Search For Light Exotic Fermions In Double-Beta Decays

The Standard Model of Particle Physics predicts the double-beta decay of certain nuclei with the emission of two active neutrinos. In this letter, we argue that double-beta decay experiments could be used to probe models with light exotic fermions through the search for spectral distortions in the electron spectrum with respect to the Standard Model expectations. We consider two concrete examples: models with light sterile neutrinos, singly produced in the double-beta decay, and models with a light Z(2)-odd fermion, pair produced due to a Z(2) symmetry. We estimate the discovery potential of a selection of double-beta decay experiments and find that future searches will test for the first time a new part of the parameter space of interest at the MeV-mass scale. (C) 2021 The Author(s). Published by Elsevier B.V.