japan vGSSI | Gran Sasso Science Institute, I-40127 L'Aquila, Italy wDepartment of Physics, Technion, IL-32000 Haifa, Israel xMETU | Middle East Technical University, TR-06800 Ankara, Turkey yINFN | Sezione di Roma, I-00185 Roma, Italy zRuder Boskovic Institute, HR-10002 Zagreb, Croatia aaAnkara University, TR-06560 Ankara, Turkey abGyeongsang National University, 900 Gazwa-dong, Jinju 660-701, Korea acCenter of Excellence for Advanced Materials and Sensing Devices, Ruder Boskovic Institute, HR-10002 Zagreb, Croatia adAichi University of Education, J-448-8542 Kariya (Aichi-Ken), Japan aeToho University, J-274-8510 Funabashi, Japan af Nihon University, J-275-8576 Narashino, Chiba, Japan 6Corresponding author. CNGS muon neutrino beam, in agreement with the 3 neutrino avour oscillation hypothesis. The OPERA neutrino interaction target, made of Emulsion Cloud Chambers, was particularly e cient in the reconstruction of electromagnetic showers. Moreover, thanks to the very high granularity of the emulsion lms, showers induced by electrons can be distinguished from those induced by 0s, thus allowing the detection of charged current interactions of electron neutrinos. In this paper the results of the search for electron neutrino events using the full dataset are reported. An improved method for the electron neutrino energy estimation is exploited. Data are compatible with the 3 neutrino avour mixing model expectations and are used to set limits on the oscillation parameters of the 3+1 neutrino mixing model, in which an additional mass eigenstate m4 is introduced. At high 10 3 eV2 is excluded for maximal mixing in appearance mode. m241 (& 0:1 eV2), an upper limit on sin2 2 e is set to 0.021 at 90% C.L. and 2 3 4 5 6 1 1 Introduction The OPERA experiment Search for e candidates Energy reconstruction Neutrino oscillation analysis 5.1 5.2 The 3 neutrino avour mixing model Analysis in the 3+1 neutrino mixing model Conclusions Introduction from 2008 to 2012 to the CERN Neutrinos to Gran Sasso (CNGS) muon neutrino beam [3, ! 4], produced at a distance of about 730 km. The analysis of a data set released in 2015 led to the identi cation of ve tau neutrino candidates with an expected background of 0.25, thus excluding the no-oscillation hypothesis with a signi cance greater than 5 [5]. The OPERA nuclear emulsion target also allows the identi cation of e (or e) charged current (CC) interactions. The outcome of the search for electron neutrinos in the data collected in 2008{2009 was reported in [6]. Here, we report the results of an improved analysis on the complete dataset, corresponding to 17.97 1019 p.o.t., which represents an increase of the exposure by a factor 3.4 with respect to the previous analysis. Data compared with the expectation from the 3 neutrino avour oscillation model allow setting an upper limit on the e appearance probability. Moreover, the hypothesis of a sterile neutrino, as hinted by LSND [7], MiniBooNE [8], reactor [9], and radio-chemical [10, 11] experiments, is tested. Limits on m241 and sin2 2 e = 4jUe4j2jU 4j2 are derived. 2 The OPERA experiment The detector was composed of two identical super modules, each consisting of a target section and a magnetic iron spectrometer. Each target section contained about 75000 Emulsion Cloud Chamber [12] modules, hereafter called bricks, for a total target mass { 1 { of about 1.25 kt. The bricks were arranged in vertical walls interleaved with planes of horizontal and vertical scintillator strips, which formed the Target Tracker (TT). Each brick consisted of 56 1-mm lead plates interspersed with 57 emulsion lms. It had a section of 10:2 cm2 and a thickness of 7.5 cm, which corresponds to 10 radiation lengths (X0). A pair of emulsion lms (Changeable Sheet or CS doublet) were packed and glued externally to the downstream face of each brick. The CS doublet acted as an interface between the cm-resolution TT and m-resolution emulsion, thus triggering the lm development. The spectrometer, instrumented with planes of Resistive Plate Chambers (RPC) and drift tube stations, provided the charge and momentum measurements of muons [13]. The CNGS beam was an almost pure muon neutrino beam having a mean energy of 17 GeV. The contaminations, in terms of CC interactions, of e , e and were 0.88%, 0.05% and 2.1%, respectively. The TT hits of an event occurring on time with the CNGS beam were used to track charged particles in the target region and to provide a calorimetric energy measurement. The TT information was exploited to rank the bricks according to their probability to contain the neutrino interaction vertex [14]. The highest probability brick was extracted from the detector. The attached CS doublet was scanned and the pattern of reconstructed tracks would either con rm the prediction of the electronic detector or act as veto and thus trigger the extraction of neighbouring bricks. In case of positive outcome, the brick emulsion lms were developed and analysed. CS tracks were then matched with those reconstructed in the downstream lms of the brick. These latter tracks were then followed upstream in the brick to their origins. Around the most upstream track disappearance point, a 1 cm2 wide area in 10 downstream and 5 upstream emulsion lms was scanned. All tracks and vertices in this volume were reconstructed, and short-lived particle decays were searched for. In case no neutrino interaction was found in the brick, the search would continue in less probable bricks. More details on the search and reconstruction of neutrino events in the bricks are given in [15{17]. 3 Search for e candidates The brick acted as a high sampling calorimeter with more than ve active layers every X0 over a total thickness of 10 X0. For most of e CC interactions, the path of the electron in the brick is long enough for the electromagnetic (e.m.) shower to develop, thus allowing its detection and reconstruction in the emulsion lms. On the other hand, the size of the standard scanned volume along the beam direction corresponds to about 1.8 X0, which is too short for the e.m. shower to develop. The search for electrons is therefore performed in an extended scanned volume applying a dedicated procedure to the 0 tagged events, i.e. events having no reconstructed three-dimensional muon track and less than 20 red TT/RPC planes. A search is performed in the CS doublet for track segments less than 2 mm apart from the extrapolation point of each track originated from the interaction vertex (primary tracks). Moreover, the direction of candidate CS tracks is required to be compatible within 150 mrad with that of the primary track. If at least three tracks are found, additional scanning along the primary track is performed aiming at the detection of an e.m. shower [6]. { 2 { bars indicate the MC statistical errors. The black line is a tted curve to the simulated e ciency points and the grey area represents the systematic error. Detected showers are carefully inspected, by visual scan, in the rst two lms downstream the interaction vertex to assess whether they are produced by a single particle. Thanks to the high granularity of the OPERA nuclear emulsions, one can recognize an epair from conversion when the e-pair tracks are separated by more than 1 m. Once the origin of the e.m. shower is con rmed as due to a single charged particle, the event is classi ed as e candidate. A classi er algorithm [18], is applied to select neutrino interaction events fully contained in the ducial volume of the detector. The e ciency of the procedure applied to search for e CC interactions, shown in gure 1, is computed by Monte Carlo (MC) simulation using the standard OPERA simulation framework [17]. Neutrino uxes are determined by FLUKA [19, 20] based simulation of the CNGS beamline [21]. Neutrino interactions in the target are generated using the GENIE v2.8.6 generator [22, 23]. The simulated e ciency points are then tted with an empirical function. Since the number of hit TT/RPC planes and presence of long tracks in neutrino events are correlated with the neutrino energy, as a result of 0 tagging criterion, a drop in the e ciency is expected at high energies. In 2008{2012, the OPERA detector had collected data corresponding to 17:97 1019 p.o.t. In total, 19505 on-time events have been registered in the target volume. Out of them, 5868 events have a reconstructed neutrino interaction vertex in the rst or second most probable brick and 1281 are tagged as 0 events. The 0 sample is further reduced to 1185 events by exclusion of not contained ones. In this sample the number of e candidates is 35. The number, Nbeam, of e candidates from CC interactions of e and e beam components is estimated using a data-driven approach from the number of observed events with { 3 { i = i = e R i "j0l( i) j = CC normalized to 1019 p.o.t. and 1 kt target mass. In brackets the e ciencies of i interactions, convoluted with the CNGS i ux and its j-type interaction cross sections, to be reconstructed as a 0l event. according to: no charged leptons, 0l, which are 0 events not identi ed as e candidates, n0l = 1185 35, Nbeam = n0l P i= ;e j=CC;NC RCeC "CeC ( e) P j R i "j0l( i) ; where "je ( i) and "j0l( i) are the e ciencies for the i interactions (i = , e), convoluted with the CNGS i ux, i , and its j-type interaction cross sections, ji (j = CC or NC), to be reconstructed as a e candidate or as a 0l event, respectively, i.e.: "je(0l)( i) = Z i j " e(0l) j dE i Z i i j dE; while Rji are the interaction rates of neutrino and antineutrino: Rji = Z i i j dE: 3- avour 0.7 ! 0.2 (syst.) events. The expected rates Rji , for 1019 p.o.t. and 1 kt target mass, and the e ciencies h"0ClC ( i) are reported in table 1. The e ciency h"CeC ( e) is 0.375. The combined systematic error from ux normalization, e/ ux ratio, detection e ciencies and cross section uncertainties is conservatively estimated as 20% and 10% for neutrino energies below and above 10 GeV, respectively. The expected value of Nbeam amounts to 30.7 0.9 (stat.) 3.1 (syst.) events. The expected numbers obtained with the above mentioned procedure are insensitive to systematic e ects on the e ciencies up to the location level being common to e and 0l events. 0 ! Background contributions are: CC interactions with ! e decays, 0 events with decay with prompt conversion, and the electron-positron pair misidenti ed as an electron. The rst type of background arises when the -lepton and its daughter electron track cannot be distinguished. It is estimated by MC simulation assuming the oscillation scheme and oscillation parameters from [24]. It amounts to The background due to 0 is derived from the data by counting the number of events ful lling the criteria for a e candidate with a converting in the second or third lead plate downstream of the interaction vertex. That number is converted into the probability to { 4 { (3.1) (3.2) (3.3) HJEP06(218)5 observe background e candidates due to conversions in the rst lead plate, taking into account the radiation length [6]. We expect 0.5 0.5 (stat.) such events. Summarizing, the expected number of background events, Nbkg, amounts to 1.2 0.5 (stat.) 0.2 (syst.), while the sum of expected events from e and e beam components and backgrounds is 31.9 Energy reconstruction The longitudinal and transverse development of e.m. showers are well parametrised both for homogeneous and sampling calorimeters [25{27], hence the energy shape analysis can be used to improve the electron energy reconstruction. The e.m. shower shape parametrisation together with the knowledge of the interaction vertex position and of the direction of the electron, obtained from the emulsion data, are the basis of the method applied in this analysis, as detailed in [28]. In this approach, the TT signals from e.m. shower, initiated by the electron at the interaction vertex, and the hadronic shower are separated. The hadronic shower energy is estimated by the calorimetric measurement in the TT. The electron energy is assessed by the shape analysis of the e.m. shower pro les. The reconstructed energy of e CC events versus their true energy is shown in gure 2. The accuracy of the energy reconstruction of e CC events can be parametrized as: which is improved with respect to previous analysis [6]. E E = 0:18 + pE(GeV) 0:55 { 5 { (4.1) 5.1 The 3 neutrino avour mixing model The neutrino oscillation phenomenon can lead to a non null di erence, nosc, between the number of observed e candidates, nobs, and that expected from the beam contamination, Nbeam and from background, Nbkg, i.e. On the other end, assuming only contributions from pected value of e candidates from oscillations, Nosc, is: ( ) ! e( e) oscillations the exwhere k is the averaged number of nuclei over the data taking period, P e is the oscillation probability, " e and e are the e detection e ciency and cross section, respectively, is the integrated ux and E is the neutrino energy. The average appearance probability, hP ei, can be derived as: hP ei = k R nosc e " e dE An optimal interval on neutrino energy, 0{40 GeV, was introduced in order to maximize the sensitivity on the average appearance probability. hP ei is model-independent, and energy interval de ned above can be used for any oscillation analysis of the OPERA data not based on the energy shape analysis. The 90% C.L. upper limit, obtained using the Feldman and Cousins method (F&C) [29], is hP ei < 3:5 A similar result, hP ei < 3:7 10 3 for neutrino energies 0 < E < 40 GeV. 10 3, is obtained using the Bayesian technique [30]. Constraints on the oscillation parameters are derived within the framework of the 3 neutrino avour oscillation model. The expected number of e candidates (N3exp) depends on the disappearance of e ( e) beam contamination and on e ( e) appearance from ( ) beam components. Both these contributions are taken into account. The oscillation probabilities are evaluated from: P ( ! ) = 4 X(U iU iU j U j ) sin2 1:27 mi2j eV2=c4 i>j L(km) E(GeV) : (5.4) Using the standard parametrization of the mixing matrix U , the values of 13, 23, CP and mi2j from [24], the expected number of e candidates, including 1.2 background events, is N exp = 34.3 3 events. Figure 3(a) shows the energy distributions of observed events, of the expectation from e and e beam components, assuming no oscillations, and of background events ( 0 and ! e). In the expectation from gure 3(b) the energy distribution of observed events is compared with e( e) ! ( ) ! e( e) oscillations channels, and from background components. The background components, 0 and ! e, are assumed to be una ected by oscillations. In the optimized neutrino energy range (0{40 GeV), the 90% C.L. upper limit on sin2 2 13 is 0.43. { 6 { (5.1) (5.2) (5.3) (a) (b) e and e components: (a) assuming no oscillations; (b) in case of 3 neutrino avour mixing with the parameters from [24]. 5.2 Analysis in the 3+1 neutrino mixing model The backgrounds arising from 0 and The excess of e and e reported by the LSND [7], MiniBooNE [8], reactor [9] and radiochemical [10, 11] experiments may be interpreted as due to the presence of light, O(1 eV/c2), sterile neutrinos. OPERA can test the hypothesis of the presence of a sterile neutrino by comparing the observed e energy spectrum with that predicted from the 3+1 neutrino mixing model. The event energy spectrum is divided in N = 6 bins, as shown in gure 3(a); ni is the number of observed events in the i-th bin. The expected number of events in each bin, i, is evaluated using GLoBES [34, 35]. Detector e ects are taken into account by smearing matrices calculated by MC simulation. The contributions from four neutrino oscillation channels are taken into account, namely e( e) ! ( ) ! e( e). ! e, de ned in section 3, are considered independent of the 3+1 mixing model parameters. Two corrective and independent factors, kj (j = 1; 2), are introduced to take into account the overall systematic uncertainties, j , on the intrinsic e and e beam components, on the e detection e ciency and on e and e cross sections. The expected number of events is i = i0(1 + kj ) where ( j = 1; if i = 1 j = 2; otherwise : (5.5) i0 is the expected number of events estimated using the nominal uxes, cross-sections and detection e ciencies. As already stated, the systematic uncertainties, j , are conservatively estimated as 20% for the rst energy bin (E 10 GeV) and 10% for E > 10 GeV. The { 7 { pro le likelihood ratio was used as test statistic, with the likelihood de ned as: 2 ln L = 2 X (ni ln i N i N i) + X2 k 2 j2 + The last term is a penalty term accounting for the current knowledge on and 2 m231 are the best t value and the 1 of interest are the squared mass di erence uncertainty, respectively. The parameters m241 and the e ective mixing sin2 2 e = 4jUe4j2jU 4j2. It is worth noting that this de nition of the e ective mixing allows a direct comparison of this analysis with short-baseline results. The non-zero value of m221 is taken into account as well as matter e ects assuming a constant Earth crust density estimated with the PREM [36, 37] onion shell model. All other oscillation parameters are treated as nuisance parameters and pro led out. The result is restricted to positive negative values are disfavoured by results on the sum of neutrino masses from cosmological surveys [38]. The resulting 90% C.L. exclusion region is shown in gure 4. An upper limit on sin2(2 e) = 0:021 is set for limit the e ective mixing for low m241 > 0:1 eV2. Moreover, OPERA contributes to m241 and excludes mixing. It must be stressed that, for small m241 values, OPERA is the only experiment having collected data in appearance mode. MINOS and Daya Bay/Bugey-3 have obtained a more stringent exclusion limit in a combined analysis of their disappearance results on, respectively, an accelerator beam and reactors e uxes. (5.6) The full OPERA data sample collected during the 2008{2012 CNGS beam runs is used to search for e candidates. Compared to the previous search [6], results reported here are based on a larger statistics and improved e CC energy resolution. Model independent results are reported to allow testing di erent oscillation hypotheses. The results are compatible with the no-oscillation hypothesis as well as with the 3 neutrino avour one. In the latter case, a 90% C.L. upper limit sin2(2 13) < 0:43 is set. For the rst time, in this paper, the results are also analysed in a 3+1 model to test the hypothesis of the existence of a sterile neutrino as suggested by several experiments. A 90% C.L. upper limit sin2(2 e) = 0:021 for m241 & 0:1 eV2 is set, thus excluding a sizable part of the region hinted by LSND and MiniBooNE experiments. OPERA is the only appearance experiment excluding neutrino mass di erence down to 4 10 3 eV2. Acknowledgments We wish to thank CERN for the successful operation of the CNGS facility and INFN for the continuous support given to the experiment through its LNGS laboratory. We warmly acknowledge funding from our national agencies: Fonds de la Recherche Scienti que-FNRS and Institut Interuniversitaire des Sciences Nucleaires for Belgium; MoSES for Croatia; CNRS and IN2P3 for France; BMBF for Germany; INFN for Italy; JSPS, MEXT, the { 8 { The 90% C.L. exclusion plot in the m241 and sin2 2 e plane is shown (black line) together with the 90% C.L. allowed region obtained by LSND (cyan) and MiniBooNE (yellow and green for and mode, respectively). The blue, red and green lines represent the 90% C.L. exclusion regions obtained by NOMAD [31], KARMEN [32] and the MINOS and DayaBay/Bugey-3 joint analysis [33], respectively. QFPU-Global COE program of Nagoya University, and Promotion and Mutual Aid Corporation for Private Schools of Japan for Japan; SNF, the University of Bern and ETH Zurich for Switzerland; the Russian Foundation for Basic Research (Grant No. 12-02-12142 o m), the Programs of the Presidium of the Russian Academy of Sciences (Neutrino Physics and Experimental and Theoretical Researches of Fundamental Interactions), and the Ministry of Education and Science of the Russian Federation for Russia, the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (Grant No. NRF-2015R1A2A 2A01004532) for Korea; and TUBITAK, the Scienti c and Technological Research Council of Turkey for Turkey (Grant No. 108T324). We thank JINR Association of Young Scientists and Specialists for partial support of this work (Grant No. 16-202-02). We thank the IN2P3 Computing Centre (CCIN2P3) for providing computing resources. 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The OPERA collaboration, N. Agafonova, A. Aleksandrov, A. Anokhina, S. Aoki, A. Ariga, T. Ariga, A. Bertolin, C. Bozza, R. Brugnera, A. Buonaura, S. Buontempo, M. Chernyavskiy, A. Chukanov, L. Consiglio, N. D’Ambrosio, G. De Lellis, M. De Serio, P. del Amo Sanchez, A. Di Crescenzo, D. Di Ferdinando, N. Di Marco, S. Dmitrievsky, M. Dracos, D. Duchesneau, S. Dusini, T. Dzhatdoev, J. Ebert, A. Ereditato, J. Favier, R. A. Fini, F. Fornari, T. Fukuda, G. Galati, A. Garfagnini, V. Gentile, J. Goldberg, Y. Gornushkin, S. Gorbunov, G. Grella, A. M. Guler, C. Gustavino, C. Hagner, T. Hara, T. Hayakawa, A. Hollnagel, B. Hosseini, K. Ishiguro, A. Iuliano, K. Jakovcic, C. Jollet, C. Kamiscioglu, M. Kamiscioglu, S. H. Kim, N. Kitagawa, B. Klicek, K. Kodama, M. Komatsu, U. Kose, I. Kreslo, F. Laudisio, A. Lauria, A. Ljubicic, A. Longhin, P. Loverre, A. Malgin, M. Malenica, G. Mandrioli, T. Matsuo, V. Matveev, N. Mauri, E. Medinaceli, F. Meisel, A. Meregaglia, S. Mikado, M. Miyanishi, F. Mizutani, P. Monacelli, M. C. Montesi, K. Morishima, M. T. Muciaccia, N. Naganawa, T. Naka, M. Nakamura, T. Nakano, K. Niwa, N. Okateva, S. Ogawa, K. Ozaki, A. Paoloni, L. Paparella, B. D. Park, L. Pasqualini, A. Pastore, L. Patrizii, H. Pessard, D. Podgrudkov, N. Polukhina, M. Pozzato, F. Pupilli, M. Roda, T. Roganova, H. Rokujo, G. Rosa, O. Ryazhskaya, O. Sato, A. Schembri, I. Shakiryanova, T. Shchedrina, H. Shibuya, E. Shibayama, T. Shiraishi, S. Simone, C. Sirignano, G. Sirri, A. Sotnikov, M. Spinetti, L. Stanco, N. Starkov, S. M. Stellacci, M. Stipcevic, P. Strolin, S. Takahashi, M. Tenti, F. Terranova, V. Tioukov, S. Vasina, P. Vilain, E. Voevodina, L. Votano, J. L. Vuilleumier, G. Wilquet, C. S. Yoon. Final results of the search for νμ → νe oscillations with the OPERA detector in the CNGS beam, Journal of High Energy Physics, 2018, 151, DOI: 10.1007/JHEP06(2018)151