Scattering of light dark matter in atomic clocks
  We present a detailed analysis of the effect of light Dark Matter (DM) on
atomic clocks, for the case where DM mass and density are such that occupation
numbers are low and DM must be considered as particles scattering off the
atoms, rather than a classical field. We show that the resulting atomic clock
frequency shifts are first order in the scattering amplitudes, and particularly
suited to constrain DM models in the regime where the DM mass $m_\chi \ll$ GeV.
We provide some rough order of magnitude estimates of sensitivity that can be
confronted to any DM model that allows for non zero differential scattering
amplitudes of the two atomic states involved in the clock.
