Shape coexistence and the search for neutrinoless double-beta decay: A case study of 72,76Ge

Abstract:  Observation of neutrinoless double-beta decay would both demonstrate the Majorana nature of the neutrino and provide experimental access to its absolute mass scale. Over the last decade, wavefunction contributions for leading neutrinoless double-beta decay candidates have been probed in a campaign of experiments utilizing transfer reactions to determine nucleon occupancies in a consistent way. While these studies have provided a great deal of information for comparison with theory, especially on contributions to the nuclear wavefunctions from competing orbitals, they lack sensitivity to the collective and shape degrees of freedom which have been shown to be an integral part of the structure of parent-daughter nuclei relevant to neutrinoless double-beta decay. In this talk, I will present results of highprecision Coulomb excitation measurements aimed at studying the various collective-shape degrees of freedom and associated phenomena. The talk will focus primarily on the electromagnetic properties of low-lying states in 72,76Ge which were investigated via multistep Coulomb excitation using the advanced gamma-ray tracking array, GRETINA and the charged particle detector, CHICO2. The influence of axial asymmetry parameter on the shape of these nuclei along with the results of multi-configuration mixing calculations carried out within the framework of the triaxial rotor model will highlighted. Most importantly, the results on 76Ge will be compared with state-of-the-art shell model calculations and recently obtained (n,n'[gamma]) data, with emphasis on demonstrating the importance of nuclear deformation in determining the nuclear decay matrix elements.