Not-so-inelastic Dark Matter

TL;DR

This paper challenges the common assumption of symmetry in inelastic dark matter models, showing that diagonal couplings are less constrained for Majorana fermions, which relaxes relic density constraints and opens new viable mass regions.

Contribution

It demonstrates that the symmetry assumption is unnecessary for Majorana fermions, allowing for relaxed relic density constraints and identifying new viable dark matter mass regions.

Findings

Two viable dark matter mass regions identified: ~hundreds of MeV and ~few GeV.

The lighter region can be tested by upcoming NA64 and Belle II experiments.

The heavier region remains difficult to probe with future experiments.

Abstract

Models of inelastic (or pseudo-Dirac) dark matter commonly assume an accidental symmetry between the left-handed and right-handed mass terms in order to suppress diagonal couplings. We point out that this symmetry is unnecessary, because for Majorana fermions the diagonal couplings are not strongly constrained. Removing the requirement of such an ad-hoc symmetry instead relaxes the relic density constraint due to additional annihilation modes. We consider a simple UV-complete model realising this setup and study constraints from (in)direct detection, beam dump experiments and colliders. We identify two viable mass regions for the dark matter mass, around a few hundred MeV and around a few GeV, respectively. The former region will be fully tested by near-future analyses of NA64 and Belle II data, while the latter turns out to be challenging to explore even with future experiments.