Langevin simulation of dark matter kinetic equilibration

TL;DR

This paper uses Langevin simulations to study how well kinetic equilibrium is maintained for scalar singlet dark matter particles around 60 GeV in the early universe, especially near low temperatures where resonant effects are significant.

Contribution

It demonstrates the application of Langevin simulations to non-equilibrium momentum distributions in cosmology, confirming that kinetic equilibrium holds down to about 1 GeV for 60 GeV dark matter.

Findings

Kinetic equilibrium persists down to ~1 GeV for 60 GeV dark matter.

Deviations from equilibrium appear as a red-tilted spectrum.

A slightly larger coupling than in equilibrium is required for correct relic abundance.

Abstract

Recently it has been questioned, notably in the context of the scalar singlet dark matter model with $m_\varphi^{ }\simeq 60$ GeV, how efficiently kinetic equilibrium is maintained if freeze-out dynamics is pushed down to low temperatures by resonant effects. We outline how Langevin simulations can be employed for addressing the non-equilibrium momentum distribution of non-relativistic particles in a cosmological background. For a scalar singlet mass $m_\varphi^{ }\simeq 60$ GeV, these simulations suggest that kinetic equilibrium is a good approximation down to $T \sim 1$ GeV, with the deviation first manifesting itself as a red-tilted spectrum. This reduces the annihilation cross section, confirming findings from other methods that a somewhat larger ($ < 20\%$) coupling than in equilibrium is needed for obtaining the correct abundance.