Neutrinoless double-beta decay and double-electron capture

The fundamental importance of searching for neutrinoless double-beta decay is widely recognized. Observation of the decay would tell us that the total lepton number is not conserved and that, consequently, neutrinos are massive Majorana fermions. The same statement could be made in the case of observing neutrinoless double-electron capture. We address the question of the sensitivity of the 0 ν ε ε decay to the effective mass of the Majorana neutrino. According to our estimates, in the case of 152Gd and 164Er the sensitivity can be comparable to the favored 0 ν β β decays of nuclei. The main uncertainty in the prediction of half-lives of the 0 ν ε ε decay stems from the lack of sufficient precision in measuring the mass difference between the parent and daughter atoms. More accurate measurements can be accomplished using the modern high-precision ion traps.