Terrestrial effects on dark matter-electron scattering experiments

A well-studied possibility is that dark matter may reside in a sector secluded from the Standard Model, except for the so-called photon portal: kinetic mixing between the ordinary and dark photons. Such interactions can be probed in dark matter direct detection experiments, and new experimental techniques involving detection of dark matter--electron scattering offer new sensitivity to sub-GeV dark matter. Typically however it is implicitly assumed that the dark matter is not altered as it traverses the Earth to arrive at the detector. In this paper we study in detail the effects of terrestrial stopping on dark photon models of dark matter, and find that they significantly reduce the sensitivity of XENON10 and DAMIC. In particular we find that XENON10 only excludes masses in the range (5--3000) MeV while DAMIC only probes (20--50) MeV. Their corresponding cross section sensitivity is reduced to a window of cross sections between (510^-38--10^-30) cm^2 for XENON10 and a small window around 10^-31 cm^2 for DAMIC. We also examine implications for a future DAMIC run.