Cosmological Constraints on Sub-GeV Dark Matter

Abstract

I present updated and novel bounds on sub-\si{GeV} dark matter mass using constraints from the CMB and BBN. Using updated values for $N_{\eff}$, D/H and \Yp, I find tighter constraints on light dark matter models where the contribution from a dark sector mediator is ignored than those previously presented in the literature. Models of this kind with dark matter coupling to photons are constrained to have masses $m_{\chi}$ above roughly \SI{1}{MeV}, and the number of equivalent neutrinos $\Delta N_{\nu}$ is constrained between $-0.25$ and $0.9$. A lower bound cannot be placed on neutrino-coupled dark matter mass in general in these models, though I find $-1\lesssim \Delta N_{\nu} \lesssim 0.45$ for this coupling. I also develop a new framework to constrain models where the contribution of a dark sector mediator is not ignored. Specifically, I apply this formalism to a model in which dark matter with mass $m_{\chi}$ couples to Standard Model photons via a $U(1)'$ dark photon with mass $3m_{\chi}$. I finally compare the results of this analysis to the sensitivities of existing and proposed direct detection experiments to determine how these constraints may impact future searches for dark matter; I find many proposed direct detection experiments are set to probe a region of parameter space that is ruled out by this constraint.