Kavli Affiliate: Gordan Krnjaic
| First 5 Authors: Asher Berlin, Gordan Krnjaic, Elena Pinetti, ,
| Summary:
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.
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