Sharp spectral features from light dark matter decay via gravity portals

TL;DR

This paper explores how light dark matter decays induced by gravity portals produce distinctive spectral features in gamma-ray and X-ray observations, providing new constraints on such models and implications for cosmic reionization.

Contribution

It introduces a novel decay mechanism for sub-GeV scalar dark matter via gravity portals, predicting sharp spectral features and setting observational limits.

Findings

Sharp spectral features in gamma-ray and X-ray spectra from dark matter decay.

Upper limits on gravity portal coupling from observational data.

Potential impact on the Universe's reionization history.

Abstract

We investigate the phenomenology of dark matter decay assuming that it is induced by non-minimal coupling to gravity, when the dark matter mass is in the sub-GeV range, i.e. below the QCD confinement scale. We show that the decay of the singlet scalar dark matter candidate produces sharp features in the photon spectrum, in the form of lines, boxes, and also in the form of a novel spectral feature, characterized by the decay into $e^+e^- \gamma$ through a contact interaction, with decay branching fractions depending only on a single parameter, namely the dark matter mass. We also derive upper limits on the strength of the gravity portal from the non-observation of sharp features in the isotropic diffuse gamma-ray spectra measured by COMPTEL, EGRET and Fermi-LAT, and the X-ray spectrum measured by INTEGRAL. Finally, we briefly comment on the impact of dark matter decay via non-minimal coupling to gravity on the reionization history of the Universe.