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

Abstract In this article we present an updated spectrum-shape analysis of the 113Cd fourfold forbidden non-unique β-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 113Cd 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 s-NME such that the experimentally known 113Cd 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 113Cd spectra. The primary analysis strategy results in significantly quenched values of the axial-vector coupling for all three nuclear models: g ‾ A ( ISM ) = 0.907 ± 0.064 , g ‾ A ( MQPM ) = 0.993 ± 0.063 and g ‾ 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 β-decays and adds to the indications for the existence of a quenching of g A in low-momentum exchange nuclear processes.