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

Abstract

Electron spectra in forbidden β decays and the quenching of the weak axial-vector coupling constant gA Kostensalo, Joel; Haaranen, Mikko; Suhonen, Jouni Kostensalo, J., Haaranen, M., & Suhonen, J. (2017). Electron spectra in forbidden β decays and the quenching of the weak axial-vector coupling constant gA. Physical Review C, 95 (4) Evolution of the electron spectra with the effective value of the weak axial-vector coupling constant g A was followed for 26 first-, second-, third-, fourth-and fifth-forbidden β − decays of odd-A nuclei by calculating the involved nuclear matrix elements (NMEs) in the framework of the microscopic quasiparticle-phonon model (MQPM). The next-to-leading-order terms were included in the β-decay shape factor of the electron spectra. The spectrum shapes of third-and fourth-forbidden nonunique decays were found to depend strongly on the value of g A , while first-and second-forbidden decays were mostly unaffected by the tuning of g A . The g A -driven evolution of the normalized β spectra was found to be quite universal, largely insensitive to the small changes in the nuclear mean field and the adopted residual many-body Hamiltonian producing the excitation spectra of the MQPM. This makes the comparison of experimental and theoretical electron spectra, coined "the spectrum-shape method" (SSM), a robust tool for extracting information on the effective values of the weak coupling constants. In this exploratory work two new experimentally interesting decays for the SSM treatment were discovered: the ground-state-to-ground-state decays of 99 Tc and 87 Rb. Comparing the experimental and theoretical spectra of these decays could shed light on the effective values of g A and g V for second-and third-forbidden nonunique decays. The measurable decay transitions of 135 Cs and 137 Cs, in turn, can be used to test the SSM in different many-body formalisms. The present work can also be considered as a (modest) step towards solving the g A problem of the neutrinoless double beta decay.

gA -driven shapes of electron spectra of forbidden β decays in the nuclear shell model

“…The only exception is the undetected fourth-forbidden β − decay branch of 50 V, for which the branching ratio has been predicted to be ≈2% by a recent shell-model calculation [36]. An earlier study [30] of the g A -driven evolution of the electron spectra of odd-A nuclei demonstrated that the second and especially the fourth-forbidden nonunique β-decay spectra depend strongly on the effective value of the weak axial-vector coupling constant. [36,45,46], so the study of these decays is somewhat challenging experimentally.…”

“…[32,33]. As in the previous SSM studies [20,21,30] we go beyond the earlier studies [32,33] and take into account the next-to-leading-order terms of the shape function. The details of the next-to-leading order shape factors are discussed in [21].…”

“…In Ref. [30] it was noticed that the shape of the spectrum was largely unaffected by modest changes in the MQPM wave function, even though the predicted half-life was affected significantly. The observations of these previous studies suggest that the SSM is very robust, quite insensitive to the details of the adopted nuclear mean field and Hamiltonian.…”

“…[30]. Only four good candidates for the application of the spectrum-shape method have been found thus far.…”

“…Only four good candidates for the application of the spectrum-shape method have been found thus far. These are the ground-state-to-ground-state decays of 87 Rb, 99 Tc, 113 Cd, and 115 In [20,30]. In the present paper we set out to find potential new candidates for the application of the SSM in light to medium-heavy nuclei using the nuclear shell model.…”