Directional dark matter detection in anisotropic Dirac materials
Dirac materials, because of unique symmetries near the Fermi surface, are a promising target for dark photon-mediated scattering and absorption of light dark matter. In this paper, we characterize the daily modulation rate of dark matter interacting with a Dirac material due to anisotropies in the crystal structure of typical Dirac materials, and we show that daily modulation is ${\cal O}(1)$ of the total rate for the Dirac material ZrTe$_5$. These modulation effects depend on the orientation of the material's Fermi velocity and permittivity tensor with respect to the dark matter wind, and they are maximized when the crystal is oriented such that the dark matter wind is completely aligned with the largest and smallest components of the Fermi velocity at two different times of the day. Because of the large modulation, any putative dark matter signal could be rapidly verified or ruled out by changing the orientation of the crystal with respect to the wind and observing how the daily modulation pattern changes.