Precision Analysis of the 136Xe Two-Neutrino ββ Spectrum in KamLAND-Zen and Its Impact on the Quenching of Nuclear Matrix Elements
Abstract
We present a precision analysis of the 136Xe two-neutrino ββ electron spectrum above 0.8 MeV, 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 2ββ nuclear matrix elements (NMEs), 231 = -0.26+0.31-0.25. 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 231 range allowed by the QRPA is excluded by the present measurement at the 90% confidence level. Our analysis reveals that predicted 231 values are sensitive to the quenching of NMEs and the competing contributions from low- and high-energy states in the intermediate nucleus. Since these aspects are also at play in neutrinoless ββ decay, 231 provides new insights towards reliable neutrinoless ββ NMEs.
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