Search for Majorana Neutrinos near the Inverted Mass Hierarchy Region with KamLAND-Zen

TL;DR

An improved search for neutrinoless double-beta (0νββ) decay of ^{136}Xe in the KamLAND-Zen experiment is presented and a significant reduction of the xenon-loaded liquid scintillator contaminant identified in previous searches is achieved.

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 $T_{1/2}^{0ν} > 1.07 \times 10^{26}$ 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 quasi-degenerate neutrino mass region.

Figures (3)

• Figure 1: (a) Vertex distribution of candidate events (black points) and reproduced $^{214}$Bi background events in a MC simulation (color histogram) for $2.3 < E < 2.7\,{\rm MeV}$ (the $0\nu\beta\beta$ window). The normalization of the MC event histogram is arbitrary. The solid and thick dashed lines indicate the shape of the IB and the 1-m-radius spherical volume, respectively. The thin dashed lines illustrate the shape of the equal-volume spherical half-shells which compose the 2-m-radius spherical fiducial volume for the $0\nu\beta\beta$ analysis. (b) An example of the energy spectrum in a volume bin with high $^{214}$Bi background events around the lower part of the IB film (shaded region in (a) at $1.47 < R < 1.53\,{\rm m}, z < 0$). (c) $R^{3}$ vertex distribution of candidate events in the $0\nu\beta\beta$ window. The curves show the best-fit background model components.
• Figure 2: (a) Energy spectrum of selected $\beta\beta$ candidates within a 1-m-radius spherical volume in Period-2 drawn together with best-fit backgrounds, the $2\nu\beta\beta$ decay spectrum, and the 90% C.L. upper limit for $0\nu\beta\beta$ decay. (b), (c) Closeup energy spectra for $2.3 < E < 3.0\,{\rm MeV}$ in Period-1 and Period-2, respectively.
• Figure 3: Effective Majorana neutrino mass $\left<m_{\beta\beta}\right>$ as a function of the lightest neutrino mass $m_{\rm lightest}$. The dark shaded regions are the predictions based on best-fit values of neutrino oscillation parameters for the normal hierarchy (NH) and the inverted hierarchy (IH), and the light shaded regions indicate the $3\sigma$ ranges calculated from the oscillation parameter uncertainties DellOro2014Capozzi2014. The horizontal bands indicate 90% C.L. upper limits on $\left<m_{\beta\beta}\right>$ with $^{136}$Xe from KamLAND-Zen (this work), and with other nuclei from Ref. Agostini2013Alfonso2015Arnold2015Barabash2011b, considering an improved phase space factor calculation Kotila2012Stoica2013 and commonly used NME calculations Rodriguez2010Menendez2009Barea2015Hyvarinen2015Meroni2013Simkovic2013Mustonen2013. The side-panel shows the corresponding limits for each nucleus as a function of the mass number.