Limits on muon-neutrino to tau-neutrino oscillations induced by a sterile neutrino state obtained by OPERA at the CNGS beam

Journal of High Energy Physics, Jun 2015

The OPERA experiment, exposed to the CERN to Gran Sasso ν μ beam, collected data from 2008 to 2012. Four oscillated ν τ Charged Current interaction candidates have been detected in appearance mode, which are consistent with ν μ → ν τ oscillations at the atmospheric Δm 2 within the “standard” three-neutrino framework. In this paper, the OPERA ν τ appearance results are used to derive limits on the mixing parameters of a massive sterile neutrino.

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Limits on muon-neutrino to tau-neutrino oscillations induced by a sterile neutrino state obtained by OPERA at the CNGS beam

Received: March muon-neutrino to tau-neutrino oscillations induced by a sterile neutrino state obtained by OPERA at the CNGS beam The OPERA collaboration The OPERA experiment, exposed to the CERN to Gran Sasso νμ beam, collected data from 2008 to 2012. Four oscillated ντ Charged Current interaction candidates have been detected in appearance mode, which are consistent with νμ → ντ oscillations at the atmospheric Δm2 within the “standard” three-neutrino framework. In this paper, the OPERA ντ appearance results are used to derive limits on the mixing parameters of a massive sterile neutrino. Oscillation; Neutrino Detectors and Telescopes; Beyond Standard Model 1 Introduction 2 3 4 Detector, beam, and data sample The OPERA collaboration The OPERA experiment [1] operated in the CERN Neutrinos to Gran Sasso (CNGS) beam produced at CERN and directed towards the Gran Sasso Underground Laboratory of INFN (LNGS), 730 km away, where the detector is located. The experiment is unique in CC interaction candidate events [4–7], consistent with the expectation of the standard ososcillation in appearance mode. In the present paper, limits are derived on the existence of a massive sterile neutrino. so-called reactor [10] and Gallium [11, 12] neutrino anomalies are also interpreted as due to the existence of a fourth sterile neutrino with mass at the eV scale. In relation to this issue, it is worth mentioning that the effective number of neutrino-like species decoupled from the primeval plasma measured by the Planck collaboration is 3.15 ± 0.23 at 95% Confidence Level (CL) [13]. In the following, a short description of the OPERA experimental setup and of the proregions in the parameter space are derived. Detector, beam, and data sample In OPERA, CNGS neutrinos interacted in a massive target made of lead plates interspaced with nuclear emulsion films acting as high accuracy tracking devices [16]. This kind of detector is historically called Emulsion Cloud Chamber (ECC). The OPERA detector is made of two identical Super Modules, each consisting of a target section and a muon magnetic spectrometer. Each target section has a mass of about 625 tons and is made of the beam direction corresponds to about 10 radiation lengths. The bricks are assembled in vertical walls instrumented with scintillator strips (Target Tracker detectors, TT) to trigger the read-out and locate neutrino interactions within the target. of the four beam components at the detector site are shown in figure 1 [19]. The data taking, based on a minimum bias interaction trigger from the TT scintillators, started in 2008 and ended in December 2012. OPERA collected data corresponding to 17.97 × 1019 protons on target (pot) with 19505 recorded events. The data sample used in this analysis is defined following the selection criteria described in [5] and corresponds to about 75% of the total statistics. Bricks selected as candidates to contain CNGS neutrino interactions are analysed following the procedure described in detail in [5]. Here we just recall the main steps of the analysis. The brick where a neutrino interaction occurred is predicted by the electronic detectors and extracted from the target by an automatic brick manipulator system. Two extra low background emulsion films (Changeable Sheets, CS) [20] located downstream of the brick act as an interface between the brick and the electronic detectors. If the measurement of the CS yields tracks related to the neutrino interaction, the emulsion films of the brick are developed. Their analysis provides the three dimensional reconstruction of the neutrino interaction and of the possible decay vertices of short-lived particles [21] with micrometric expected background in the analysed sample amounts to 0.23 ± 0.05 events. The absence oscillations in the atmospheric sector, computed within a simplified two-flavour scheme is 2.30 ± 0.46 (2.21 ± 0.44) assuming normal (inverted) hierarchy of neutrino masses; the 2.43 × 10−3 eV2 (|Δm2| = 2.38 × 10−3 eV2) [2], where Δm2 is defined as m23 − number is obtained by rescaling the value given in [7] for |Δm232| ≈ |Δm231| ≈ |(Δmm21+2m| 22=). By including the background, 2.53 ± 0.46 (2.44 ± 0.44) events are expected in total. The atmospheric oscillation parameters. The observation of four events is compatible with In presence of a fourth sterile neutrino with mass m4, the oscillation probability is a function of the 4 × 4 mixing matrix U and of the three squared mass differences. Defining C = 2|Uμ3||Uτ3|, Δij = 1.27 Δmi2j L/E (i,j = 1,2,3,4), φμτ = P (E) = C2 sin2 Δ31 + sin2 2θμτ sin2 Δ41 − C sin 2θμτ sin φμτ sin2 Δ31 sin 2Δ41 Observed neutrino oscillation anomalies [23], if interpreted in terms of one additional values, since negative values are disfavoured by results on the sum of neutrino masses respectively. The oscillation probability P (E) can thus be approximated to [23]: factor proportional to the fraction of the analysed sample and to the target mass. 2 distribution of the log likelihood ratio test statistics: q = −2 ln(Le(φμτ , sin2 2θμτ )/L0 ), where L0 = e−n nn /n! In figure 2(a) the 90% CL exclusion limits are presented for both normal and inverted figure 3 together with the unitarity bound (|Uμ4|2 + |Uτ4|2 ≤ 1). Kamiokande collaboration recently published new limits based on the analysis of atmo0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 (dashed line) and for the profile likelihood (continuous line). The unitarity bound (grey line) is also shown. Bands are drawn to indicate the excluded regions. 2 (e41102 OPERA NH OPERA IH CHORUS NOMAD normal (NH, dashed red) and inverted (IH, solid blue) hierarchy of the three standard neutrino masses. The exclusion plots by NOMAD [14] and CHORUS [15] are also shown. Bands are drawn to indicate the excluded regions. ments: CDHS [25], CCFR [26], MiniBooNE+SciBooNE [27] and MINOS [28], as well as by neutral current interaction rate comparison between the near and the far detector [29] and more recently by the Super-Kamiokande analysis on matter effects for up-going atmospheric ues, the likelihood has been computed using the GLoBES software [30, 31], which takes approximated by a constant value estimated with the PREM [32, 33] onion shell model. tions at short-baselines obtained by the NOMAD [14] and CHORUS [15] experiments are dard neutrino masses, with a 10% systematic error deriving from the uncertainties on the 10−3 eV2 would disappear. A narrow region is excluded at 90% CL at Δm241 ≈ 10−3 eV2 for the normal hierarchy probability due to the presence of the sterile neutrino. Instead, the oscillation probability the analysis was repeated following this assumption. The exclusion plots obtained in this way are similar to those of figure 4, but with hierarchies exchanged. It is worth underlining that the results obtained in the 3+1 model, shown in figures 2 and 3, are independent of P (E|CP conservation) ≈ sin2 2θeff · sin2(1.27 Δme2ff L/E) In this framework, with 4 observed events and 2.53 ± 0.46 events expected from the normal hierarchy of standard oscillations, including background, the upper limit on the number of pearance at a baseline of 730 km in the CNGS beam. Exploiting its unique capability to consistent with the expected number of oscillation events in the standard three-neutrino framework, 2.30 ± 0.46 (2.21 ± 0.44), for the normal (inverted) mass hierarchy and 0.23 ± 0.05 background events. In this paper we present limits on the existence of a sterile neutrino in the 3+1 neutrino We 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 acknowledge funding from our national agencies: Fonds de la Recherche Scientique-FNRS and Institut InterUniversitaire des Sciences Nucleaires for Belgium, MoSES for Croatia, CNRS and IN2P3 for France, BMBF for Germany, INFN for Italy, JSPS (Japan Society for the Promotion of Science), MEXT (Ministry of Education, Culture, Sports, Science and Technology), QFPU (Global COE programme of Nagoya University, Quest for Fundamental Principles in the Universe supported by JSPS and MEXT) and Promotion and Mutual Aid Corporation for Private Schools of Japan for Japan, SNF, the University of Bern for Switzerland, the Russian Foundation for Basic Research (grant no. 19-02-00213-a, 12-02-12142 ofim), the Programs of the Presidium of the Russian Academy of Sciences Neutrino physics and Experimental and theoretical researches of fundamental interactions connected with work on the accelerator of CERN, the Programs of Support of Leading Schools (grant no. 3110.2014.2), and the Ministry of Education and Science of the Russian Federation for Russia and the National Research Foundation of Korea Grant (NRF-2013R1A1A2061654) for Korea. 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N. Agafonova, A. Aleksandrov, A. Anokhina, S. Aoki, A. Ariga, T. Ariga, D. Bender, A. Bertolin, I. Bodnarchuk, C. Bozza, R. Brugnera, A. Buonaura, S. Buontempo, B. Büttner, M. Chernyavsky, 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. Dmitrievski, M. Dracos, D. Duchesneau, S. Dusini, T. Dzhatdoev, J. Ebert, A. Ereditato, R. A. Fini, T. Fukuda, G. Galati, A. Garfagnini, J. Goldberg, Y. Gornushkin, G. Grella, A. M. Guler, C. Gustavino, C. Hagner, T. Hara, A. Hollnagel, B. Hosseini, K. Ishiguro, K. Jakovcic, C. Jollet, C. Kamiscioglu, M. Kamiscioglu, J. H. Kim, S. H. Kim, N. Kitagawa, B. Klicek, K. Kodama, M. Komatsu, U. Kose, I. Kreslo, A. Lauria, A. Ljubicic, A. Longhin, A. Malgin, M. Malenica, G. Mandrioli, T. Matsuo, V. Matveev, N. Mauri, E. Medinaceli, A. Meregaglia, S. Mikado, P. Monacelli, M. C. Montesi, K. Morishima, M. T. Muciaccia, N. Naganawa, T. Naka, M. Nakamura, T. Nakano, Y. Nakatsuka, K. Niwa, S. Ogawa, T. Omura, K. Ozaki, A. Paoloni, L. Paparella, B. D. Park, I. G. 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. Shakirianova, T. Shchedrina, A. Sheshukov, H. Shibuya, T. Shiraishi, G. Shoziyoev, S. Simone, M. Sioli, C. Sirignano, G. Sirri, M. Spinetti, L. Stanco, N. Starkov, S. M. Stellacci, M. Stipcevic, P. Strolin, S. Takahashi, M. Tenti, F. Terranova, V. Tioukov, S. Tufanli, P. Vilain, M. Vladymyrov, L. Votano, J. L. Vuilleumier, G. Wilquet, B. Wonsak, C. S. Yoon, S. Zemskova, The OPERA collaboration. Limits on muon-neutrino to tau-neutrino oscillations induced by a sterile neutrino state obtained by OPERA at the CNGS beam, Journal of High Energy Physics, 2015, 69, DOI: 10.1007/JHEP06(2015)069