Dynamic freeze-in: impact of thermal masses and cosmological phase transitions on dark matter production

Abstract

The cosmological abundance of dark matter can be significantly influenced by the temperature dependence of particle masses and vacuum expectation values. We illustrate this point in three simple freeze-in models. The first one, which we call kinematically induced freeze-in, is based on the observation that the effective mass of a scalar temporarily becomes very small as the scalar potential undergoes a second order phase transition. This opens dark matter production channels that are otherwise forbidden. The second model we consider, dubbed vev-induced freeze-in, is a fermionic Higgs portal scenario. Its scalar sector is augmented compared to the Standard Model by an additional scalar singlet, S, which couples to dark matter and temporarily acquires a vacuum expectation value (a twostep phase transition or "vev flip-flop"). While S = 0, the modified coupling structure in the scalar sector implies that dark matter production is significantly enhanced compared to the S = 0 phases realised at very early times and again today. The third model, which we call mixing-induced freeze-in, is similar in spirit, but here it is the mixing of dark sector fermions, induced by non-zero S , that temporarily boosts the dark matter production rate. For all three scenarios, we carefully dissect the evolution of the dark sector in the early Universe. We compute the DM relic abundance as a function of the model parameters, emphasising the importance of thermal corrections and the proper treatment of phase transitions in the calculation.

Forbidden frozen-in dark matter

“…Since Higgs portal scenarios are typically constrained by a variety of limits, we briefly review them and apply them to the considered model provide in order to identify new regions that are allowed by forbidden freeze-in. This scenario is similar to case III of [13], which closely resembles our model as the particle content is similar. The main difference, however, is the vacuum expectation value (VEV) structure.…”

“…This kind of effect we believe was first identified for gravitino [11] and subsequently axino production [12]. More recently it was described in a more generic context in [13,14].…”

“…4. 13 The direct S →χχ decay width is given by eq. (2.21) and is suppressed by the very small Yukawa coupling y χ .…”

“…This allows us to isolate the pure forbidden freeze-in regime without the impact of the SM Higgs VEV, thus simplifying the analysis and exploring the forbidden freeze-in independently. We additionally explore a different mass region than [13], which leads to a distinct phenomenology for the portal particle.…”

Spontaneous freeze out of dark matter from an early thermal phase transition

“…The possibility that the dark-matter mass might be a time-dependent quantity was studied in the past in the context of Variable-Mass Particles (VAMPs) in which dark-matter particles interact with a quintessence field [73][74][75]. Moreover, the effect of thermal corrections to the potential of a dark scalar was also used in the context of the so-called Flip-Flop vev mechanism [76,77], super-cool dark matter [78], or in the forbidden freeze-in scenario [79], where a second order phase transition is used to kinematically open or close certain annihilation or decay channels in the early universe.…”