Migdal effect and photon Bremsstrahlung: improving the sensitivity to
  light dark matter of liquid argon experiments
  The search for dark matter weakly interacting massive particles with noble
liquids has probed masses down and below a GeV/c^2. The ultimate limit is
represented by the experimental threshold on the energy transfer to the nuclear
recoil. Currently, the experimental sensitivity has reached a threshold
equivalent to a few ionization electrons. In these conditions, the contribution
of a Bremsstrahlung photon or a so-called Migdal electron due to the sudden
acceleration of a nucleus after a collision might be sizable. In the present
work, we use a Bayesian approach to study how these effects can be exploited in
experiments based on liquid argon detectors. In particular, taking inspiration
from the DarkSide-50 public spectra, we develop a simulated experiment to show
how the Migdal electron and the Bremsstrahlung photon allow to push the
experimental sensitivity down to masses of 0.1 GeV/c^2, extending the search
region for dark matter particles of previous results. For these masses we
estimate the effect of the Earth shielding that, for strongly interacting dark
matter, makes any detector blind. Finally, we show how the sensitivity scales
for higher exposure.
