Cooling in a Dissipative Dark Sector
  The possibility of a subdominant component of dark matter dissipating energy
could lead to dramatic new phenomenology such as the formation of a dark disk.
One rigorous way to assess this possibility and settle the debate on its
feasibility is to include the dissipative dark component in a numerical
hydrodynamical simulation. A necessary input to such a simulation is a
prescription including energy dissipation rates of different processes and
rates of processes that change the number densities of dark ions and atoms. In
this article, we study the simplest dissipative dark sector which consists of a
dark electron and proton, both charged under a dark gauged U(1). We present
approximate analytic formulas for energy loss rates due to Compton scattering,
bremsstrahlung, recombination, collisional ionization and collisional
excitation as well as the rates of number density change. We also include the
heating rate due to photoionization. The work serves as the first step to
realize a numerical simulation including a dissipative dark sector, which
hopefully can shed more light on the formation and properties of a dark disk
originating from dark matter self-interactions.
