Journal
PHYSICAL REVIEW LETTERS
Volume 122, Issue 19, Pages -Publisher
AMER PHYSICAL SOC
DOI: 10.1103/PhysRevLett.122.192501
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Funding
- JSPS KAKENHI [21000001, 26104002, 18K03639]
- World Premier International Research Center Initiative (WPI Initiative), MEXT, Japan
- Netherlands Organisation for Scientific Research
- U.S. Department of Energy (DOE) [DE-AC02-05CH11231]
- DOE
- NSF
- NII
- JICFuS
- CNS-RIKEN joint project for large-scale nuclear structure calculations
- U.S. DOE [DE-FG02-00ER41132]
- Slovak Research and Development Agency [APVV-14-0524]
- European Regional Development Fund-Project Engineering applications of microworld physics [CZ.02.1.01/0.0/0.0/16_019/0000766]
- MEXT
- Grants-in-Aid for Scientific Research [18K03639, 26104002] Funding Source: KAKEN
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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.
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