UNC-CH Physics and Astronomy Colloquium

Akaa Daniel Ayangeakaa, United States Naval Academy

“Nuclear Structure and the search for neutrinoless doublebeta decay (0νββ): A case study of ^72,76Ge”

The observation of neutrinoless double-beta decay (0νββ) would simultaneously demonstrate the Majorana nature of the neutrino and provide experimental access to its absolute mass scale. Over the last decade, wavefunction contributions for leading (0νββ) 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 are shown to be an integral component of the structure of parent-daughter nuclei relevant to 0νββ. In this talk, I will present results from high-precision 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 the axial asymmetry parameter on the shape of these nuclei along with the results of multiconfiguration mixing calculations carried out within the framework of the triaxial rotor model will be highlighted. Most importantly, new experimental evidence characterizing the precise nature of triaxial deformation in ⁷⁶Ge will be presented. The results will also be compared with state-of-the-art shell model calculations and recently obtained (n,n’γ) data, with emphasis on demonstrating the importance of nuclear deformation in determining the nuclear decay matrix elements relevant to neutrinoless double-beta decay (0νββ).