Constraints on dark matter-electron scattering from molecular cloud ionization
Anirudh Prabhu, Carlos Blanco
Abstract
We demonstrate that ionization of ${\mathrm{H}}_{2}$ by dark matter in dense molecular clouds can provide strong constraints on the scattering strength of dark matter with electrons. Molecular clouds have high UV-optical attenuation, shielding them from ultraviolet and x-ray photons. Their chemical and thermal evolution are governed by low-energy cosmic rays. Dark matter with mass $\ensuremath{\gtrsim}4\text{ }\text{ }\mathrm{MeV}$ can ionize ${\mathrm{H}}_{2}$, contributing to the observed ionization rate. We require that the dark matter-induced ionization rate of ${\mathrm{H}}_{2}$ not exceed the observed cosmic-ray ionization rate ${\ensuremath{\zeta}}^{{\mathrm{H}}_{2}}$, in diffuse molecular clouds as well as dense molecular clouds such as L1551 in the Taurus cloud complex. This allows us to place strong constraints on the dark matter-electron cross section ${\overline{\ensuremath{\sigma}}}_{e}$ that complement existing astrophysical constraints and probe the strongly interacting parameter space where terrestrial and underground direct-detection experiments lose sensitivity. We show that constraints from molecular clouds combined with planned balloon and satellite-based experiments would strongly constrain the fractional abundance of dark matter that interacts strongly with electrons. We comment on future modeling and observational efforts that may improve our bounds.