  Conventional dark matter direct detection experiments set stringent
constraints on dark matter by looking for elastic scattering events between
dark matter particles and nuclei in underground detectors. However these
constraints weaken significantly in the sub-GeV mass region, simply because
light dark matter does not have enough energy to trigger detectors regardless
of the dark matter-nucleon scattering cross section. Even if future experiments
lower their energy thresholds, they will still be blind to parameter space
where dark matter particles interact with nuclei strongly enough that they lose
enough energy and become unable to cause a signal above the experimental
threshold by the time they reach the underground detector. Therefore in case
dark matter is in the sub-GeV region and strongly interacting, possible
underground scatterings of dark matter with terrestrial nuclei must be taken
into account because they affect significantly the recoil spectra and event
rates, regardless of whether the experiment probes DM via DM-nucleus or
DM-electron interaction. To quantify this effect we present the publicly
available Dark Matter Simulation Code for Underground Scatterings (DaMaSCUS), a
Monte Carlo simulator of DM trajectories through the Earth taking underground
scatterings into account. Our simulation allows the precise calculation of the
density and velocity distribution of dark matter at any detector of given depth
and location on Earth. The simulation can also provide the accurate recoil
spectrum in underground detectors as well as the phase and amplitude of the
diurnal modulation caused by this shadowing effect of the Earth, ultimately
relating the modulations expected in different detectors, which is important to
decisively conclude if a diurnal modulation is due to dark matter or an
irrelevant background.
