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Limits on Majoron-emitting double-beta decays of Xe-136 in the KamLAND-Zen experiment

KamLAND-Zen Collaboration, :, A. Gando, Y. Gando, H. Hanakago, H. Ikeda, K. Inoue, R. Kato, M. Koga, S. Matsuda, T. Mitsui, T. Nakada, K. Nakamura, A. Obata, A. Oki, Y. Ono, I. Shimizu, J. Shirai, A. Suzuki, Y. Takemoto, K. Tamae, K. Ueshima, H. Watanabe, B. D. Xu, S. Yamada, H. Yoshida, A. Kozlov, S. Yoshida, T. I. Banks, J. A. Detwiler, S. J. Freedman, B. K. Fujikawa, K. Han, T. O'Donnell, B. E. Berger, Y. Efremenko, H. J. Karwowski, D. M. Markoff, W. Tornow, S. Enomoto, M. P. Decowski

Abstract

We present limits on Majoron-emitting neutrinoless double-beta decay modes based on an exposure of 112.3 days with 125 kg of Xe-136. In particular, a lower limit on the ordinary (spectral index n = 1) Majoron-emitting decay half-life of Xe-136 is obtained as T_{1/2}^{0νχ^{0}} > 2.6 x 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) x 10^{-5}. This excludes a previously unconstrained region of parameter space and strongly limits the possible contribution of ordinary Majoron emission modes to 0νββdecay for neutrino masses in the inverted hierarchy scheme.

Limits on Majoron-emitting double-beta decays of Xe-136 in the KamLAND-Zen experiment

Abstract

We present limits on Majoron-emitting neutrinoless double-beta decay modes based on an exposure of 112.3 days with 125 kg of Xe-136. In particular, a lower limit on the ordinary (spectral index n = 1) Majoron-emitting decay half-life of Xe-136 is obtained as T_{1/2}^{0νχ^{0}} > 2.6 x 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) x 10^{-5}. This excludes a previously unconstrained region of parameter space and strongly limits the possible contribution of ordinary Majoron emission modes to 0νββdecay for neutrino masses in the inverted hierarchy scheme.

Paper Structure

This paper contains 2 equations, 3 figures, 2 tables.

Figures (3)

  • Figure 1: KamLAND-Zen visible energy spectra for different $^{136}$Xe decay modes, characterized by the spectral index $n$. The resolution-limited line [$\sigma$ = ($6.6 \pm 0.3)\%/\sqrt{E({\rm MeV})}$] at the $Q$ value indicates the $0\nu\beta\beta$ decay peak without Majoron emission.
  • Figure 2: Energy spectrum of selected $\beta\beta$ decay candidates (data points) together with the best-fit backgrounds (gray dashed line) and $2\nu\beta\beta$ decay (purple solid line), and the 90% C.L. upper limit for $0\nu\beta\beta$ decay and Majoron-emitting $0\nu\beta\beta$ decays for each spectral index. The red line depicts the sum of the $2\nu\beta\beta$ decay and background spectra. Numerical results are reported in Table \ref{['table:limit']}. The best-fit has a $\chi^{2}$/d.o.f. = 100.4/87 for the full fit range $0.5 < E < 4.8~{\rm MeV}$.
  • Figure 3: Event rate variation in the energy regions (a) $1.2 < E < 2.0~{\rm MeV}$ ($2\nu\beta\beta$ window) and (b) $2.2 < E < 3.0~{\rm MeV}$ ($0\nu\beta\beta$ window). The fitted curves correspond to the expected variations for the hypotheses that all the events in the $0\nu\beta\beta$ window are solely from one of the background candidates, $^{\rm 110}$Ag$^{m}$ (solid line), $^{208}$Bi (dotted line), or unsupported $^{88}$Y (dot-dashed line).