  Scattering of sub-GeV dark matter (DM) particles with hydrogen atoms is
studied in this paper. The interactions of DM with electrons and nucleons are
both included and formulated in a general framework based on nonrelativistic
effective field theory. On the assumption of same dark matter coupling
strengths, it is found that DM-electron interactions dominate the inelastic
atomic transitions to discrete excited states and ionization continuum around
the threshold regions, and DM-nucleon interactions become more important with
increasing energy and dominate in elastic scattering. The conclusion should
apply, qualitatively, to practical detector species so that electronic and
nuclear contributions in DM scattering processes can be disentangled, while
issues including binding effects and recoil mechanism in many-body systems will
require further detailed calculations.
