Abstract
Effective values of the axial-vector coupling constant have lately attracted much attention due to the prominent role of in determining the half-lives of double decays, in particular their neutrinoless mode. The half-life method, i.e., comparing the calculated half-lives to the corresponding experimental ones, is the most widely used method to access the effective values of . The present paper investigates the possibilities offered by a complementary method: the spectrum-shape method (SSM). In the SSM, comparison of the shapes of the calculated and measured electron spectra of forbidden nonunique decays yields information on the magnitude of . In parallel, we investigate the impact of the next-to-leading-order terms of the -decay shape function and the radiative corrections on the half-life method and the SSM by analyzing the fourfold forbidden decays of and by using three nuclear-structure theory frameworks; namely, the nuclear shell model, the microscopic interacting boson-fermion model, and the microscopic quasiparticle-phonon model. The three models yield a consistent result, , when the SSM is applied to the decay of for which -spectrum data are available. At the same time the half-life method yields results which are in tension with each other and the SSM result.
- Received 10 November 2016
DOI:https://doi.org/10.1103/PhysRevC.95.024327
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