Excited bound states and their role in dark matter production

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.

21 pages + references, 10 figure; v2: minor presentational improvements, Eq. (A5) corrected, matches journal version

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• Conversion-Driven Leptogenesis: A Testable Theory of Dark Matter and Baryogenesis at the Electroweak Scale
• Unitarity in the non-relativistic regime and implications for dark matter
• Probing conversion-driven freeze-out at the LHC
• t-channel dark matter at the LHC -- a whitepaper
• Flavoured Majorana Dark Matter then and now: From freeze-out scenarios to LHC signatures
• The role of unitarisation on dark-matter freeze-out via metastable bound states
• Radiation back-reaction during dark-matter freeze-out via metastable bound states
• Darkonia at Colliders
• BSFfast: Rapid computation of bound-state effects on annihilation in the early Universe
• Unitarity violation and restoration in radiative bound-state formation