Confirmation of gA quenching using the revised spectrum-shape method for the analysis of the 113Cd β-decay as measured with the COBRA demonstrator

In this article we present an updated spectrum-shape analysis of the Cd-113 fourfold forbidden non unique beta-decay transition in order to address the quenching of the weak axial-vector coupling g(A) in low-momentum exchange nuclear processes. The experimental data were collected in a dedicated low threshold run with the COBRA demonstrator at the LNGS and resulted in 44 individual Cd-113 spectra. These data are evaluated in the context of three nuclear model frameworks based on a revised version of the spectrum-shape method and the conserved vector current hypothesis. The novel idea devised in the present work is to fit the value of the small relativistic nuclear matrix element (s-NME) driving the nuclear model calculations, which remained essentially as a free parameter in previous studies. This is done by tuning the nuclear structure calculations and making use of the interplay of g(A) and the sNME such that the experimentally known Cd-113 half-life gets reproducible by the different frameworks. In this way, a best fit s-NME value can be derived for each of the considered nuclear models, which finally enters the template calculations used to perform the spectrum-shape analysis for each of the obtained Cd-113 spectra. The primary analysis strategy results in significantly quenched values of the axial vector coupling for all three nuclear models: (g) over bar (A)(ISM) = 0.907 +/- 0.064, (g) over bar (A)(MQPM) = 0.993 +/- 0.063 and (g) over bar (A)(IBFM-2) = 0.828 +/- 0.140. Moreover, with our data-driven approach one of the main shortcomings of the spectrum-shape method has been resolved. This achievement is a milestone in the description of strongly forbidden beta-decays and adds to the indications for the existence of a quenching of g(A) in low-momentum exchange nuclear processes. (C) 2021 The Authors. Published by Elsevier B.V.

Automated summary

This study presents an updated spectrum-shape analysis of the Cd-113 fourfold forbidden beta-decay transition to investigate the quenching of the weak axial-vector coupling g(A) in low-momentum exchange nuclear processes. By tuning the nuclear structure calculations and leveraging the interplay of g(A) and the s-NME, reproducible Cd-113 half-life values were obtained for three different nuclear models. The analysis revealed significantly quenched values of the axial vector coupling and resolved a main shortcoming of the spectrum-shape method, providing insights into the existence of a quenching of g(A) in low-momentum exchange nuclear processes.