Triaxial nuclear shapes from simple ratios of electric-quadrupole matrix elements

Abstract

Theoretical models often invoke triaxial nuclear shapes to explain elusive collective phenomena, but such assumptions are usually difficult to confirm experimentally. The only direct measurements of the nuclear axial asymmetry $γ$ is based on rotational invariants of zero-coupled products of the electric-quadrupole (E2) operator, the Kumar-Cline sum rule analysis, which generally require knowledge of a large number of E2 matrix elements connecting the state of interest. We propose an alternative assumptions-free method to determine $γ$ of even-even rotating nuclei using only two E2 matrix elements, which are among the easiest to measure. This approach is based on a simple description of nuclear rotation, where the underlying assumptions of the Davydov-Filippov model are either empirically proven or unnecessary. The $γ$ values extracted here are found in agreement with the values deduced from Kumar-Cline sum rules measurements (where available), providing further evidence that the proposed approach represents a reliable, model-independent deduction of $γ$. The technique was applied to more than 60 deformed even-even rotating nuclei and the results indicate that rotating nuclei generally exhibit well-defined stable axially-asymmetric shapes.