Spin-dependent dark matter-electron interactions

Detectors with low thresholds for electron recoil open a new window to direct searches of sub-GeV dark matter (DM) candidates. In the past decade, many strong limits on DM-electron interactions have been set, but most on the one which is spin-independent (SI) of both dark matter and electron spins. In this work, we study DM-atom scattering through a spin-dependent (SD) interaction at leading order (LO), using well-benchmarked, state-of-the-art atomic many-body calculations. Exclusion limits on the SD DM-electron cross section are derived with data taken from experiments with xenon and germanium detectors at leading sensitivities. In the DM mass range of 0.1-10 GeV, the best limits set by the XENON1T experiment: o -e < 10-41-10-40 cm2, are comparable to the ones drawn on DM-neutron and DM-proton at slightly oSD) bigger DM masses. The detector's responses to the LO SD and SI interactions are analyzed. In nonrelativistic limit, a constant ratio between them leads to an indistinguishability of the SD and SI recoil energy spectra. Relativistic calculations however show the scaling starts to break down at a few hundreds of eV, where the spin-orbit effects become sizable. We discuss the prospects of disentangling the SI and SD components in DM-electron interactions via spectral shape measurements, as well as having spin-sensitive experimental signatures without SI backgrounds.

Automated summary

Detectors with low thresholds for electron recoil open a new window to direct searches of sub-GeV dark matter candidates. In this work, we study DM-atom scattering through a spin-dependent interaction at leading order using well-benchmarked atomic many-body calculations. Exclusion limits on the spin-dependent DM-electron cross section are derived with data from xenon and germanium detectors. The best limits set by the XENON1T experiment are comparable to the ones on DM-neutron and DM-proton at slightly bigger DM masses. The detector's responses to the spin-dependent and spin-independent interactions are analyzed.