Plasma dark matter direct detection
  Dark matter in spiral galaxies like the Milky Way may take the form of a dark
plasma. Hidden sector dark matter charged under an unbroken $U(1)'$ gauge
interaction provides a simple and well defined particle physics model realising
this possibility. The assumed $U(1)'$ neutrality of the Universe then implies
(at least) two oppositely charged dark matter components with self-interactions
mediated via a massless "dark photon" (the $U(1)'$ gauge boson). In addition to
nuclear recoils such dark matter can give rise to keV electron recoils in
direct detection experiments. In this context, the detailed physical properties
of the dark matter plasma interacting with the Earth is required. This is a
complex system, which is here modelled as a fluid governed by the
magnetohydrodynamic equations. These equations are numerically solved for some
illustrative examples, and implications for direct detection experiments
discussed. In particular, the analysis presented here leaves open the
intriguing possibility that the DAMA annual modulation signal is due primarily
to electron recoils (or even a combination of electron recoils and nuclear
recoils). The importance of diurnal modulation (in addition to annual
modulation) as a means of probing this kind of dark matter is also emphasised.
