Abstract
We present a precision analysis of the two-neutrino electron spectrum above 0.8 MeV, based on high-statistics data obtained with the KamLAND-Zen experiment. An improved formalism for the two-neutrino rate allows us to measure the ratio of the leading and subleading nuclear matrix elements (NMEs), . 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 range allowed by the QRPA is excluded by the present measurement at the 90% confidence level. Our analysis reveals that predicted 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 decay, provides new insights toward reliable neutrinoless NMEs.
- Received 13 January 2019
- Revised 11 March 2019
DOI:https://doi.org/10.1103/PhysRevLett.122.192501
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.
Published by the American Physical Society