Constraint on0νββMatrix Elements from a Novel Decay Channel of the Scissors Mode: The Case ofGd154

The nucleus $^{154}\mathrm{Gd}$ is located in a region of the nuclear chart where rapid changes of nuclear deformation occur as a function of particle number. It was investigated using a combination of $\ensuremath{\gamma}$-ray scattering experiments and a $\ensuremath{\gamma}\ensuremath{\gamma}$-coincidence study following electron capture decay of $^{154}\mathrm{Tb}^{m}$. A novel decay channel from the scissors mode to the first excited ${0}^{+}$ state was observed. Its transition strength was determined to $B(M1;{1}_{sc}^{+}\ensuremath{\rightarrow}{0}_{2}^{+})=0.031(4){\ensuremath{\mu}}_{N}^{2}$. The properties of the scissors mode of $^{154}\mathrm{Gd}$ imply a much larger matrix element than previously thought for the neutrinoless double-$\ensuremath{\beta}$ decay to the ${0}_{2}^{+}$ state in such a shape-transitional region. Theory indicates an even larger effect for $^{150}\mathrm{Nd}$.