Generalizing Minimal Dark Matter: Millicharge or Decay

TL;DR

This paper explores generalizations of the Minimal Dark Matter framework, including decaying and millicharged dark matter candidates, analyzing their spectra and constraints from gamma-ray data to extend the model's viability.

Contribution

It introduces two new directions for Minimal Dark Matter: allowing decay by lowering the cutoff scale and considering millicharged stable candidates, expanding the model's scope.

Findings

Gamma-ray data constrain the cutoff to be above the GUT scale.

A Dirac SU(2)_L triplet is the least constrained millicharged candidate.

Decaying quintuplet models are viable with cutoff scales above the GUT scale.

Abstract

The Minimal Dark Matter framework classifies viable Dark Matter (DM) candidates that are obtained by simply augmenting the Standard Model of particle interactions with a new multiplet, without adding new ad hoc symmetries to make the DM stable. The model has no free parameters and is therefore extremely predictive; moreover, recent studies singled out a Majorana $SU(2)_\text{L}$ quintuplet as the only viable candidate. The model can be constrained by both direct and indirect DM searches, with present time gamma-ray line searches in the Galactic Center being particularly sensitive. It is therefore timely to critically review this paradigm and point out possible generalizations. We propose and explore two distinct directions. One is to lower the cutoff of the model, which was originally fixed at the Planck scale, to allow for decays of the DM quintuplet. We analyze the decay spectrum of this candidate in detail and show that gamma-ray data constrain the cutoff to lie above the GUT scale. Another possibility is to abandon the assumption of DM electric neutrality in favor of absolutely stable, millicharged DM candidates. We explicitly study a few examples, and find that a Dirac $SU(2)_\text{L}$ triplet is the candidate least constrained by indirect searches.