Indications of neutrino oscillation in K2K experiment

This thesis presents the first results of analysis dedicated to neutrino oscillation with the disappearance of muon neutrino in a long baseline neutrino experiment, the K2K experiment. The analysis is based on the data taken June 1999 to July 2001, in correspondence to total number of protons on target of 4.79× 1019. In the analysis, the neutrino flux and energy spectrum measurements in the detectors at near site, the beam extrapolation from near to far site validated by a pion monitor measurements, and the observation in far site are all in use with the correlation between energies and interaction cross-sections taken into account. The results indicate the existence of neutrino oscillation νμ 6→ νμ: a reduction of νμ flux together with a distortion of the energy spectrum. Fifty-six beam neutrino events are observed in Super-Kamiokande, 250 km from the neutrino production point, with an expectation of 80.1 −5.4 in the case of no neutrino oscillation. Twenty-nine one ring μ-like events are used to reconstruct the neutrino energy spectrum, which is better matched to the expected spectrum with neutrino oscillation than without. The probability that the observed flux at Super-Kamiokande is explained by statistical fluctuation without neutrino oscillation is less than 1%. The oscillation parameters at the best-fit point are obtained to be (sin 2θ, ∆m2) = (1.0, 2.8 × 10−3 eV) for the two flavor oscillation. The 90% C.L. region for the mass squared difference, ∆m2, at sin 2θ = 1 is estimated to be (1.5−3.9)× 10−3 eV. These results are consistent with the results of the atmospheric neutrino observations. Acknowledgements This thesis could not be achieved without any supports. First of all, I would like to express my appreciation to Prof. Koichiro Nishikawa, the supervisor of this thesis and the spokesperson of K2K experiment. He involved me with the most exciting experiments in high energy physics, K2K and Super-Kamiokande, and gave me a great chance to study the most important parts of neutrino physics and high energy physics in the world. It has been an extraordinary time in physics and for me. He has guided me through thick and thin to become a physicist. I would also like to make an appreciation to Prof. Tsuyoshi Nakaya. He has also been one of advisers for this thesis. He has been giving me a number of things interesting about high energy physics. I could learn a lot of things from basic knowledges to technical issues. I thank Prof. Takashi Kobayashi for his many advices all the time since the very beginning of my graduate school life. He has encouraged me to join the HARP experiment at CERN and gave me very precious experiences to enjoy physics abroad and to make discussions with the overseas physicists. I wish to extend my appreciation to Prof. Y. Totsuka, Prof. K. Nakamura, Prof. Y. Suzuki, Prof. C. K. Jung, Prof. R. J. Wilkes, and Prof. M. Sakuda. They have been managed the K2K experiment to accomplish my research. My study strongly depends on the hard works of many collaborators. For 1KT part, Prof. Y. Itow gave me a lot of instruction about the detector, the analysis software, and the physics issues. I also want to thank Dr. M. Miura, Dr. S. Mine, Dr. S. Nakayama, Ms. J. Zalipska, Dr. A. K. Ichikawa, Dr. C. Mauger, and other 1KT group members for their supports for the stable data taking and analysis. For the beam monitoring, I was supported by Beam Monitor Group, especially Dr. T. Hasegawa, Dr. T. Maruyama, and Dr. J. Hill. I also thank the SciFi and MRD Groups, particularly Prof. M. Sakuda, Dr. T. Ishii, Dr. T. Ishida, Dr. T. Inagaki, Dr. T. Iwashita, and Ms. H. Yokoyama for their instructions on the system and the analysis. The analysis for Super-Kamiokande data is supported by Prof. K. Kaneyuki, Dr. S. Yamada, and all of Super-Kamiokande collaborators. I express my gratitude for their great works on the detector calibrations and data analysis. The complete works for the data taking have been performed by Prof. T. Kobayashi, Dr. Y. Hayato, Dr. M. Yoshida, and many other people, and here I want to thank them. I also give another thank to Dr. Y. Hayato for his supports in the simulation works. I really thank Mr. H. Maesaka, Mr. M. Hasegawa, and Mr. S. Yamamoto for supporting me in my research as collaborating students from Kyoto University. K2K is made possible by the inventiveness and the diligent efforts of the KEK-PS machine and beam channel groups. I would like to give my special thanks to them for their perfect beam operation. Thanks to all of the kind collaborators, I could spend very precious, fruitful, and happy time in the K2K experiment. For the HARP experiment, which unfortunately could not be included in this thesis, I would like to thank Prof. J. J. Gomez Cadenas who has taken care of me about the data analysis and my stay in CERN as a analysis coordinator and also as a K2K collaborator. I also express my appreciation to Dr. A. Cervera Villanueva who has also co-worked with me on the analysis. I’m very gratitude to Prof. J. Panman and Prof. F. Dydak, spokespersons of HARP experiment, for their kindly inviting us to the experiment. I also want to give thanks to all the other HARP collaborators for their kindness during my stay in HARP. I could have very precious and valuable time and experience there. I am deeply grateful to all the members of my laboratory in Kyoto University; To Prof. N. Sasao for giving me the first educations on the high energy physics. To Prof. H. Sakamoto, Dr. R. Kikuchi, Dr. T. Nomura, and Dr. M. Yokoyama for encouraging me in my research life and giving me great advices. To Mr. Y. Honda, Mr. A. Shima, and Mr. E. Shin’ya as colleagues at my laboratory. To Mr. M. Suehiro, Dr. Y. Takeuchi, Dr. K. Murakami, Dr.