Probing the Nature of Heavy Neutral Leptons in Direct Searches and Neutrinoless Double Beta Decay

TL;DR

This paper explores how future experiments like DUNE and LEGEND-1000 can jointly investigate the properties of Heavy Neutral Leptons, including their Majorana or Dirac nature and CP violation, through combined direct and neutrinoless double beta decay searches.

Contribution

It provides a detailed analysis of the complementarity between direct searches and neutrinoless double beta decay in probing HNL properties within a standard Seesaw framework.

Findings

Combined searches improve sensitivity to HNL nature.

DUNE and LEGEND-1000 can distinguish Majorana from quasi-Dirac neutrinos.

CP-violating phases can be constrained through joint analysis.

Abstract

Heavy Neutral Leptons (HNLs) are a popular extension of the Standard Model to explain the lightness of neutrino masses and the matter-antimatter asymmetry through leptogenesis. Future direct searches, such as fixed target setups like DUNE, and neutrinoless double beta decay are both expected to probe the regime of active-sterile neutrino mixing in a standard Seesaw scenario of neutrino mass generation for HNL masses around m_N <~ 1 GeV. Motivated by this, we analyse the complementarity between future direct searches and neutrinoless double beta decay to probe the nature of HNLs, i.e., whether they are Majorana or quasi-Dirac states, and CP-violating phases in the sterile neutrino sector. Following an analytic discussion of the complementarity, we implement a generic fixed target experiment modelling DUNE. We perform a statistical study in how a combined search for HNLs in direct searches and neutrinoless double beta decay, using DUNE and LEGEND-1000 as representative examples, can probe the nature of sterile neutrinos.