Speaker
Description
We propose a minimal and testable framework for light fermion dark matter in the MeV–GeV mass range, charged under a dark $U(1)_D$ gauge symmetry that is spontaneously broken by a scalar field. The resulting dark-sector phase transition induces a mass gap that activates a forbidden annihilation channel into slightly heavier dark states, thereby setting the dark matter relic abundance while naturally suppressing late-time annihilation. As a consequence, the scenario evades stringent cosmic microwave background and indirect detection bounds that typically exclude thermal light dark matter. The same symmetry-breaking transition is strongly first order, generating a stochastic gravitational wave background potentially observable by upcoming space-based interferometers and pulsar timing arrays. We show that reproducing the observed relic abundance tightly correlates the dark matter mass with the nucleation temperature of the phase transition, linking dark-sector dynamics, relic abundance, and gravitational wave signatures in a unified framework.