Reviving MeV-GeV indirect detection with inelastic dark matter

Thermal relic dark matter below <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:mo>∼</a:mo><a:mn>10</a:mn><a:mtext> </a:mtext><a:mtext> </a:mtext><a:mi>GeV</a:mi></a:mrow></a:math> 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 <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:msub><c:mi>χ</c:mi><c:mn>1</c:mn></c:msub></c:math> and a slightly heavier excited state <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:msub><e:mi>χ</e:mi><e:mn>2</e:mn></e:msub></e:math> 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, <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:msub><g:mi>χ</g:mi><g:mn>1</g:mn></g:msub><g:msub><g:mi>χ</g:mi><g:mn>1</g:mn></g:msub><g:mo stretchy="false">→</g:mo><g:msub><g:mi>χ</g:mi><g:mn>2</g:mn></g:msub><g:msub><g:mi>χ</g:mi><g:mn>2</g:mn></g:msub></g:math> upscattering can efficiently regenerate a cosmologically long-lived Galactic population of <j:math xmlns:j="http://www.w3.org/1998/Math/MathML" display="inline"><j:msub><j:mi>χ</j:mi><j:mn>2</j:mn></j:msub></j:math>, whose subsequent coannihilations with <l:math xmlns:l="http://www.w3.org/1998/Math/MathML" display="inline"><l:msub><l:mi>χ</l:mi><l:mn>1</l:mn></l:msub></l:math> give rise to observable gamma-rays in the <n:math xmlns:n="http://www.w3.org/1998/Math/MathML" display="inline"><n:mo>∼</n:mo><n:mn>1</n:mn><n:mtext> </n:mtext><n:mtext> </n:mtext><n:mi>MeV</n:mi><n:mo>−</n:mo><n:mn>100</n:mn><n:mtext> </n:mtext><n:mtext> </n:mtext><n:mi>MeV</n:mi></n:math> 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. Published by the American Physical Society 2024