An extensive, model-independent analysis of the nature of triaxial deformation in ${}^{76}\mathrm{Ge}$, a candidate for neutrinoless double-beta ($0\nu \beta \beta$) decay, was carried out following multistep Coulomb excitation. Shape parameters deduced on the basis of a rotational-invariant sum-rule analysis provided considerable insight into the underlying collectivity of the ground-state and $\gamma$ bands. Both sequences were determined to be characterized by the same $\beta$ and $\gamma$ deformation parameter values. In addition, compelling evidence for low-spin, rigid triaxial deformation in ${}^{76}\mathrm{Ge}$ was obtained for the first time from the analysis of the statistical fluctuations of the quadrupole asymmetry deduced from the measured $E2$ matrix elements. These newly determined shape parameters are important input and constraints for calculations aimed at providing, with suitable accuracy, the nuclear matrix elements relevant to $0\nu \beta \beta$.

• Received 14 June 2019

DOI:https://doi.org/10.1103/PhysRevLett.123.102501