Signatures of Earth-scattering in the direct detection of Dark Matter

TL;DR

This paper introduces an analytic model and a publicly available code to account for Earth-scattering effects in direct dark matter detection, revealing location-dependent rate modulations that could help identify dark matter properties.

Contribution

The authors develop a self-consistent analytic calculation of Earth-scattering effects on dark matter particles, including deflections, and provide the EarthShadow code for practical use.

Findings

Earth-scattering can both reduce and increase detection rates depending on location.

Earth's rotation causes daily modulation in detection rates, sensitive to latitude and interaction type.

Signatures of Earth-scattering could help confirm dark matter detection and determine interaction properties.

Abstract

Direct detection experiments search for the interactions of Dark Matter (DM) particles with nuclei in terrestrial detectors. But if these interactions are sufficiently strong, DM particles may scatter in the Earth, affecting their distribution in the lab. We present a new analytic calculation of this `Earth-scattering' effect in the regime where DM particles scatter at most once before reaching the detector. We perform the calculation self-consistently, taking into account not only those particles which are scattered away from the detector, but also those particles which are deflected towards the detector. Taking into account a realistic model of the Earth and allowing for a range of DM-nucleon interactions, we present the EarthShadow code, which we make publicly available, for calculating the DM velocity distribution after Earth-scattering. Focusing on low-mass DM, we find that Earth-scattering reduces the direct detection rate at certain detector locations while increasing the rate in others. The Earth's rotation induces a daily modulation in the rate, which we find to be highly sensitive to the detector latitude and to the form of the DM-nucleon interaction. These distinctive signatures would allow us to unambiguously detect DM and perhaps even identify its interactions in regions of the parameter space within the reach of current and future experiments.