Search for Light Exotic Fermions in Double-Beta Decays

TL;DR

This paper explores how double-beta decay experiments can be used to detect light exotic fermions, such as sterile neutrinos and Z2-odd fermions, by analyzing spectral distortions in electron spectra, opening new avenues for particle physics research.

Contribution

It proposes using double-beta decay experiments to search for light exotic fermions, extending the experimental reach to MeV-mass scale particles beyond the Standard Model.

Findings

Future experiments can probe new parameter space for light sterile neutrinos.

Spectral distortions in electron spectra can indicate the presence of exotic fermions.

Potential to discover or constrain light exotic fermions at MeV-mass scale.

Abstract

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.