Neutrinoless double beta decay versus other probes of heavy sterile neutrinos

TL;DR

This paper compares neutrinoless double beta decay constraints on heavy sterile neutrinos with other experimental bounds, highlighting the impact of mass splitting and CP phases on lepton number violation detection.

Contribution

It introduces a phenomenological parametrisation of the seesaw model to analyze sterile neutrino scenarios and their experimental constraints.

Findings

Neutrinoless double beta decay constraints depend on sterile neutrino mass splitting and CP phases.

The sensitivity of neutrinoless double beta decay is highest for sterile neutrinos around 0.1-10 GeV.

Constraints vary significantly between Majorana and pseudo-Dirac sterile neutrino scenarios.

Abstract

We make a comparative study of the neutrinoless double beta decay constraints on heavy sterile neutrinos versus other direct and indirect constraints from both lepton number conserving and violating processes, as a sensitive probe of the extent of lepton number violation and possible interference effects in the sterile sector. We introduce a phenomenological parametrisation of the simplified one-generation seesaw model with one active and two sterile neutrino states in terms of experimentally measurable quantities, such as active-sterile neutrino mixing angles, CP phases, masses and mass splittings. This simple parametrisation enables us to analytically derive a spectrum of possible scenarios between the canonical seesaw with purely Majorana heavy neutrinos and inverse seesaw with pseudo-Dirac ones. We then go on to constrain the simplified parameters of this model from various experiments at the energy, intensity and cosmic frontiers. We emphasise that the constraints from lepton number violating processes strongly depend on the mass splitting between the two sterile states and the relative CP phase between them. This is particularly relevant for neutrinoless double beta decay, which is weakened for small mass splitting and opposite CP parities between the sterile states. On the other hand, neutrinoless double beta decay is especially sensitive for Majorana sterile neutrinos with masses around $0.1-10$ GeV.