HNL mass degeneracy: implications for low-scale seesaws, LNV at colliders and leptogenesis

TL;DR

This paper investigates how small mass splittings in low-scale seesaw models influence lepton number violation, collider signals, and leptogenesis, offering insights to distinguish between different seesaw variants.

Contribution

It analyzes the parameter space of low-scale seesaw mechanisms, highlighting how mass splittings affect phenomenology and can differentiate between inverse and linear seesaw models.

Findings

Small mass splittings can lead to observable lepton-number-violating signals.

Degenerate states may enhance low-scale leptogenesis and dark matter production.

Different seesaw variants exhibit distinct phenomenological signatures.

Abstract

Low-scale seesaw variants protected by lepton number symmetry provide a natural explanation of the smallness of neutrino masses but, unlike their higher-scale counterparts, with potentially testable phenomenology. The approximate lepton number symmetry arranges the heavy neutrinos in pseudo-Dirac pairs, which might be accessible at collider or even beam dump experiments if their mass is low enough and their mixing with the active neutrinos sufficiently large. Despite their pseudo-Dirac nature, their small mass splittings may lead to oscillations that prevent the cancellation of their potential lepton-number-violating signals. Interestingly, these small splittings may also resonantly enhance the production of a lepton number asymmetry for low-scale leptogenesis scenarios or, for extremely degenerate states, lead to an asymmetry large enough to resonantly produce a keV sterile neutrino dark matter candidate with the correct relic abundance via the Shi-Fuller mechanism. In this work we explore the parameter space of the different low-scale seesaw mechanisms and study the size of these splittings, given their important and interesting phenomenological consequences. While all low-scale seesaw variants share the same dimension 5 and 6 operators when integrating out the heavy states, we point out that the mass splitting of the pseudo-Dirac pairs are very different in different realizations such as the inverse or linear seesaw. This different phenomenology could offer a way to discriminate between low-scale seesaw realizations.