Measurement of neutrino oscillation with Kamland: Evidence of spectral distortion

Measurement of Neutrino Oscillation with KamLAND: Evidence of Spectral Distortion T. Araki, 1 K. Eguchi, 1 S. Enomoto, 1 K. Furuno, 1 K. Ichimura, 1 H. Ikeda, 1 K. Inoue, 1 K. Ishihara, 1, ∗ T. Iwamoto, 1, † T. Kawashima, 1 Y. Kishimoto, 1 M. Koga, 1 Y. Koseki, 1 T. Maeda, 1 T. Mitsui, 1 M. Motoki, 1 K. Nakajima, 1 H. Ogawa, 1 K. Owada, 1 J.-S. Ricol, 1 I. Shimizu, 1 J. Shirai, 1 F. Suekane, 1 A. Suzuki, 1 K. Tada, 1 O. Tajima, 1 K. Tamae, 1 Y. Tsuda, 1 H. Watanabe, 1 J. Busenitz, 2 T. Classen, 2 Z. Djurcic, 2 G. Keefer, 2 K. McKinny, 2 D.-M. Mei, 2, ‡ A. Piepke, 2 E. Yakushev, 2 B.E. Berger, 3 Y.D. Chan, 3 M.P. Decowski, 3 D.A. Dwyer, 3 S.J. Freedman, 3 Y. Fu, 3 B.K. Fujikawa, 3 J. Goldman, 3 F. Gray, 3 K.M. Heeger, 3 K.T. Lesko, 3 K.-B. Luk, 3 H. Murayama, 3, § A.W.P. Poon, 3 H.M. Steiner, 3 L.A. Winslow, 3 G.A. Horton-Smith, 4 C. Mauger, 4 R.D. McKeown, 4 P. Vogel, 4 C.E. Lane, 5 T. Miletic, 5 P.W. Gorham, 6 G. Guillian, 6 J.G. Learned, 6 J. Maricic, 6 S. Matsuno, 6 S. Pakvasa, 6 S. Dazeley, 7 S. Hatakeyama, 7 A. Rojas, 7 R. Svoboda, 7 B.D. Dieterle, 8 J. Detwiler, 9 G. Gratta, 9 K. Ishii, 9 N. Tolich, 9 Y. Uchida, 9, ¶ M. Batygov, 10 W. Bugg, 10 Y. Efremenko, 10 Y. Kamyshkov, 10 A. Kozlov, 10 Y. Nakamura, 10 C.R. Gould, 11 H.J. Karwowski, 11 D.M. Markoff, 11 J.A. Messimore, 11 K. Nakamura, 11 R.M. Rohm, 11 W. Tornow, 11 R. Wendell, 11 A.R. Young, 11 M.-J. Chen, 12 Y.-F. Wang, 12 and F. Piquemal 13 (The KamLAND Collaboration) Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan Department of Physics and Astronomy, University of Alabama, Tuscaloosa, Alabama 35487, USA Physics Department, University of California at Berkeley and Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA W. K. Kellogg Radiation Laboratory, California Institute of Technology, Pasadena, California 91125, USA Physics Department, Drexel University, Philadelphia, Pennsylvania 19104, USA Department of Physics and Astronomy, University of Hawaii at Manoa, Honolulu, Hawaii 96822, USA Department of Physics and Astronomy, Louisiana State University, Baton Rouge, Louisiana 70803, USA Physics Department, University of New Mexico, Albuquerque, New Mexico 87131, USA Physics Department, Stanford University, Stanford, California 94305, USA Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37996, USA Triangle Universities Nuclear Laboratory, Durham, North Carolina 27708, USA and Physics Departments at Duke University, North Carolina State University, and the University of North Carolina at Chapel Hill Institute of High Energy Physics, Beijing 100039, People’s Republic of China CEN Bordeaux-Gradignan, IN2P3-CNRS and University Bordeaux I, F-33175 Gradignan Cedex, France (Dated: June 13, 2004) arXiv:hep-ex/0406035 v1 13 Jun 2004 We present an improved measurement of the oscillation between the first two neutrino families based on a 766.3 ton-year exposure of KamLAND to reactor anti-neutrinos. KamLAND observes 258 events with ν e en- ergies above 3.4 MeV compared to 365.2 events expected in the absence of neutrino oscillation. The confidence level for reactor ν e disappearance is now 99.995%. The observed energy spectrum disagrees with the expected spectral shape in the absence of neutrino oscillation at the 99.9% confidence level but agrees with the distortion expected from ν e oscillation effects. A two-neutrino oscillation analysis of the KamLAND data gives a best-fit point at ∆m 2 = 8.3×10 −5 eV 2 and tan 2 θ = 0.41. A global analysis of data from KamLAND and solar neutrino experiments yields ∆m 2 = 8.2 +0.6 ×10 −5 eV 2 and tan 2 θ = 0.40 +0.09 , the most precise determination to date. PACS numbers: 14.60.Pq, 26.65.+t, 28.50.Hw The first measurement of reactor anti-neutrino disappear- ance by KamLAND [1] suggested that solar neutrino flavor transformation through the Mikheyev-Smirnov-Wolfenstein (MSW) [2] matter effect has a direct correspondence to anti-neutrino oscillation in vacuum. KamLAND and solar- neutrino experiments have restricted the oscillation parame- ter space for the first two families, eliminating all but the large-mixing-angle (LMA-MSW) solution. The LMA solu- tion was confined to two small regions conventionally named “LMA I” and “LMA II” [3] for the lower ∆m 2 ∼7×10 −5 eV 2 and higher ∆m 2 ∼2×10 −4 eV 2 bands respectively. A com- bined analysis [4] of the latest results from SNO, other solar neutrino experiments, and the previous KamLAND result dis- favored LMA II at greater than 99% C.L. This Letter reports on new results based on a factor of three longer exposure time and analysis improvements allowing a 33% larger fiducial vol- ume. There were large variations in the reactor power produc- tion in Japan in 2003, providing an opportunity to study the anti-neutrino flux modulation at the KamLAND site. The KamLAND experiment consists of 1 kton of ultra-pure liquid scintillator (LS) contained in a transparent nylon-based balloon suspended in non-scintillating oil. The balloon is sur- rounded by an array of 1879 photomultiplier tubes (PMT’s) mounted on the inner surface of an 18-m-diameter spherical stainless-steel containment vessel. Electron anti-neutrinos are detected via the inverse β-decay reaction, ν e + p → e + + n, with a 1.8 MeV ν e energy threshold. The prompt scintillation light from the e + gives an estimate of the incident ν e energy,