Limits on Majoron-Emitting Double-Beta Decays of 136Xe in KamLAND-Zen

Limits on Majoron-Emitting Double-Beta Decays 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 (KamLAND-Zen Collaboration) Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan Kavli Institute for the Physics and Mathematics of the Universe (WPI), University of Tokyo, Kashiwa, 277-8583, 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: May 30, 2012) We present limits on Majoron-emitting neutrinoless double-beta decay modes based on an exposure of 112.3 days with 125 kg of 136 Xe. In particular, a lower limit on the ordinary (spectral index n = 1) Majoron- emitting decay half-life of 136 Xe is obtained as T 1/2 > 2.6 × 10 24 yr at 90% C.L., a factor of five more stringent than previous limits. The corresponding upper limit on the effective Majoron-neutrino coupling, using a range of available nuclear matrix calculations, is g ee < (0.8 − 1.6) × 10 −5 . This excludes a previously unconstrained region of parameter space and strongly limits the possible contribution of ordinary Majoron emis- sion modes to 0νββ decay for neutrino masses in the inverted hierarchy scheme. PACS numbers: 23.40.−s, 21.10.Tg, 14.60.Pq, 27.60.+j 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. The search for neutrinoless double-beta (0νββ) decay is the best probe of the Majorana nature of the neutrino known at present. The observation of this process would immediately imply total lepton number violation and the equivalence of the neutrino and the antineutrino, irrespective of the mech- anism by which the decay is mediated [1]. Although most current experimental efforts focus on the detection of 0νββ decay mediated by light Majorana neutrino exchange, many other mechanisms have been proposed. Some exotic mod- els [2, 3] predict decays proceeding through the emission of massless Nambu-Goldstone (NG) bosons, referred to as Ma- jorons. Precise measurements of the invisible decay width of the Z boson in LEP showed that traditional Majoron models require severe fine-tuning [4]. However, a number of addi- tional models have been proposed which avoid such fine tun- ing, including modes in which the Majoron can carry leptonic charge, and need not be a NG boson [5], or in which 0νββ decay proceeds through the emission of two Majorons [6]. These models predict different shapes for the spectrum of the summed energy of the two emitted β’s. In this report we an- alyze the spectrum obtained from a 38.6 kg-year exposure of Xe with KamLAND-Zen [7] to derive new limits on sev- eral of these decay modes. Table I summarizes ten Majoron models [6, 8–10], which can be divided into two categories: (I) lepton number violat- ing models, and (II) lepton number conserving models. The table indicates also whether the corresponding 0νββ decay is accompanied by the emission of one or two Majorons: (A, Z) → (A, Z + 2) + 2e − + χ 0 , (A, Z) → (A, Z + 2) + 2e − + 2χ 0 . The main distinguishing features of the models are listed in the third, fourth, and fifth columns: whether the Majoron is a NG boson or not, its leptonic charge (L), and the model’s spectral index (n), respectively. The spectral index is de-