  Scattering of light dark matter (LDM) particles with atomic electrons is
studied in the context of effective field theory. Contact and long-range
interactions between dark matter and an electron are both considered. A
state-of-the-art many-body method is used to evaluate the spin-independent
atomic ionization cross sections of LDM-electron scattering, with an estimated
error about 20%. New upper limits are derived on parameter space spanned by LDM
mass and effective coupling strengths using data from the CDMSlite, XENON10,
XENON100, and XENON1T experiments. Comparison with existing calculations shows
the importance of atomic structure. Two aspects particularly important are
relativistic effect for inner-shell ionization and final-state free electron
wave function which sensitively depends on the underlying atomic approaches.
