Make Dark Matter Charged Again
  We revisit constraints on dark matter that is charged under a $U(1)$ gauge
group in the dark sector, decoupled from Standard Model forces. We find that
the strongest constraints in the literature are subject to a number of
mitigating factors. For instance, the naive dark matter thermalization
timescale in halos is corrected by saturation effects that slow down
isotropization for modest ellipticities. The weakened bounds uncover
interesting parameter space, making models with weak-scale charged dark matter
viable, even with electromagnetic strength interaction. This also leads to the
intriguing possibility that dark matter self-interactions within small dwarf
galaxies are extremely large, a relatively unexplored regime in current
simulations. Such strong interactions suppress heat transfer over scales larger
than the dark matter mean free path, inducing a dynamical cutoff length scale
above which the system appears to have only feeble interactions. These effects
must be taken into account to assess the viability of darkly-charged dark
matter. Future analyses and measurements should probe a promising region of
parameter space for this model.
