The Sommerfeld enhancement for scalar particles and application to sfermion co-annihilation regions

TL;DR

This paper investigates how the Sommerfeld enhancement affects the relic density of neutralinos in supersymmetric models, providing formulas, computational methods, and a public tool for accurate predictions in co-annihilation scenarios.

Contribution

It introduces a general approach to include Sommerfeld effects in relic density calculations and applies it to sfermion co-annihilation regions, with significant results and a public computational package.

Findings

Sommerfeld effects can alter relic density calculations by several percent to a factor of 5.

The impact is especially large in stop co-annihilation regions.

A publicly available numerical package is provided for broader use.

Abstract

We study the impact of the Sommerfeld enhancement on the thermal relic density of the lightest neutralino in the case of large co-annihilation effects with a scalar particle. The proper way of including the Sommerfeld effect in this case is discussed, and the appropriate formulas for a general scenario with a set of particles with arbitrary masses and (off-)diagonal interactions are provided. We implement these results to compute the relic density in the neutralino sfermion co-annihilation regions in the mSUGRA framework. We find non-negligible effects in whole sfermion co-annihilation regimes. For stau co-annihilations the correction to the relic density is of the order of several per cent, while for stop co-annihilations is much larger, reaching a factor of 5 in some regions of the parameter space. A numerical package for computing the neutralino relic density including the Sommerfeld effect in a general MSSM setup is made public available.