Molecular clouds constraints on sub-GeV DM and asteroid-mass PBHs

Abstract

We show that the ionization of molecular clouds provides a novel probe of dark matter scenarios producing low-energy $e^+e^-$ pairs through annihilation, decay, or Hawking evaporation of primordial black holes. We derive constraints on MeV-scale dark matter with masses between $\sim1$ and $100$ MeV, as well as on primordial black holes in the mass range $10^{14}$--$3\times10^{17}$g. By modeling the propagation of electrons and positrons inside molecular clouds, we show that uncertainties in charged-particle transport constitute the main limitation of this method. Nevertheless, for the most physically motivated propagation scenarios, the resulting constraints remain competitive with the strongest bounds currently available. We also identify the molecular-cloud properties that maximize the sensitivity to dark matter-induced ionization and discuss how larger samples of clouds, together with improved modeling of cosmic-ray ionization and cloud structure, could substantially enhance the reach of this technique. Our results establish molecular-cloud ionization as a promising and complementary probe of sub-GeV dark matter and evaporating primordial black holes.