Dark Matter in the Earth and the Sun -- Simulating Underground
  Scatterings for the Direct Detection of Low-Mass Dark Matter
  If DM-matter scatterings are assumed to occur in a detector's target
material, collisions will naturally take place inside the bulk of planets and
stars as well. For large cross sections, these scatterings might occur in the
Earth or Sun even prior to the detection. In this thesis, we study the impact
of these pre-detection scatterings on direct searches of light DM with the use
of Monte Carlo (MC) simulations. By simulating the trajectories and scatterings
of many individual DM particles in the Earth or Sun, we determine the local
distortions of the statistical properties of DM at any detector caused by
elastic DM-nucleus collisions. Scatterings inside the Earth distort the
underground DM density and velocity distribution. Any detector moves
periodically through these inhomogeneities due to the Earth's rotation, and the
expected event rate will vary throughout a sidereal day. Using MC simulations,
we can determine the exact amplitude and phase of this diurnal modulation for
any experiment. For even higher scattering probabilities, collisions in the
overburden above the typically underground detectors start to attenuate the
incoming DM flux. The critical cross section above which an experiment loses
sensitivity to DM itself is determined for a variety of DM-nucleus and
DM-electron scattering experiments and different interaction types.
Furthermore, we develop the idea that sub-GeV DM particles can enter the Sun,
gain kinetic energy by colliding on hot nuclei and get reflected with great
speeds. By deriving an analytic expressions for the particle flux from solar
reflection via a single scattering, we demonstrate the prospects of future
experiments to probe reflected DM and extend their sensitivity to lower masses
than accessible by halo DM alone. Finally, we present first results for MC
simulations of solar reflections taking into account the effect of multiple
scatterings.
