Abstract
We present a precision analysis of the ^{136}Xe two-neutrino ββ electron spectrum above 0.8MeV, based on high-statistics data obtained with the KamLAND-Zen experiment. An improved formalism for the two-neutrino ββ rate allows us to measure the ratio of the leading and subleading 2νββ nuclear matrix elements (NMEs), ξ_{31}^{2ν}=-0.26_{-0.25}^{+0.31}. Theoretical predictions from the nuclear shell model and the majority of the quasiparticle random-phase approximation (QRPA) calculations are consistent with the experimental limit. However, part of the ξ_{31}^{2ν} range allowed by the QRPA is excluded by the present measurement at the 90%confidence level. Our analysis reveals that predicted ξ_{31}^{2ν} values are sensitive to the quenching of NMEs and the competing contributions from low- and high-energy states in the intermediate nucleus. Because these aspects are also at play in neutrinoless ββ decay, ξ_{31}^{2ν} provides new insights toward reliable neutrinoless ββ NMEs.
Highlights
We present a precision analysis of the 136Xe two-neutrino ββ electron spectrum above 0.8 MeV, based on highstatistics data obtained with the KamLAND-Zen experiment
Theoretical predictions from the nuclear shell model and the majority of the quasiparticle random-phase approximation (QRPA) calculations are consistent with the experimental limit
Our analysis reveals that predicted ξ231ν values are sensitive to the quenching of nuclear matrix elements (NMEs) and the competing contributions from low- and high-energy states in the intermediate nucleus
Summary
We present a precision analysis of the 136Xe two-neutrino ββ electron spectrum above 0.8 MeV, based on highstatistics data obtained with the KamLAND-Zen experiment. The nuclear shell model and QRPA typically reproduce experimental 2νββ rates and predict nonmeasured ones [17,18,31,32,33].
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