Dark matter decays from non-minimal coupling to gravity

TL;DR

This paper shows that non-minimal coupling to gravity can induce decay of dark matter particles, even if they are stable in flat spacetime, and provides bounds on this effect from experiments.

Contribution

It introduces the concept of a gravity portal causing dark matter decay via non-minimal coupling and calculates decay widths and experimental limits for a singlet scalar dark matter candidate.

Findings

Gravity portal induces dark matter decay in curved spacetime.

Decay branching ratios depend only on dark matter mass.

Experimental limits strongly constrain non-minimal coupling parameters.

Abstract

We consider the Standard Model extended with a dark matter particle in curved spacetime, motivated by the fact that the only current evidence for dark matter is through its gravitational interactions, and we investigate the impact on the dark matter stability of terms in the Lagrangian linear in the dark matter field and proportional to the Ricci scalar. We show that this "gravity portal" induces decay even if the dark matter particle only has gravitational interactions, and that the decay branching ratios into Standard Model particles only depend on one free parameter: the dark matter mass. We study in detail the case of a singlet scalar as dark matter candidate, which is assumed to be absolutely stable in flat spacetime due to a discrete $Z_2$ symmetry, but which may decay in curved spacetimes due to a $Z_2$-breaking non-minimal coupling to gravity. We calculate the dark matter decay widths and we set conservative limits on the non-minimal coupling parameter from experiments. The limits are very stringent and suggest that there must exist an additional mechanism protecting the singlet scalar from decaying via this gravity portal.