Nuclear observables to constrain neutrinoless double-beta decay*

Observing neutrinoless double-beta (0nbb) decay is the most promising way to detect lepton number violation in the laboratory, and it would imply that neutrinos are its own antiparticle. The decay half-life naturally depends on a nuclear matrix element that needs to be calculated theoretically. A good knowledge of this matrix element is key for the planning of 0nbb decay experiments, and also to extract information on the neutrino mass once 0nbb decay is observed.

At the moment, predicted matrix-element values depend on the nuclear many-body method used to calculate them. These approaches need to be tested against other observables. Useful tests are Gamow-Teller beta decays, and especially two-neutrino double-beta decays, which share initial and final nuclear states with 0nbb decay. In addition, a comparison to muon capture or inelastic neutrino scattering probe the momentum transfer regime relevant for 0nbb decay.  Finally, double charge-exchange experiments, planned at RIKEN and other labs, can be sensitive to double Gamow-Teller transitions, whose matrix elements have been found to be correlated to 0nbb decay matrix elements.