A map of the non-thermal WIMP

TL;DR

This paper investigates how elastic scattering influences the relic abundance of non-thermal WIMPs produced after reheating, revealing new dependencies on scattering rates and categorizing different parameter regimes affecting dark matter relic density.

Contribution

It explores the intermediate regimes between thermalization and no annihilation, highlighting the importance of elastic scattering in determining non-thermal WIMP relic abundance, especially for p-wave annihilators.

Findings

Relic density depends on both annihilation and elastic scattering rates.

Elastic scattering significantly affects the relic abundance of p-wave dark matter.

Parameter space is categorized into regions with distinct relic abundance mechanisms.

Abstract

We study the effect of the elastic scattering on the non-thermal WIMP, which is produced by direct decay of heavy particles at the end of reheating. The non-thermal WIMP becomes important when the reheating temperature is well below the freeze-out temperature. Usually, two limiting cases have been considered. One is that the produced high energetic dark matter particles are quickly thermalized due to the elastic scattering with background radiations. The corresponding relic abundance is determined by the thermally averaged annihilation cross-section at the reheating temperature. The other one is that the initial abundance is too small for the dark matter to annihilate so that the final relic is determined by the initial amount itself. We study the regions between these two limits, and show that the relic density depends not only on the annihilation rate, but also on the elastic scattering rate. Especially, the relic abundance of the p-wave annihilating dark matter crucially relies on the elastic scattering rate because the annihilation cross-section is sensitive to the dark matter velocity. We categorize the parameter space into several regions where each region has distinctive mechanism for determining the relic abundance of the dark matter at the present Universe. The consequence on the (in)direct detection is also studied.