Spin-dependent dark matter-electron interactions

TL;DR

This paper investigates spin-dependent dark matter-electron interactions using atomic many-body calculations, deriving new exclusion limits from xenon and germanium detectors, and discusses potential methods to distinguish SD from SI interactions spectrally.

Contribution

It provides the first detailed analysis of spin-dependent DM-electron interactions at leading order with state-of-the-art atomic calculations and experimental data, highlighting differences from spin-independent interactions.

Findings

Best limits on SD cross section: 10^{-41} - 10^{-40} cm^2 for 0.1-10 GeV DM

SD and SI recoil spectra are indistinguishable nonrelativistically, but differ at higher energies

Relativistic effects like spin-orbit coupling affect spectral scaling at a few hundred eV

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: $\sigma_e^{\textrm{(SD)}}<10^{-41}-10^{-40}\,\textrm{cm}^2$ 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 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 background.