On thermal corrections to near-threshold annihilation

TL;DR

This paper studies how finite-temperature effects modify the annihilation of non-relativistic dark particles, revealing significant impacts on bound states and annihilation rates relevant for dark matter relic abundance calculations.

Contribution

It provides a resummed framework to incorporate thermal corrections into near-threshold annihilation spectra, including effects on bound states and gauge interactions, for TeV-scale dark particles.

Findings

Bound states melt below freeze-out temperature.

Attractive interactions can increase annihilation rates by up to 80 times.

Thermal effects significantly alter the annihilation spectrum and rates.

Abstract

We consider non-relativistic "dark" particles interacting through gauge boson exchange. At finite temperature, gauge exchange is modified in many ways: virtual corrections lead to Debye screening; real corrections amount to frequent scatterings of the heavy particles on light plasma constituents; mixing angles change. In a certain temperature and energy range, these effects are of order unity. Taking them into account in a resummed form, we estimate the near-threshold spectrum of kinetically equilibrated annihilating TeV scale particles. Weakly bound states are shown to "melt" below freeze-out, whereas with attractive strong interactions, relevant e.g. for gluinos, bound states boost the annihilation rate by a factor 4...80 with respect to the Sommerfeld estimate, thereby perhaps helping to avoid overclosure of the universe. Modestly non-degenerate dark sector masses and a way to combine the contributions of channels with different gauge and spin structures are also discussed.