The impact of a strongly first-order phase transition on the abundance of thermal relics
Carroll Wainwright, Stefano Profumo
TL;DR
This paper addresses how a strongly first-order electroweak phase transition (EWPT) can dilute the thermal relic density of particles that freeze out before the transition, potentially reconciling MSSM neutralino dark matter with the observed cold dark matter density. The authors define a dilution factor driven by entropy injection during the EWPT, derive general thermodynamic relations for both near-equilibrium and strongly non-equilibrium transitions, and determine the transition temperature via bubble nucleation, with $S_3/T$ of order $130$–$140$ at the electroweak scale. They apply the framework to the MSSM, performing a parameter scan and using DarkSUSY to compute the neutralino relic abundance and freeze-out temperature, identifying regions where overabundant neutralinos can be salvaged by EWPT. They find that heavy (multi-TeV) wino- or higgsino-like neutralinos and bino-like neutralinos across broader masses can be compatible with the observed dark matter density given a suitable dilution, with implications for LHC tests and the structure of the EW sector.
Abstract
We study the impact of a strongly first-order electro-weak phase transition on the thermal relic abundance of particle species that could constitute the dark matter and that decoupled before the phase transition occurred. We define a dilution factor induced by generic first-order phase transitions, and we explore the parameter space of the minimal supersymmetric extension to the Standard Model to determine which phase transition temperatures and dilution factors are relevant for the lightest neutralino as a dark matter candidate. We then focus on a specific toy-model setup that could give rise to a strongly first-order electro-weak phase transition, and proceed to a detailed calculation of dilution factors and transition temperatures, comparing our findings to actual neutralino dark matter models. Typical models that would produce an excessive thermal relic density and that can be salvaged postulating a strongly first-order electro-weak phase transition include massive (multi-TeV) wino or higgsino-like neutralinos, as well as bino-like neutralinos in a wider mass range, with masses as low as 400 GeV. If LHC data indicate an inferred thermal neutralino relic abundance larger than the cold dark matter density, the mismatch could thus potentially be explained by electro-weak scale physics that will also be thoroughly explored with collider experiments in the near future.