Self-interacting asymmetric dark matter coupled to a light massive dark
  photon
  Dark matter (DM) with sizeable self-interactions mediated by a light species
offers a compelling explanation of the observed galactic substructure;
furthermore, the direct coupling between DM and a light particle contributes to
the DM annihilation in the early universe. If the DM abundance is due to a dark
particle-antiparticle asymmetry, the DM annihilation cross-section can be
arbitrarily large, and the coupling of DM to the light species can be
significant. We consider the case of asymmetric DM interacting via a light (but
not necessarily massless) Abelian gauge vector boson, a dark photon. In the
massless dark photon limit, gauge invariance mandates that DM be
multicomponent, consisting of positive and negative dark ions of different
species which partially bind in neutral dark atoms. We argue that a similar
conclusion holds for light dark photons; in particular, we establish that the
multi-component and atomic character of DM persists in much of the parameter
space where the dark photon is sufficiently light to mediate sizeable DM
self-interactions. We discuss the cosmological sequence of events in this
scenario, including the dark asymmetry generation, the freeze-out of
annihilations, the dark recombination and the phase transition which gives mass
to the dark photon. We estimate the effect of self-interactions in DM haloes,
taking into account this cosmological history. We place constraints based on
the observed ellipticity of large haloes, and identify the regimes where DM
self-scattering can affect the dynamics of smaller haloes, bringing theory in
better agreement with observations. Moreover, we estimate the cosmological
abundance of dark photons in various regimes, and derive pertinent bounds.
