Search for light exotic fermions in double-beta decays

Matteo Agostini,Elisabetta Bossio,Alejandro Ibarra,Xabier Marcano

doi:10.1016/j.physletb.2021.136127

Matteo Agostini, Elisabetta Bossio + Show 2 more

Open Access

https://doi.org/10.1016/j.physletb.2021.136127

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Abstract

The Standard Model of Particle Physics predicts the double-β decay of certain nuclei with the emission of two active neutrinos. In this letter, we argue that double-β 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-β decay, and models with a light Z2-odd fermion, pair produced due to a Z2 symmetry. We estimate the discovery potential of a selection of double-β 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.

Highlights

Many models of new physics contain new spin 1/2 particles, singlets under the Standard Model gauge group, possibly related to the mechanism of neutrino mass generation, or to the dark matter of the Universe
We argue that double-β 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
In a variant of this scenario, the singlet fermion is furnished with a Z2 symmetry so it can only be produced in pairs

Summary

INTRODUCTION

Many models of new physics contain new spin 1/2 particles, singlets under the Standard Model gauge group, possibly related to the mechanism of neutrino mass generation, or to the dark matter of the Universe. This opens the possibility to observe a double-β decay final state in which a standard neutrino is replaced by a sterile neutrino (νN ββ decay): (A, Z) → (A, Z + 2) + 2e + ν + N , with a modified spectrum due to the sterile neutrino mass This is the same principle as the one used in kink searches with single-β decays [11,12,13,14,15], which currently provide the strongest laboratory bounds in our mass range of interest [2, 3, 16, 17]. We will derive sensitivity projections for the most promising 0νββ experiments, assuming the systematic uncertainties and experimental performance achieved by running experiments

DOUBLE-β DECAY INTO STERILE NEUTRINOS

DOUBLE-β DECAY INTO Z2-ODD SINGLET FERMIONS

DISCOVERY POTENTIAL OF FUTURE 0νββ DECAY EXPERIMENTS

RESULTS AND OUTLOOK