Joint CMB and BBN Constraints on Light Dark Sectors with Dark Radiation

Understand

Dark sectors provide a compelling theoretical framework for thermally producing sub-GeV dark matter, and motivate an expansive new accelerator and direct-detection experimental program.

• We demonstrate the power of constraining such dark sectors using the measured effective number of neutrino species, $N_\text{eff}$, from the Cosmic Microwave Background (CMB) and primordial elemental abundances from Big Bang Nucleosynthesis (BBN).
• As a concrete example, we consider a dark matter particle of arbitrary spin that interacts with the Standard Model via a massive dark photon, accounting for an arbitrary number of light degrees of freedom in the dark sector.
• We exclude dark matter masses below $\sim$ 4 MeV at 95% confidence for all dark matter spins and dark photon masses.