Dark matter-electron scattering in materials: Sum rules and heterostructures

In recent years, a growing experimental program has begun to search for sub-GeV dark matter through its scattering with electrons. An associated theoretical challenge is to compute the dark matter scattering rate in experimental targets, and to find materials with large scattering rates. In this paper we point out that, if dark matter scatters through a mediator that couples to electromagnetic charge, then electromagnetic sum rules place limits on the achievable scattering rates. These limits serve as a useful sanity check for calculations, as well as setting a theoretical target for proposed detection methods. Motivated by this analysis, we explore how conductor-dielectric heterostructures can result in enhanced scattering rates compared to bulk conductors, for dark matter masses ???MeV. These effects could be especially important in computing the scattering rates from thin-film targets, e.g., superconducting detectors such as super-conducting nanowire single-photon detectors, transition edge sensors, or microwave kinetic inductance detectors, for which the scattering rate could be enhanced by orders of magnitude at low enough dark matter masses, as well as introducing or enhancing directional dependence.

Automated summary

In recent years, there has been an increase in experimental programs searching for sub-GeV dark matter through its scattering with electrons. However, calculating the dark matter scattering rate and finding materials with large scattering rates poses a theoretical challenge. This paper suggests that limits on achievable scattering rates can be determined through electromagnetic sum rules if dark matter scatters through a mediator that couples to electromagnetic charge. These limits not only serve as a useful sanity check for calculations, but also provide a theoretical target for proposed detection methods. In light of this analysis, the study explores how conductor-dielectric heterostructures can result in enhanced scattering rates compared to bulk conductors for dark matter masses of ???MeV. These effects could be particularly important in thin-film targets, such as superconducting detectors, where the scattering rate could be significantly increased at low enough dark matter masses and introduce or enhance directional dependence.