title: Search for Majorana Neutrinos Near the Inverted Mass Hierarchy Region with KamLAND-Zen.
abstract: We present an improved search for neutrinoless double-beta (0νββ) decay of ^{136}Xe in the KamLAND-Zen experiment. Owing to purification of the xenon-loaded liquid scintillator, we achieved a significant reduction of the ^{110m}Ag contaminant identified in previous searches. Combining the results from the first and second phase, we obtain a lower limit for the 0νββ decay half-life of T_{1/2}^{0ν}>1.07×10^{26}  yr at 90% C.L., an almost sixfold improvement over previous limits. Using commonly adopted nuclear matrix element calculations, the corresponding upper limits on the effective Majorana neutrino mass are in the range 61-165 meV. For the most optimistic nuclear matrix elements, this limit reaches the bottom of the quasidegenerate neutrino mass region.
authors: Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan.
authors: A
authors: A
authors: Gando
authors: Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan.
authors: Y
authors: Y
authors: Gando
authors: Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan.
authors: T
authors: T
authors: Hachiya
authors: Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan.
authors: A
authors: A
authors: Hayashi
authors: Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan.
authors: S
authors: S
authors: Hayashida
authors: Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan.
authors: H
authors: H
authors: Ikeda
authors: Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan.|Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study, The University of Tokyo, Kashiwa, Chiba 277-8583, Japan.
authors: K
authors: K
authors: Inoue
authors: Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan.
authors: K
authors: K
authors: Ishidoshiro
authors: Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan.
authors: Y
authors: Y
authors: Karino
authors: Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan.|Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study, The University of Tokyo, Kashiwa, Chiba 277-8583, Japan.
authors: M
authors: M
authors: Koga
authors: Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan.
authors: S
authors: S
authors: Matsuda
authors: Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan.
authors: T
authors: T
authors: Mitsui
authors: Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan.|Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study, The University of Tokyo, Kashiwa, Chiba 277-8583, Japan.
authors: K
authors: K
authors: Nakamura
authors: Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan.
authors: S
authors: S
authors: Obara
authors: Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan.
authors: T
authors: T
authors: Oura
authors: Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan.
authors: H
authors: H
authors: Ozaki
authors: Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan.
authors: I
authors: I
authors: Shimizu
authors: Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan.
authors: Y
authors: Y
authors: Shirahata
authors: Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan.
authors: J
authors: J
authors: Shirai
authors: Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan.
authors: A
authors: A
authors: Suzuki
authors: Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan.
authors: T
authors: T
authors: Takai
authors: Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan.
authors: K
authors: K
authors: Tamae
authors: Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan.
authors: Y
authors: Y
authors: Teraoka
authors: Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan.
authors: K
authors: K
authors: Ueshima
authors: Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan.
authors: H
authors: H
authors: Watanabe
authors: Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study, The University of Tokyo, Kashiwa, Chiba 277-8583, Japan.
authors: A
authors: A
authors: Kozlov
authors: Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study, The University of Tokyo, Kashiwa, Chiba 277-8583, Japan.
authors: Y
authors: Y
authors: Takemoto
authors: Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan.
authors: S
authors: S
authors: Yoshida
authors: Faculty of Integrated Arts and Science, University of Tokushima, Tokushima 770-8502, Japan.
authors: K
authors: K
authors: Fushimi
authors: Physics Department, University of California, Berkeley, and Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.
authors: T I
authors: TI
authors: Banks
authors: Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study, The University of Tokyo, Kashiwa, Chiba 277-8583, Japan.|Physics Department, University of California, Berkeley, and Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.
authors: B E
authors: BE
authors: Berger
authors: Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study, The University of Tokyo, Kashiwa, Chiba 277-8583, Japan.|Physics Department, University of California, Berkeley, and Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.
authors: B K
authors: BK
authors: Fujikawa
authors: Physics Department, University of California, Berkeley, and Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.
authors: T
authors: T
authors: O'Donnell
authors: Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
authors: L A
authors: LA
authors: Winslow
authors: Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study, The University of Tokyo, Kashiwa, Chiba 277-8583, Japan.|Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37996, USA.
authors: Y
authors: Y
authors: Efremenko
authors: Triangle Universities Nuclear Laboratory, Durham, North Carolina 27708, USA; Physics Departments at Duke University, Durham, North Carolina 27708, USA; North Carolina Central University, Durham, North Carolina 27707, USA; and The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, USA.
authors: H J
authors: HJ
authors: Karwowski
authors: Triangle Universities Nuclear Laboratory, Durham, North Carolina 27708, USA; Physics Departments at Duke University, Durham, North Carolina 27708, USA; North Carolina Central University, Durham, North Carolina 27707, USA; and The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, USA.
authors: D M
authors: DM
authors: Markoff
authors: Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study, The University of Tokyo, Kashiwa, Chiba 277-8583, Japan.|Triangle Universities Nuclear Laboratory, Durham, North Carolina 27708, USA; Physics Departments at Duke University, Durham, North Carolina 27708, USA; North Carolina Central University, Durham, North Carolina 27707, USA; and The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, USA.
authors: W
authors: W
authors: Tornow
authors: Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study, The University of Tokyo, Kashiwa, Chiba 277-8583, Japan.|Center for Experimental Nuclear Physics and Astrophysics, University of Washington, Seattle, Washington 98195, USA.
authors: J A
authors: JA
authors: Detwiler
authors: Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study, The University of Tokyo, Kashiwa, Chiba 277-8583, Japan.|Center for Experimental Nuclear Physics and Astrophysics, University of Washington, Seattle, Washington 98195, USA.
authors: S
authors: S
authors: Enomoto
authors: Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study, The University of Tokyo, Kashiwa, Chiba 277-8583, Japan.|Nikhef and the University of Amsterdam, Science Park, Amsterdam 1098XG, The Netherlands.
authors: M P
authors: MP
authors: Decowski