Reviving MeV-GeV Indirect Detection with Inelastic Dark Matter

TL;DR

This paper proposes a new inelastic dark matter model that predicts detectable gamma-ray signals in the MeV range, potentially allowing for indirect detection of sub-GeV thermal relic dark matter with upcoming telescopes.

Contribution

It introduces a cosmologically viable inelastic dark matter model with late-time annihilation signatures detectable by future MeV gamma-ray telescopes.

Findings

Proposed a model where inelastic scattering regenerates a detectable dark matter population.

Showed that upcoming telescopes could probe much of the model's parameter space.

Demonstrated potential for discovery and model discrimination with future observations.

Abstract

Thermal relic dark matter below $\sim 10 \ \text{GeV}$ is excluded by cosmic microwave background data if its annihilation to visible particles is unsuppressed near the epoch of recombination. Usual model-building measures to avoid this bound involve kinematically suppressing the annihilation rate in the low-velocity limit, thereby yielding dim prospects for indirect detection signatures at late times. In this work, we investigate a class of cosmologically-viable sub-GeV thermal relics with late-time annihilation rates that are detectable with existing and proposed telescopes across a wide range of parameter space. We study a representative model of inelastic dark matter featuring a stable state $\chi_1$ and a slightly heavier excited state $\chi_2$ whose abundance is thermally depleted before recombination. Since the kinetic energy of dark matter in the Milky Way is much larger than it is during recombination, $\chi_1 \chi_1 \to \chi_2 \chi_2$ upscattering can efficiently regenerate a cosmologically long-lived Galactic population of $\chi_2$, whose subsequent coannihilations with $\chi_1$ give rise to observable gamma-rays in the $\sim 1 \ \text{MeV} - 100 \ \text{MeV}$ energy range. We find that proposed MeV gamma-ray telescopes, such as e-ASTROGAM, AMEGO, and MAST, would be sensitive to much of the thermal relic parameter space in this class of models and thereby enable both discovery and model discrimination in the event of a signal at accelerator or direct detection experiments.