Fermion Dark Matter Effect on Electroweak Phase Transition

TL;DR

This paper investigates how adding fermions to a scalar dark matter model influences the electroweak phase transition, finding that fermions tend to weaken the transition and prevent it from becoming strongly first-order.

Contribution

It is the first study to analyze the impact of fermions on the electroweak phase transition within a singlet scalar dark matter framework.

Findings

Fermions reduce the strength of the electroweak phase transition.

Finite temperature effects enable a weak first-order transition, absent in high-temperature approximation.

The model remains consistent with dark matter detection constraints.

Abstract

The addition of extra scalars to the Standard Model (SM) of particle physics enriches the vacuum structure and consequently gives rise to strong first-order phase transitions (EWPT) in the early universe. We raise the question that how the EWPT is affected by the addition of fermions in models beyond the SM, and address this question by studying the EWPT in a dark matter model comprising a singlet scalar and two Dirac fermions. The singlet scalar develops a nonzero vacuum expectation value (VEV), and the lighter fermion plays the role of the dark matter. The model evades the stringent direct detection bounds due to the presence of two fermions. We first show that applying the high-temperature approximation, no first-order phase transition is found. Then we demonstrate that when including the full finite temperature corrections to the effective potential, the first-order phase transition becomes possible, nevertheless, all the phase transitions will be weak. We therefore deduce that the addition of fermions reduces the strength of the EWPT.