Excited bound states and their role in dark matter production

TL;DR

This paper investigates how highly excited bound states influence dark matter production in the early Universe, revealing that they can significantly alter freeze-out dynamics and dark matter abundance.

Contribution

It introduces a novel method to include a vast number of bound state processes in dark matter models, enhancing the understanding of freeze-out and depletion mechanisms.

Findings

Highly excited states prevent particle freeze-out in non-Abelian theories.

Bound states cause mediator depletion to persist until decay, affecting dark matter density.

Dark matter mass constraints can be relaxed by an order of magnitude due to bound state effects.

Abstract

We explore the impact of highly excited bound states on the evolution of number densities of new physics particles, specifically dark matter, in the early Universe. Focusing on dipole transitions within perturbative, unbroken gauge theories, we develop an efficient method for including around a million bound state formation and bound-to-bound transition processes. This enables us to examine partial-wave unitarity and accurately describe the freeze-out dynamics down to very low temperatures. In the non-Abelian case, we find that highly excited states can prevent the particles from freezing out, supporting a continuous depletion in the regime consistent with perturbativity and unitarity. We apply our formalism to a simplified dark matter model featuring a colored and electrically charged $t$-channel mediator. Our focus is on the regime of superWIMP production which is commonly characterized by a mediator freeze-out followed by its late decay into dark matter. In contrast, we find that excited states render mediator depletion efficient all the way until its decay, introducing a dependence of the dark matter density on the mediator lifetime as a novel feature. The impact of bound states on the viable dark matter mass can amount to an order of magnitude, relaxing constraints from Lyman-$\alpha$ observations.