Bringing Isolated Dark Matter Out of Isolation: Late-time Reheating and Indirect Detection

TL;DR

This paper explores how non-thermal early universe histories can enhance small-scale dark matter structures, boosting annihilation signals and enabling indirect detection of otherwise elusive dark matter candidates like Binos.

Contribution

It demonstrates that non-thermal cosmologies lead to microhalo formation, significantly increasing dark matter annihilation signals and providing new avenues for indirect detection, especially for Bino dark matter.

Findings

Enhanced small-scale structure from non-thermal histories boosts annihilation signals.

Current gamma-ray observations constrain Bino dark matter up to ~300 GeV.

Microhalo formation can make otherwise undetectable dark matter detectable.

Abstract

In standard cosmology, the growth of structure becomes significant following matter-radiation equality. In non-thermal histories, where an effectively matter-dominated phase occurs due to scalar oscillations prior to Big Bang Nucleosynthesis, a new scale at smaller wavelengths appears in the matter power spectrum. Density perturbations that enter the horizon during the matter-dominated phase grow linearly with the scale factor prior to the onset of radiation domination, which leads to enhanced inhomogeneity on small scales if dark matter thermally and kinetically decouples during the matter-dominated phase. The microhalos that form from these enhanced perturbations significantly boost the self-annihilation rate for dark matter. This has important implications for indirect detection experiments: the larger annihilation rate will result in observable signals from dark matter candidates that are usually deemed untestable. As a proof of principle, we consider Binos in heavy supersymmetry with an intermediate extended Higgs sector and all other superpartners decoupled. We find that these isolated Binos, which lie under the neutrino floor, can account for the dark matter relic density while also leading to observable predictions for Fermi-LAT. Current limits on the annihilation cross section from Fermi-LAT's observations of dwarf spheroidal galaxies may already constrain Bino dark matter up to masses $\mathcal{O}(300)$ GeV, depending on the internal structure of the microhalos. More extensive constraints are possible with improved gamma-ray bounds and boost calculations from $N$-body simulations.