Observation of Electron Neutrino Appearance in a Muon Neutrino Beam
K. Abe, J. Adam, H. Aihara, T. Akiri, C. Andreopoulos, S. Aoki, A. Ariga, T. Ariga, S. Assylbekov, D. Autiero, M. Barbi, G. J. Barker, G. Barr, M. Bass, M. Batkiewicz, F. Bay, S. W. Bentham, V. Berardi, B. E. Berger, S. Berkman, I. Bertram, S. Bhadra, F. d. M. Blaszczyk, A. Blondel, C. Bojechko, S. Bordoni, S. B. Boyd, D. Brailsford, A. Bravar, C. Bronner, N. Buchanan, R. G. Calland, J. Caravaca Rodríguez, S. L. Cartwright, R. Castillo, M. G. Catanesi, A. Cervera, D. Cherdack, G. Christodoulou, A. Clifton, J. Coleman, S. J. Coleman, G. Collazuol, K. Connolly, L. Cremonesi, A. Dabrowska, I. Danko, R. Das, S. Davis, P. de Perio, G. De Rosa, T. Dealtry, S. R. Dennis, C. Densham, F. Di Lodovico, S. Di Luise, O. Drapier, T. Duboyski, K. Duffy, F. Dufour, J. Dumarchez, S. Dytman, M. Dziewiecki, S. Emery, A. Ereditato, L. Escudero, A. J. Finch, L. Floetotto, M. Friend, Y. Fujii, Y. Fukuda, A. P. Furmanski, V. Galymov, A. Gaudin, S. Giffin, C. Giganti, K. Gilje, D. Goeldi, T. Golan, J. J. Gomez-Cadenas, M. Gonin, N. Grant, D. Gudin, D. R. Hadley, A. Haesler, M. D. Haigh, P. Hamilton, D. Hansen, T. Hara, M. Hartz, T. Hasegawa, N. C. Hastings, Y. Hayato, C. Hearty, R. L. Helmer, M. Hierholzer, J. Hignight, A. Hillairet, A. Himmel, T. Hiraki, et al. (239 additional authors not shown)
aa r X i v : . [ h e p - e x ] A p r Observation of Electron Neutrino Appearance in a Muon Neutrino Beam
K. Abe, J. Adam, H. Aihara,
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T. Akiri, C. Andreopoulos, S. Aoki, A. Ariga, T. Ariga, S. Assylbekov, D. Autiero, M. Barbi, G.J. Barker, G. Barr, M. Bass, M. Batkiewicz, F. Bay, S.W. Bentham, V. Berardi, B.E. Berger, S. Berkman, I. Bertram, S. Bhadra, F.d.M. Blaszczyk, A. Blondel, C. Bojechko, S. Bordoni, S.B. Boyd, D. Brailsford, A. Bravar, C. Bronner, N. Buchanan, R.G. Calland, J. Caravaca Rodr´ıguez, S.L. Cartwright, R. Castillo, M.G. Catanesi, A. Cervera, D. Cherdack, G. Christodoulou, A. Clifton, J. Coleman, S.J. Coleman, G. Collazuol, K. Connolly, L. Cremonesi, A. Dabrowska, I. Danko, R. Das, S. Davis, P. de Perio, G. De Rosa, T. Dealtry,
44, 35
S.R. Dennis,
54, 44
C. Densham, F. Di Lodovico, S. Di Luise, O. Drapier, T. Duboyski, K. Duffy, F. Dufour, J. Dumarchez, S. Dytman, M. Dziewiecki, S. Emery, A. Ereditato, L. Escudero, A.J. Finch, L. Floetotto, M. Friend, ∗ Y. Fujii, ∗ Y. Fukuda, A.P. Furmanski, V. Galymov, A. Gaudin, S. Giffin, C. Giganti, K. Gilje, D. Goeldi, T. Golan, J.J. Gomez-Cadenas, M. Gonin, N. Grant, D. Gudin, D.R. Hadley, A. Haesler, M.D. Haigh, P. Hamilton, D. Hansen, T. Hara, M. Hartz,
23, 50
T. Hasegawa, ∗ N.C. Hastings, Y. Hayato, C. Hearty, † R.L. Helmer, M. Hierholzer, J. Hignight, A. Hillairet, A. Himmel, T. Hiraki, S. Hirota, J. Holeczek, S. Horikawa, K. Huang, A.K. Ichikawa, K. Ieki, M. Ieva, M. Ikeda, J. Imber, J. Insler, T.J. Irvine, T. Ishida, ∗ T. Ishii, ∗ S.J. Ives, K. Iyogi, A. Izmaylov,
16, 22
A. Jacob, B. Jamieson, R.A. Johnson, J.H. Jo, P. Jonsson, C.K. Jung, ‡ A.C. Kaboth, T. Kajita, ‡ H. Kakuno, J. Kameda, Y. Kanazawa, D. Karlen,
51, 50
I. Karpikov, E. Kearns,
3, 23, ‡ M. Khabibullin, A. Khotjantsev, D. Kielczewska, T. Kikawa, A. Kilinski, J. Kim, J. Kisiel, P. Kitching, T. Kobayashi, ∗ L. Koch, A. Kolaceke, A. Konaka, L.L. Kormos, A. Korzenev, K. Koseki, ∗ Y. Koshio, ‡ I. Kreslo, W. Kropp, H. Kubo, Y. Kudenko, § S. Kumaratunga, R. Kurjata, T. Kutter, J. Lagoda, K. Laihem, I. Lamont, M. Laveder, M. Lawe, M. Lazos, K.P. Lee, C. Licciardi, T. Lindner, C. Lister, R.P. Litchfield, A. Longhin, L. Ludovici, M. Macaire, L. Magaletti, K. Mahn, M. Malek, S. Manly, A.D. Marino, J. Marteau, J.F. Martin, T. Maruyama, ∗ J. Marzec, E.L. Mathie, V. Matveev, K. Mavrokoridis, E. Mazzucato, M. McCarthy, N. McCauley, K.S. McFarland, C. McGrew, C. Metelko, M. Mezzetto, P. Mijakowski, C.A. Miller, A. Minamino, O. Mineev, S. Mine, A. Missert, M. Miura, ‡ L. Monfregola, S. Moriyama, ‡ Th.A. Mueller, A. Murakami, M. Murdoch, S. Murphy, J. Myslik, T. Nagasaki, T. Nakadaira, ∗ M. Nakahata,
46, 23
T. Nakai, K. Nakamura,
23, 14, ∗ S. Nakayama, ‡ T. Nakaya,
25, 23
K. Nakayoshi, ∗ D. Naples, C. Nielsen, M. Nirkko, K. Nishikawa, ∗ Y. Nishimura, H.M. O’Keeffe, R. Ohta, ∗ K. Okumura,
47, 23
T. Okusawa, W. Oryszczak, S.M. Oser, R.A. Owen, Y. Oyama, ∗ V. Palladino, V. Paolone, D. Payne, G.F. Pearce, O. Perevozchikov, J.D. Perkin, Y. Petrov, L.J. Pickard, E.S. Pinzon Guerra, C. Pistillo, P. Plonski, E. Poplawska, B. Popov, ¶ M. Posiadala, J.-M. Poutissou, R. Poutissou, P. Przewlocki, B. Quilain, E. Radicioni, P.N. Ratoff, M. Ravonel, M.A.M. Rayner, A. Redij, M. Reeves, E. Reinherz-Aronis, F. Retiere, A. Robert, P.A. Rodrigues, P. Rojas, E. Rondio, S. Roth, A. Rubbia, D. Ruterbories, R. Sacco, K. Sakashita, ∗ F. S´anchez, F. Sato, E. Scantamburlo, K. Scholberg, ‡ J. Schwehr, M. Scott, Y. Seiya, T. Sekiguchi, ∗ H. Sekiya, ‡ D. Sgalaberna, M. Shiozawa,
46, 23
S. Short, Y. Shustrov, P. Sinclair, B. Smith, R.J. Smith, M. Smy, J.T. Sobczyk, H. Sobel,
5, 23
M. Sorel, L. Southwell, P. Stamoulis, J. Steinmann, B. Still, Y. Suda, A. Suzuki, K. Suzuki, S.Y. Suzuki, ∗ Y. Suzuki,
46, 23
T. Szeglowski, R. Tacik,
39, 50
M. Tada, ∗ S. Takahashi, A. Takeda, Y. Takeuchi,
24, 23
H.K. Tanaka, ‡ H.A. Tanaka, † M.M. Tanaka, ∗ D. Terhorst, R. Terri, L.F. Thompson, A. Thorley, S. Tobayama, W. Toki, T. Tomura, Y. Totsuka, ∗∗ C. Touramanis, T. Tsukamoto, ∗ M. Tzanov, Y. Uchida, K. Ueno, A. Vacheret, M. Vagins,
23, 5
G. Vasseur, T. Wachala, A.V. Waldron, C.W. Walter, ‡ D. Wark,
44, 17
M.O. Wascko, A. Weber,
44, 35
R. Wendell, ‡ R.J. Wilkes, M.J. Wilking, C. Wilkinson, Z. Williamson, J.R. Wilson, R.J. Wilson, T. Wongjirad, Y. Yamada, ∗ K. Yamamoto, C. Yanagisawa, †† S. Yen, N. Yershov, M. Yokoyama, ‡ T. Yuan, A. Zalewska, J. Zalipska, L. Zambelli, K. Zaremba, M. Ziembicki, E.D. Zimmerman, M. Zito, and J. ˙Zmuda (The T2K Collaboration) University of Alberta, Centre for Particle Physics, Department of Physics, Edmonton, Alberta, Canada University of Bern, Albert Einstein Center for Fundamental Physics,Laboratory for High Energy Physics (LHEP), Bern, Switzerland Boston University, Department of Physics, Boston, Massachusetts, U.S.A. University of British Columbia, Department of Physics and Astronomy, Vancouver, British Columbia, Canada University of California, Irvine, Department of Physics and Astronomy, Irvine, California, U.S.A. IRFU, CEA Saclay, Gif-sur-Yvette, France University of Colorado at Boulder, Department of Physics, Boulder, Colorado, U.S.A. Colorado State University, Department of Physics, Fort Collins, Colorado, U.S.A. Duke University, Department of Physics, Durham, North Carolina, U.S.A. Ecole Polytechnique, IN2P3-CNRS, Laboratoire Leprince-Ringuet, Palaiseau, France ETH Zurich, Institute for Particle Physics, Zurich, Switzerland University of Geneva, Section de Physique, DPNC, Geneva, Switzerland H. Niewodniczanski Institute of Nuclear Physics PAN, Cracow, Poland High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki, Japan Institut de Fisica d’Altes Energies (IFAE), Bellaterra (Barcelona), Spain IFIC (CSIC & University of Valencia), Valencia, Spain Imperial College London, Department of Physics, London, United Kingdom INFN Sezione di Bari and Universit`a e Politecnico di Bari, Dipartimento Interuniversitario di Fisica, Bari, Italy INFN Sezione di Napoli and Universit`a di Napoli, Dipartimento di Fisica, Napoli, Italy INFN Sezione di Padova and Universit`a di Padova, Dipartimento di Fisica, Padova, Italy INFN Sezione di Roma and Universit`a di Roma “La Sapienza”, Roma, Italy Institute for Nuclear Research of the Russian Academy of Sciences, Moscow, Russia Kavli Institute for the Physics and Mathematics of the Universe (WPI),Todai Institutes for Advanced Study, University of Tokyo, Kashiwa, Chiba, Japan Kobe University, Kobe, Japan Kyoto University, Department of Physics, Kyoto, Japan Lancaster University, Physics Department, Lancaster, United Kingdom University of Liverpool, Department of Physics, Liverpool, United Kingdom Louisiana State University, Department of Physics and Astronomy, Baton Rouge, Louisiana, U.S.A. Universit´e de Lyon, Universit´e Claude Bernard Lyon 1, IPN Lyon (IN2P3), Villeurbanne, France Miyagi University of Education, Department of Physics, Sendai, Japan National Centre for Nuclear Research, Warsaw, Poland State University of New York at Stony Brook, Department of Physics and Astronomy, Stony Brook, New York, U.S.A. Okayama University, Department of Physics, Okayama, Japan Osaka City University, Department of Physics, Osaka, Japan Oxford University, Department of Physics, Oxford, United Kingdom UPMC, Universit´e Paris Diderot, CNRS/IN2P3, Laboratoire dePhysique Nucl´eaire et de Hautes Energies (LPNHE), Paris, France University of Pittsburgh, Department of Physics and Astronomy, Pittsburgh, Pennsylvania, U.S.A. Queen Mary University of London, School of Physics and Astronomy, London, United Kingdom University of Regina, Department of Physics, Regina, Saskatchewan, Canada University of Rochester, Department of Physics and Astronomy, Rochester, New York, U.S.A. RWTH Aachen University, III. Physikalisches Institut, Aachen, Germany University of Sheffield, Department of Physics and Astronomy, Sheffield, United Kingdom University of Silesia, Institute of Physics, Katowice, Poland STFC, Rutherford Appleton Laboratory, Harwell Oxford, and Daresbury Laboratory, Warrington, United Kingdom University of Tokyo, Department of Physics, Tokyo, Japan University of Tokyo, Institute for Cosmic Ray Research, Kamioka Observatory, Kamioka, Japan University of Tokyo, Institute for Cosmic Ray Research, Research Center for Cosmic Neutrinos, Kashiwa, Japan Tokyo Metropolitan University, Department of Physics, Tokyo, Japan University of Toronto, Department of Physics, Toronto, Ontario, Canada TRIUMF, Vancouver, British Columbia, Canada University of Victoria, Department of Physics and Astronomy, Victoria, British Columbia, Canada University of Warsaw, Faculty of Physics, Warsaw, Poland Warsaw University of Technology, Institute of Radioelectronics, Warsaw, Poland University of Warwick, Department of Physics, Coventry, United Kingdom University of Washington, Department of Physics, Seattle, Washington, U.S.A. University of Winnipeg, Department of Physics, Winnipeg, Manitoba, Canada Wroclaw University, Faculty of Physics and Astronomy, Wroclaw, Poland York University, Department of Physics and Astronomy, Toronto, Ontario, Canada (Dated: April 17, 2014)The T2K experiment has observed electron neutrino appearance in a muon neutrino beam pro-duced 295 km from the Super-Kamiokande detector with a peak energy of 0.6 GeV. A total of 28electron neutrino events were detected with an energy distribution consistent with an appearancesignal, corresponding to a significance of 7.3 σ when compared to 4.92 ± parameters including three mixing angles θ , θ , θ , a mass difference ∆ m and a CP violatingphase δ CP . In this neutrino oscillation scenario, assuming | ∆ m | = 2 . × − eV , sin θ = 0 . m > m < θ = 0 . +0 . − . (0 . +0 . − . ) is obtained at δ CP = 0. When combining the result with the current best knowledge of oscillation parametersincluding the world average value of θ from reactor experiments, some values of δ CP are disfavoredat the 90% CL. PACS numbers: 14.60.Pq, 14.60.Lm, 25.30.Pt, 29.40.Ka
Introduction —The discovery of neutrino oscillationsusing atmospheric neutrinos was made by Super-Kamiokande in 1998 [1]. Since then, many other exper-iments have confirmed the phenomenon of neutrino os-cillations through various disappearance modes of flavortransformations. However, to date, there has not been anobservation of the explicit appearance of a different neu-trino flavor from neutrinos of another flavor through neu-trino oscillations. In 2011, the T2K collaboration pub-lished the first indication of electron neutrino appearancefrom a muon neutrino beam at 2.5 σ significance based ona data set corresponding to 1 . × protons on target(POT) [2, 3]. This result was followed by the publicationof further evidence for electron neutrino appearance at3.1 σ in early 2013 [4]. This paper presents new resultsfrom the T2K experiment that establish, at greater than5 σ , the observation of electron-neutrino appearance froma muon-neutrino beam.In a three-flavor framework, neutrino oscillations aredescribed by the PMNS matrix [5, 6] which is parame-terized by three mixing angles θ , θ , θ , and a CPviolating phase δ CP . In this framework the probabilityfor ν µ → ν e oscillation can be expressed [7] as P ( ν µ → ν e ) ≃ sin θ sin θ sin ∆ m L E − sin 2 θ sin 2 θ θ sin ∆ m L E sin θ sin ∆ m L E sin δ CP + (CP even term , solar term , matter effect term) , (1) where L is the neutrino propagation distance and E isthe neutrino energy. The measurement of ν µ → ν e oscil-lations is of particular interest because this mode is sensi-tive to both θ and δ CP . The first indication of non-zero θ was published by T2K [3] based on the measurementof ν µ → ν e oscillations. More recently, indications of ν µ → ν e oscillations were also reported by the MINOSexperiment [12]. The value of θ is now precisely knownto be 9 . ± . ◦ from measurements of ν e disappearancein reactor neutrino experiments [8–11]. Using the reac-tor measurement of θ , the ν µ → ν e appearance modecan be used to explore CP violation, which has yet tobe observed in the lepton sector. CP violation, as shownin Equation 1, is governed by the second term and canbe as large as 27% of the first term for the T2K exper-imental setup when using current values of the neutrinooscillation parameters. T2K Experiment —T2K operates at the J-PARC facil-ity in Tokai, Japan. A muon neutrino beam is produced from the decay of charged pions and kaons generated by30 GeV protons hitting a graphite target and focusedby three magnetic horns. Downstream of the horns isthe decay volume, 96 meters in length, followed by thebeam dump and muon monitors (MUMON). The neu-trino beam illuminates an on-axis detector and off-axisdetectors positioned at an angle of 2.5 ◦ relative to thebeam direction. The resulting energy spectrum, peakedat 0.6 GeV for the off-axis detectors, reduces the ν e con-tamination and the feed-down backgrounds to the ν e appearance signal from higher energy neutrinos. Thenear detector complex at 280 meters from the targetis used to measure the neutrino beam direction, spec-trum, and composition before oscillations and to mea-sure neutrino cross sections. The complex consists of anon-axis detector (INGRID) and a suite of off-axis detec-tors (ND280) that reside within a 0.2 T magnet [2]. TheSuper-Kamiokande (SK) 50 kt water Cherenkov detec-tor, situated 295 km away, is used to detect the oscillatedneutrinos.The results presented here are based on data takenfrom January 2010 to May 2013. During this period theproton beam power has steadily increased and reached220 kW continuous operation with a world record of1 . × protons per pulse. The total neutrino beamexposure at SK corresponds to 6 . × POT.
Neutrino Beam Flux —The neutrino beam flux [13]is predicted by modeling interactions of the primarybeam protons in a graphite target using external hadronproduction data from the CERN NA61/SHINE exper-iment [14, 15] and the FLUKA2008 package [16, 17].GEANT3 [18] with GCALOR [19] simulates propagationof the secondary/tertiary pions and kaons, and their de-cays into neutrinos. The ν e component (including a smallamount of ν e ) in the beam is estimated to be less than1% of the flux below 1.5 GeV, and constitutes an irre-ducible background to the ν e appearance search. Thiscomponent is generated predominantly by the decay ofmuons for E ν < E ν > ± Neutrino Interaction Simulations and Cross SectionParameters —The NEUT neutrino interaction genera-tor [21] is used to simulate neutrino interactions in theINGRID, ND280, and SK detectors. At interaction ener-gies typical of the T2K beam, the dominant charged cur-rent (CC) interactions are charged current quasi-elastic(CCQE) and single resonant pion production. The crosssection parameterization can be divided into two cat-egories: parameters common to interactions at bothND280 and SK, and parameters evaluated separately forthe two detectors. Parameters in the first category com-prise the axial masses for CCQE ( M QEA ) and single res-onant pion production ( M RESA ), and normalizations forCCQE, CC single pion, and neutral current (NC) 1 π interactions. Parameters in the second category are typ-ically related to the interaction target—primarily carbonat ND280 and oxygen at SK—and include Fermi mo-mentum, binding energy, and spectral function modelingfor the CCQE nuclear model. Also in this category arenormalizations for other CC and NC cross sections, the ν e /ν µ CC cross section ratio, pion production parame-ters, and final state interactions of pions exiting the nu-cleus. External data sets, primarily from [22–24], areused to determine the initial values and prior uncertain-ties of the parameters [4].
ND280 Measurements, Flux and Common Cross Sec-tion fits —The energy spectrum of the neutrino beam andthe neutrino cross section parameters are constrained us-ing ν µ CC interactions in ND280. The fine-grained de-tectors (FGDs) [25] are scintillator trackers that serveas the primary neutrino target, and the momentum andidentity of the particles emerging from the interaction aredetermined by the time projection chambers (TPCs) [26]interleaved with the FGDs. The muon is assumed tobe the highest-momentum, negative-curvature track thatemerges from the FGD fiducial volume with an energy de-position consistent with a muon in the TPC downstreamof the FGD. Tracks found in the TPC upstream of theFGD are used to veto external background events.The ND280 analysis includes many improvements overthe previous T2K electron neutrino appearance measure-ment [4]. Candidate events are now divided into threesamples: CC-0 π , dominated by CCQE interactions; CC-1 π + , dominated by CC resonant pion production; andCC-other. The samples are defined by the number of pi-ons in the observed final state. A π + can be identifiedin one of three ways: an FGD+TPC track with positivecurvature and a TPC charge deposition consistent witha pion, an FGD-contained track with a charge depositionconsistent with a pion, or a delayed energy deposit dueto a decay electron from stopped π + → µ + in the FGD. Muon momentum (MeV/c) N u m b e r o f e v e n t s Muon momentum (MeV/c) D a t a / M C FIG. 1. The muon momentum distribution for the ND280CC-0 π sample (upper). The black points represent the data,the blue histogram shows the MC prediction before data con-straint, and the red histogram shows the MC prediction afterconstraint. The lower plot shows the ratio of data to MC forthe pre- and post-constraint cases. To tag a π − , only negative curvature FGD+TPC tracksare used. A π is identified if there exists a track in theTPC with a charge deposition consistent with an electronfrom a γ conversion. Events containing no pions are clas-sified as CC-0 π , events with exactly one π + and no π − or π are classified as CC-1 π + , and all other CC eventsare classified as CC-other. There are 17369, 4047, and4173 data events in the CC-0 π , CC-1 π + , and CC-othersamples, respectively. The ND280 data set used for thisanalysis corresponds to 5 . × POT.The three samples are fit with 25 beam flux parametersat ND280 (11 E ν µ , 5 E ¯ ν µ , 7 E ν e , and 2 E ¯ ν e bins), 21 crosssection parameters (5 in common with SK, and 16 usedonly for ND280), as well as 210 parameters describing theND280 detector systematics (10 momentum × π , CC-1 π + , and CC-other samples, re-spectively. A χ goodness-of-fit test returns a p-value of0.66, indicating no disagreement between the data andthe prediction using best-fit parameters. Figure 1 showsthe muon momentum distribution of the CC-0 π sample,and the improvement in data and MC agreement whenusing the best-fit parameters.The fit to the ND280 data gives estimates for 22 beamflux parameters at SK, the 5 common cross section pa-rameters, and their covariance. Using the ND280 infor-mation reduces the uncertainty on the expected numberof electron-like events at SK due to the propagated pa-rameters from 25.9% to 2.9%. SK Measurements —The SK detector is composed ofan inner detector (ID) and an outer detector (OD). TheID has a water fiducial volume (FV) of 22.5 kt that isequipped with 11129 photomultiplier tubes (PMT) andis surrounded by the 2 m wide OD. Neutrino events atSK are selected if the Cherenkov ring is consistent withan energy above 30 MeV in the ID with low activityin the OD to reject any entering background or exitingevents. These events are labeled fully-contained (FC).The FC fiducial volume (FCFV) sample is obtained byapplying the further cut that the event vertex is at least2 m away from the ID tank wall. A timing cut of − µ s relative to the first beam bunch arrival is applied todistinguish T2K data from other neutrino samples suchas atmospheric neutrino interactions. The timing cutreduces the contamination from other neutrino sourcesto 0.0085 events in the full sample.To select ν e interaction candidate events in the FCFVsample, a single electron-like Cherenkov ring is required.The reconstructed electron momentum ( p e ) is requiredto exceed 100 MeV/c to eliminate decay-electrons fromstopping muons generated by CC interactions and pi-ons in NC interactions. In addition, events are requiredto have a reconstructed neutrino energy ( E rec ν ) below1250 MeV. Nearly all of the oscillated ν e signal eventsare below this value, while most of the intrinsic beam ν e background events have higher energies. The E rec ν iscalculated assuming a CCQE interaction as E rec ν = m p − ( m n − E b ) − m e + 2( m n − E b ) E e m n − E b − E e + p e cos θ e ) , (2)where m n ( m p ) is the neutron (proton) mass, E b is theneutron binding energy in oxygen (27 MeV), m e is theelectron mass, E e is its energy, and θ e is the angle of theelectron direction relative to the beam direction.The final selection criterion removes additional π background events using a new reconstruction algorithm,based on an extension of the model described in Refer-ence [27], to determine the kinematics of all final stateparticles. The new algorithm is a maximum-likelihoodfit in which charge and time probability density func-tions (PDFs) are constructed for every PMT hit for agiven particle hypothesis with a set of 7 parameters:the vertex position, the timing, the direction and themomentum. Multiple-particle fit hypotheses are con-structed by summing the charge contributions from eachconstituent particle. Different neutrino final states aredistinguished by comparing the best-fit likelihood result-ing from the fit of each hypothesis. To separate π events from ν e CC events, both the reconstructed π mass ( m π ) and the ratio of the best-fit likelihoods ofthe π and electron fits ( L π /L e ) are used. Figure 2shows the ln( L π /L e ) vs π mass distribution for signal ν e -CC events and events containing a π in the MC sam-ple, as well as the rejection cut line. Events that satisfyln( L π /L e ) < − . × m π (MeV/c ) constitutethe final ν e candidate sample. This cut removes 69% of the π background events relative to the previous T2K ν e appearance selection, with only a 2% loss in signalefficiency [3]. ) Mass (MeV/c π ) e / L π l n ( L Signal e ν Background π FIG. 2. The ln( L π /L e ) vs m π distribution is shown for bothsignal ν e -CC events (boxes) and background events containinga π (blue scale). The red line indicates the location of the π rejection cut. Events in the upper right corner are rejected. A summary of the number of events passing each se-lection cut is shown in Table I. After all cuts, the to-tal number of candidate ν e events selected in data is 28,which is significantly larger than the 4.92 ± θ = 0. For sin θ = 0 . δ CP = 0, theexpected number is 21.6, as shown in Table I. TABLE I. The expected number of signal and backgroundevents passing each selection stage assuming sin θ = 0 . θ = 0 . | ∆ m | = 2 . × − eV , δ CP = 0, and∆ m >
0, compared to the observed number in data. In-teractions in the true FV are based on the MC truth informa-tion while all other numbers are based on the reconstructedinformation and have been rounded off after addition to avoidrounding error.Selection Data ν µ → ν e ν µ + ν µ ν e + ν e NC TotalCC CC CC MCInteractions in FV - 27.1 325.7 16.0 288.1 656.8FCFV 377 26.2 247.8 15.4 83.0 372.4+Single-ring 193 22.7 142.4 9.8 23.5 198.4+ e -like PID 60 22.4 5.6 9.7 16.3 54.2+ p e > / c 57 22.0 3.7 9.7 14.0 49.4+No decay- e
44 19.6 0.7 7.9 11.8 40.0+ E rec ν < π -like 28 17.3 0.1 3.2 1.0 21.6 The systematic uncertainty due to the SK selectioncuts is evaluated using various data and MC samples.The uncertainty for both the FC and the FV selectionis 1%. The decay-electron rejection cut has errors of0.2-0.4%, depending on neutrino flavor and interactiontype. The uncertainties for the single electron-like ringselection and π rejection are estimated by using the SKatmospheric neutrino data and SK cosmic-ray muons.Electron-neutrino CC-enriched control samples based onthese cuts were prepared, and the differences betweenMC predictions and data are used to extract the system-atic uncertainty. The uncertainty associated with the π background is determined by constructing a hybridsample with either an electron-like ring taken from theatmospheric data sample or from decay-electrons selectedin the stopping muon data sample, and a MC-generatedgamma ray assuming π kinematics. The selection cutsystematic uncertainty is calculated to be 1.6% for signalevents and 7.3% for background events. The total SKselection uncertainty is 2.1% for the ν e candidate eventsassuming sin θ = 0 . ν e event sample, 15% of the remaining π backgroundis due to events where one of the π decay photons is ab-sorbed in a PN interaction. A systematic uncertainty of100% is assumed for the normalization of the PN crosssection. Oscillation Analysis —The neutrino oscillation param-eters are evaluated using a binned extended maximum-likelihood fit. The likelihood consists of four components:a normalization term ( L norm ), a term for the spectrumshape ( L shape ), a systematics term ( L syst ), and a con-straint term ( L const ) from other measurements, L ( N obs , ~x, ~o, ~f ) = L norm ( N obs ; ~o, ~f ) × L shape ( ~x ; ~o, ~f ) ×L syst ( ~f ) × L const ( ~o ) , (3)where N obs is the number of observed events, ~x is a set ofkinematic variables, ~o represents oscillation parameters,and ~f describes systematic uncertainties. In the fit, thelikelihood is integrated over the nuisance parameters toobtain a marginalized likelihood for the parameters ofinterest. L norm is calculated from a Poisson distribution us-ing the mean value from the predicted number of MCevents. L syst ( ~f ) constrains the 27 systematic parametersfrom the ND280 fit, the SK-only cross section parame-ters, and the SK selection efficiencies. Table II showsthe uncertainties on the predicted number of signal ν e events. The L shape term uses x =( p e , θ e ) to distinguishthe ν e signal from backgrounds. An alternative analysisuses x = E recν , the reconstructed neutrino energy. In or-der to combine the results presented in this letter withother measurements to better constrain sin θ and δ CP ,the L const term can also be used to apply additional con-straints on sin θ , sin θ and ∆ m . TABLE II. The uncertainty (RMS/mean in %) on the pre-dicted number of signal ν e events for each group of systematicuncertainties for sin θ = 0 . ν interaction uncertainties are those coming from parts of theneutrino interaction model that cannot be constrained withND280.Error source [%] sin θ = 0 . θ = 0Beam flux and near detector 2.9 4.8(w/o ND280 constraint) (25.9) (21.7)Uncorrelated ν interaction 7.5 6.8Far detector and FSI+SI+PN 3.5 7.3Total 8.8 11.1 A ng l e ( d e g r ee s ) DataBest fitBackground component
Momentum (MeV/c)0 500 1000 150000.20.40.60.81
DataBest fit
FIG. 3. The ( p e , θ e ) distribution for ν e candidate events withthe MC prediction using the primary method best-fit value ofsin θ = 0 .
140 (normal hierarchy).
The following oscillation parameters are fixed in theanalysis: sin θ = 0 . m = 7 . × − eV [29],sin θ = 0 . | ∆ m | = 2 . × − eV [30] and δ CP = 0.For the normal (inverted) hierarchy case, the best-fitvalue with a 68% confidence level (CL) is sin θ =0 . +0 . − . (0 . +0 . − . ). Figure 3 shows the best-fit re-sult, with the 28 observed ν e events. The alternativeanalysis using E recν and a profile likelihood method pro-duces consistent best-fit values and nearly identical confi-dence regions. Figure 4 shows the E rec ν distribution withthe MC prediction for the best-fit θ value in the alter-native analysis.The significance for a non-zero θ is calculated to be7.3 σ , using the difference of log likelihood values betweenthe best-fit θ value and θ = 0. An alternative methodof calculating the significance, by generating a large num-ber of toy MC experiments assuming θ = 0, also returnsa value of 7.3 σ . These significances were calculated us-ing a test statistic having fixed values for θ and δ CP . Reconstructed neutrino energy (MeV)0 500 1000 1500 2000 ca nd i d a t e e v e n t s e ν N u m b e r o f DataBest fitBackground component
Fit region < 1250 MeV>
FIG. 4. The E rec ν distribution for ν e candidate events withthe MC prediction at the best fit of sin θ = 0 .
144 (normalhierarchy) by the alternative binned E rec ν analysis. ) π ( C P δ
68% CL90% CLBest fit range σ PDG2012 1 >0 m ∆ -1-0.500.51 θ sin ) π ( C P δ <0 m ∆ -1-0.500.51 FIG. 5. The 68% and 90% CL allowed regions for sin θ ,as a function of δ CP assuming normal hierarchy (top) andinverted hierarchy (bottom). The solid line represents thebest fit sin θ value for given δ CP values. The values ofsin θ and ∆ m are varied in the fit with the constraintfrom [30]. The shaded region shows the average θ valuefrom the PDG2012 [8]. For any values for these parameters, consistent with theirpresent uncertainties, the significance remains above 7 σ .As the precision of this measurement increases, the un-certainty from other oscillation parameters becomes in-creasingly important. The uncertainties on θ and ∆ m are taken into account in the fit by adding a L const termand marginalizing the likelihood over θ and ∆ m . The L const term is the likelihood as a function of sin θ and ∆ m , obtained from the T2K ν µ disappearance mea-surement [30]. The value of δ CP and the hierarchy areheld fixed in the fit. Performing the fit for all values of δ CP , the allowed 68% and 90% CL regions for sin θ are obtained as shown in Figure 5. For δ CP = 0 andnormal (inverted) hierarchy case, the best-fit value witha 68% CL is sin θ = 0 . +0 . − . (0 . +0 . − . ). Withthe current statistics, the correlation between the ν µ dis-appearance and ν e appearance measurements in T2K isnegligibly small.Constraints on δ CP are obtained by combining our re-sults with the θ value measured by reactor experiments.The additional likelihood constraint term on sin θ isdefined as exp {− (sin θ − . / (2(0 . )) } , where0.098 and 0.013 are the averaged value and the error ofsin θ from PDG2012 [8]. The −
2∆ ln L curve as afunction of δ CP is shown in Figure 6, where the likeli-hood is marginalized over sin θ , sin θ and ∆ m .The combined T2K and reactor measurements prefer δ CP = − π/
2. The 90% CL limits shown in Figure 6are evaluated by using the Feldman-Cousins method [31]in order to extract the excluded region. The data ex-cludes δ CP between 0.19 π and 0.80 π ( − π and − . π ,and − . π and π ) with normal (inverted) hierarchy at90% CL.The maximum value of −
2∆ ln L is 3.38 (5.76) at δ CP = π/ δ CP = − π/
2, sin θ = 0 . θ = 0 . m = 2 . × − eV . The MCaveraged value of −
2∆ ln L at δ CP = π/ π and 0.63 π (0.09 π and 0.90 π ) radians fornormal (inverted) hierarchy case. Conclusions —T2K has made the first observation ofelectron neutrino appearance in a muon neutrino beamwith a peak energy of 0.6 GeV and a baseline of 295 km.With the fixed parameters | ∆ m | = 2 . × − eV ,sin θ = 0 . δ CP = 0, and ∆ m > m < θ = 0 . +0 . − . (0 . +0 . − . ) isobtained, with a significance of 7.3 σ over the hypothesisof sin θ = 0. When combining the T2K result withthe world average value of θ from reactor experiments,some values of δ CP are disfavored at the 90% CL.T2K will continue to take data to measure the neutrinooscillation parameters more precisely and to further ex-plore CP violation in the lepton sector.We thank the J-PARC staff for superb accelerator per-formance and the CERN NA61 collaboration for provid-ing valuable particle production data. We acknowledgethe support of MEXT, Japan; NSERC, NRC and CFI,Canada; CEA and CNRS/IN2P3, France; DFG, Ger-many; INFN, Italy; Ministry of Science and Higher Edu- ) π ( CP δ -1 -0.5 0 0.5 1 l n L ∆ - >0 m ∆ <0 m ∆ >0) m ∆
90% CL ( <0) m ∆
90% CL (
FIG. 6. The −
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