We revisit the WIMP-type dark matter scattering on electrons that results in atomic ionization and can manifest itself in a variety of existing direct-detection experiments. Unlike the WIMP-nucleon scattering, where current experiments probe typical interaction strengths much smaller than the Fermi constant, the scattering on electrons requires a much stronger interaction to be detectable, which in turn requires new light force carriers. We account for such new forces explicitly, by introducing a mediator particle with scalar or vector couplings to dark matter and to electrons. We then perform state-of-the-art numerical calculations of atomic ionization relevant to the existing experiments. Our goals are to consistently take into account the atomic physics aspect of the problem (e.g., the relativistic effects, which can be quite significant) and to scan the parameter space - the dark matter mass, the mediator mass, and the effective coupling strength - to see if there is any part of the parameter space that could potentially explain the DAMA modulation signal. While we find that the modulation fraction of all events with energy deposition above 2 keV in NaI can be quite significant, reaching ∼50%, the relevant parts of the parameter space are excluded by the XENON10 and XENON100 experiments.
Bibliographical noteFunding Information:
The authors would like to thank J. Berengut, R. Budnik, A. Derevianko, G. Gribakin, R. Lang, M. Schumann, and I. Yavin for helpful discussions. This work was supported by the Australian Research Council, the Perimeter Institute for Theoretical Physics, and National Science Foundation Grant No. PHY-1506424. B.M.R., V.V.F., and M.P. are grateful to the Mainz Institute for Theoretical Physics (MITP) for its hospitality and support. B.M.R. is grateful to the Perimeter Institute for Theoretical Physics, where part of this work was completed, for its hospitality and financial support. M.P. gratefully acknowledges the support of the Gordon Godfrey fellowship and UNSW Australia. Research at the Perimeter Institute is supported by the Government of Canada through Industry Canada and by the Province of Ontario through the Ministry of Economic Development & Innovation.
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