Forbidden nonuniqueβdecays and effective values of weak coupling constants

Abstract

Forbidden nonunique β decays and effective values of weak coupling constantsHaaranen, Mikko; Srivastava, P. C.; Suhonen, Jouni Haaranen, M., Srivastava, P. C., & Suhonen, J. (2016 Forbidden nonunique β decays feature shape functions that are complicated combinations of different nuclear matrix elements and phase-space factors. Furthermore, they depend in a very nontrivial way on the values of the weak coupling constants, g V for the vector part and g A for the axial-vector part. In this work we include also the usually omitted second-order terms in the shape functions to see their effect on the computed decay half-lives and electron spectra (β spectra). As examples we study the fourth-forbidden nonunique ground-state-to-ground-state β − decay branches of 113 Cd and 115 In using the microscopic quasiparticle-phonon model and the nuclear shell model. A striking new feature that is reported in this paper is that the calculated shape of the β spectrum is quite sensitive to the values of g V and g A and hence comparison of the calculated with the measured spectrum shape opens a way to determine the values of these coupling constants. This article is designed to show the power of this comparison, coined spectrum-shape method (SSM), by studying the two exemplary β transitions within two different nuclear-structure frameworks. While the SSM seems to confine the g V values close to the canonical value g V = 1.0, the values of g A extracted from the half-life data and by the SSM emerge contradictory in the present calculations. This calls for improved nuclear-structure calculations and more measured data to systematically employ SSM for determination of the effective value of g A in the future.

Electron spectra in forbiddenβdecays and the quenching of the weak axial-vector coupling constantgA

“…[17]) an interesting new feature was found regarding the weak coupling constants g V and g A . The shape of the electron spectra of the fourth-forbidden nonunique ground-state-to-ground-state β − decays of 113 Cd and 115 In depend strongly on the value of g V and g A due to the complicated shape factor featuring vector, axial-vector, and mixed vector-axial-vector parts containing a number of different NMEs and phase-space factors.…”

“…The shapes of the theoretical and experimental spectra can be compared to find the effective values of the coupling constants in a new method called the spectrum-shape method (SSM). The study [17] used two nuclear models; namely, the MQPM [18,19] and the nuclear shell model (NSM) [20][21][22]. When the computed electron spectra of 113 Cd were compared with the experimental spectrum of Ref.…”

“…As in Refs. [17] and [24] we take into account the next-to-leading-order terms in the shape factor, and so the spectra of unique-forbidden decays also depend on the axial-vector coupling constant in a nontrivial way. Based on the conclusions of Ref.…”

“…Based on the conclusions of Ref. [17], we adopt the CVC value g V = 1.00 in the calculations. In the analyzes we use the normalized electron spectra (the normalized spectra are also used when comparing with available experimental data) for which the integrated area under the spectrum curve is unity.…”

Gamow-Teller response in the configuration space of a density-functional-theory–rooted no-core configuration-interaction model

“…This study paves a way for more systematic studies of nuclear β-decay rates, for exploring forbidden β decays, and for tackling the double-β-decay process within the DFT-NCCI framework. With forbidden β decays, the spectrum-shape method may offer valuable hints for the g A quenching puzzle [52]. Although the correspondence to experimental results was generally found to be rather good, the underlying effective interaction used to construct the EDF has its limitations.…”