A Fast Earth-scattering Formalism for Light Dark Matter with Dark Photon Mediators

While Dark Matter (DM) is typically assumed to interact only very weakly with the particles of the Standard Model, many direct detection experiments are currently exploring regions of parameter space where DM can have a large scattering cross section. In this scenario, DM may scatter in the atmosphere and Earth before reaching the detector, leading to a distortion of the DM flux and a daily modulation of the signal rate as the detector is shielded by more or less of the Earth at different times of day. This modulation is a distinctive signature of strongly-interacting DM and provides a powerful method of discriminating against time-independent backgrounds. However, the calculation of these Earth-scattering effects by Monte Carlo methods is computationally intensive, inhibiting a systematic exploration of the DM parameter space. Here, we present a semi-analytic formalism for calculating Earth-scattering effects, for models of MeV-mass DM which interacts via a dark photon mediator, and release the associated code Verne2. This formalism assumes that DM travels along straight-line trajectories until it scatters and is reflected back along its incoming path, along us to taking into account the affects of both attenuation and reflection in the Earth. We compare this formalism with the results of full Monte Carlo simulations for cross sections within reach of current and future DM-electron scattering searches. We find that Verne2 is accurate to better than 10-30%, making it suitable for performing signal modeling in the search for daily modulation, while reducing the computational cost by a factor of $\sim10^4$ compared to full Monte Carlo simulations.