Majorana vs Pseudo-Dirac Neutrinos at the ILC

TL;DR

This paper investigates how future colliders like the ILC can detect tiny mass splittings in pseudo-Dirac neutrinos, revealing lepton number violation and linking to light neutrino masses through displaced decay signals.

Contribution

It demonstrates that the ILC can measure very small Majorana mass splittings via charge asymmetry in displaced decays, surpassing current experimental bounds.

Findings

Charge asymmetry at ILC can probe tiny mass splittings.

Displaced decay signals provide a golden channel for detection.

Sensitivity exceeds current neutrinoless double beta decay limits.

Abstract

Neutrino masses could originate in seesaw models testable at colliders, with light mediators and an approximate lepton number symmetry. The minimal model of this type contains two quasi-degenerate Majorana fermions forming a pseudo-Dirac pair. An important question is to what extent future colliders will have sensitivity to the splitting between the Majorana components, since this quantity signals the breaking of lepton number and is connected to the light neutrino masses. We consider the production of these neutral heavy leptons at the ILC, where their displaced decays provide a golden signal: a forward-backward charge asymmetry, which depends crucially on the mass splitting between the two Majorana components. We show that this observable can constrain the mass splitting to values much lower than current bounds from neutrinoless double beta decay and natural loop corrections.