Spin-dependent dark matter-electron interactions
C.-P. Liu, Chih-Pan Wu, Jiunn-Wei Chen, Hsin-Chang Chi, Mukesh Kumar Pandey, L. Singh, H. T. Wong
Abstract
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: ${\ensuremath{\sigma}}_{e}^{(\mathrm{SD})}<{10}^{\ensuremath{-}41}--{10}^{\ensuremath{-}40}\text{ }\text{ }{\mathrm{cm}}^{2}$, are comparable to the ones drawn on DM-neutron and DM-proton at slightly 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.