Measurement of the Double-Beta Decay Half-life of 136Xe in KamLAND-Zen
2013; American Physical Society; Linguagem: Inglês
ISSN
1092-0145
Autores Tópico(s)Particle physics theoretical and experimental studies
ResumoMeasurement of the Double-Beta Decay Half-life of 136 Xe in KamLAND-Zen A. Gando, 1 Y. Gando, 1 H. Hanakago, 1 H. Ikeda, 1 K. Inoue, 1, 2 R. Kato, 1 M. Koga, 1, 2 S. Matsuda, 1 T. Mitsui, 1 T. Nakada, 1 K. Nakamura, 1, 2 A. Obata, 1 A. Oki, 1 Y. Ono, 1 I. Shimizu, 1 J. Shirai, 1 A. Suzuki, 1 Y. Takemoto, 1 K. Tamae, 1 K. Ueshima, 1 H. Watanabe, 1 B.D. Xu, 1 S. Yamada, 1 H. Yoshida, 1 A. Kozlov, 2 S. Yoshida, 3 T.I. Banks, 4 J.A. Detwiler, 4 S.J. Freedman, 2, 4 B.K. Fujikawa, 2, 4 K. Han, 4 T. O’Donnell, 4 B.E. Berger, 5 Y. Efremenko, 2, 6 H.J. Karwowski, 7 D.M. Markoff, 7 W. Tornow, 7 S. Enomoto, 2, 8 and M.P. Decowski 2, 9 (The KamLAND-Zen Collaboration) Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan Institute for the Physics and Mathematics of the Universe, Tokyo University, Kashiwa 277-8568, Japan Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan Physics Department, University of California, Berkeley, and Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA Department of Physics, Colorado State University, Fort Collins, Colorado 80523, 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 Central University, and the University of North Carolina at Chapel Hill Center for Experimental Nuclear Physics and Astrophysics, University of Washington, Seattle, Washington 98195, USA Nikhef and the University of Amsterdam, Science Park, Amsterdam, the Netherlands (Dated: January 31, 2012) We present results from the KamLAND-Zen double-beta decay experiment based on an exposure of 77.6 days with 129 kg of 136 Xe. The measured two-neutrino double-beta decay half-life of 136 Xe is T 1/2 0.02(stat) ± 0.14(syst) × 10 yr, consistent with a recent measurement by EXO-200. We also obtain a lower limit for the neutrinoless double-beta decay half-life, T 1/2 > 5.7 × 10 24 yr at 90% C.L. PACS numbers: 23.40.-s, 21.10.Tg, 14.60.Pq DISCLAIMER: This document was prepared as an ac- count of work sponsored by the United States Govern- ment. While this document is believed to contain correct information, neither the United States Government nor any agency thereof, nor the Regents of the University of California, nor any of their employees, makes any war- ranty, express or implied, or assumes any legal responsi- bility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial prod- uct, process, or service by its trade name, trademark, manufacturer, or otherwise, does not necessarily consti- tute or imply its endorsement, recommendation, or fa- voring by the United States Government or any agency thereof, or the Regents of the University of California. The views and opinions of authors expressed herein do not nec- essarily state or reflect those of the United States Govern- ment or any agency thereof or the Regents of the Univer- sity of California. Determining m ββ from a 0νββ decay half-life requires knowledge of the decay’s phase-space factor (G 0ν ) and nu- clear matrix element (M 0ν ). G 0ν can be calculated ex- actly, but to date all estimations of M 0ν must rely on model- based approximations possessing difficult-to-quantify uncer- tainties. The two-neutrino double-beta (2νββ) decay half- life, if known, can be used to constrain some relevant model parameters, reducing some sources of uncertainty [3, 4]. The first direct measurement of the 2νββ decay half-life of Xe, recently reported by EXO-200 [5], was significantly below previously published lower limits [6, 7]. This Letter on the first results from the KamLAND-Zen (KamLAND Zero- Neutrino Double-Beta Decay) experiment reports a new mea- surement of the 136 Xe 2νββ decay half-life, as well as im- proved limits on 0νββ mode. The data presented were col- lected between October 12, 2011, and January 2, 2012. Majorana neutrinos are a natural feature of many high- energy theoretical models. However, the only viable experi- mental probe of this property at present is neutrinoless double- beta (0νββ) decay. Observation of this lepton-number violat- ing nuclear process would definitively establish the Majorana nature of the neutrino, and would be a profound discovery [1]. In addition, since the rate of this process increases with the square of the effective neutrino mass m ββ ≡ Σ i U ei m ν i , its measurement would provide information on the absolute neutrino mass scale. Searches for 0νββ decay have been in- KamLAND-Zen (Fig. 1) is a modification of the exist- ing KamLAND detector carried out in the summer of 2011. The ββ source/detector is 13 tons of Xe-loaded liquid scin- tillator (Xe-LS) contained in a 3.08-m-diameter spherical in- ner balloon (IB). The IB is constructed from 25-µm-thick transparent nylon film and is suspended at the center of the KamLAND detector [8] by 12 film straps of the same material. The IB is surrounded by 1 kton of liquid scintillator (LS) con- tained in a 13-m-diameter spherical outer balloon (OB) made of 135-µm-thick nylon/EVOH (ethylene vinyl alcohol copoly- vigorated by recent measurements of neutrino mass splittings by oscillation experiments, which require at least one neutrino mass above ∼50 meV [2]. This scale is within the reach of present-day efforts.
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