$\mu$DM: a new mechanism for the baryon-dark matter coincidence

Abstract

For a relativistically decoupled thermal relic, the dark-matter energy-to-entropy ratio scales as ${\rho }_{\mathrm{DM}}/s\sim 10^{-3}{m}_{\mathrm{DM}}$. This is the familiar hot-dark-matter abundance relation. Immediately after weak-sphaleron freeze-out, the baryon asymmetry can be parametrized as ${\rho }_{\mathrm{baryon}}/s\sim 10^{-4}|{\mu }_B|$. Here ${\mu }_B$ is the baryon-number chemical potential. We propose a new class of models, called chemical-potential-matched dark matter ($\mu$DM), in which $|{\mu }_B|\sim m_{\mathrm{DM}}$, thereby providing a new route to the baryon-dark matter coincidence. As a concrete example of $\mu$DM, we consider an Affleck-Dine field whose excitations constitute dark matter. We also briefly discuss spontaneous electroweak baryogenesis associated with axion-like-particle walls. In the absence of entropy dilution, this coincidence mechanism generically points to dark matter in the eV-keV mass range. Its momentum distribution can nevertheless be cold if thermal production is dominated by Bose-enhanced stimulated emission. Alternatively, entropy dilution after both the dark-matter and baryon yields are fixed allows masses up to the MeV scale.

Comments: 15 pages, 4figures, comments welcome!