Forbidden dark matter assisted by first-order phase transition and associated gravitational waves
arXiv:2601.12319
Abstract
We propose a simple yet testable framework for light fermion dark matter (DM) with mass in the MeV--GeV range, charged under a dark gauge symmetry. Before symmetry breaking, DM annihilates excessively into massless dark gauge bosons, resulting in an under-abundant relic and hence creating severe tension with cosmic microwave background (CMB) and indirect detection constraints. This is naturally remedied by a strongly first-order phase transition (FOPT) in the dark sector, triggered by a scalar field , which gives rise to masses for the dark gauge boson () and the physical scalar (). To achieve the correct relic abundance while evading indirect detection constraints, the -wave annihilation channel is strictly maintained in the kinematically forbidden regime. Crucially, due to the gauge structure, the usual annihilation is strictly absent at tree level and only arises at the one-loop level. Within this framework, we explore two distinct possibilities. When the loop-induced channel is also kinematically forbidden, it predominantly determines the relic density. Conversely, when the channel is kinematically allowed, a rich interplay emerges between the tree-level forbidden process and the loop-level allowed process, with the dominant contribution being dictated sensitively by the DM mass and gauge coupling. The late-time annihilations are highly suppressed in both of these channels, either by the energetic threshold of the forbidden channel or the -wave velocity suppression of the loop-induced final state process, rendering the scenario entirely safe from CMB and indirect search bounds...
12+5 pages, 11 captioned figures, 2 tables