Constraint on 0νββ matrix elements from a novel decay channel of the scissors mode: the case of Gd154

The nucleus Gd154 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 γ-ray scattering experiments and a γγ-coincidence study following electron capture decay of Tbm154. 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→0+2)=0.031(4)μ2N. The properties of the scissors mode of Gd154 imply a much larger matrix element than previously thought for the neutrinoless double-β decay to the 0+2 state in such a shape-transitional region. Theory indicates an even larger effect for Nd150.