A Phase-Transition Portal to Enhanced Dark Matter Annihilation

Abstract

We present a thermal dark matter (DM) scenario in which a phase transition after freeze-out changes both the DM spectrum and the dominant annihilation process, allowing an enhanced present day annihilation rate with suppressed direct detection. We realize this mechanism in a two-Higgs-doublet model extended by a complex scalar singlet, a Majorana fermion, and a heavier Dirac fermion. Before the transition, the Majorana state freezes out through $p$-wave annihilation into singlet scalars. The subsequent transition changes the dark fermion masses and mixing, enabling $s$-wave annihilation through a pseudoscalar portal. The surviving relic differs in mass and composition from the original state. We find relic compatible solutions with present day annihilation rates more than an order of magnitude above the canonical thermal value, without tuning to a mediator resonance, while satisfying direct detection bounds.