Testing Thermal-Relic Dark Matter with a Dark Photon Mediator

In light of recent DAMIC-M results, we present the status of thermal-relic dark matter $\chi$ coupled to a kinetically-mixed dark photon $A^\prime$. In the predictive "direct annihilation" regime, $m_{A'} > m_\chi$, the relic abundance depends on the kinetic mixing parameter, and there is a minimum value compatible with thermal freeze out. Using only electron and nuclear recoil direct detection results, we find that for complex scalar dark matter, the direct-annihilation regime is now excluded for nearly all values of $m_\chi$; the only exception is the resonant annihilation regime where $m_{A'}\approx 2 m_\chi$. Direct annihilation relic targets for other representative models, including Majorana and Pseudo-Dirac candidates, remain viable across a wide range of model parameters, but will be tested with a combination of dedicated accelerator searches in the near future. In the opposite "secluded annihilation" regime, where $m_\chi > m_{A'}$, this scenario is excluded by cosmic microwave background measurements for all $m_\chi \lesssim 30$ GeV. Similar conclusions in both the direct and secluded regimes hold for all anomaly-free vector mediators that couple to the first generation of electrically-charged Standard Model particles.