Precision Analysis of the Xe-136 Two-Neutrino beta beta Spectrum in KamLAND-Zen and Its Impact on the Quenching of Nuclear Matrix Elements

AbstraktAbstract

We present a precision analysis of the Xe-136 two-neutrino beta beta electron spectrum above 0.8 MeV, based on high-statistics data obtained with the KamLAND-Zen experiment. An improved formalism for the two-neutrino beta beta rate allows us to measure the ratio of the leading and subleading 2 nu beta beta nuclear matrix elements (NMEs), xi(2 nu)(31) = -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 xi(2 nu)(31) range allowed by the QRPA is excluded by the present measurement at the 90% confidence level. Our analysis reveals that predicted xi(2 nu)(31) 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 beta beta decay, xi(2 nu)(31) provides new insights toward reliable neutrinoless beta beta NMEs.

We present a precision analysis of the Xe-136 two-neutrino beta beta electron spectrum above 0.8 MeV, based on high-statistics data obtained with the KamLAND-Zen experiment. An improved formalism for the two-neutrino beta beta rate allows us to measure the ratio of the leading and subleading 2 nu beta beta nuclear matrix elements (NMEs), xi(2 nu)(31) = -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 xi(2 nu)(31) range allowed by the QRPA is excluded by the present measurement at the 90% confidence level. Our analysis reveals that predicted xi(2 nu)(31) 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 beta beta decay, xi(2 nu)(31) provides new insights toward reliable neutrinoless beta beta NMEs.