We study in detail sub-GeV dark matter scattering off electrons in xenon,including the expected electron recoil spectra and annual modulation spectra.We derive improved constraints using low-energy XENON10 and XENON100ionization-only data. For XENON10, in addition to including electron-recoildata corresponding to about $1-3$ electrons, we include for the first timeevents with $\gtrsim 4$ electrons. Assuming the scattering is momentumindependent, this strengthens a previous cross-section bound by almost an orderof magnitude for dark matter masses above 50 MeV. The available XENON100 datacorresponds to events with $\gtrsim 4$ electrons, and leads to a constraintthat is comparable to the XENON10 bound above 50 MeV. We demonstrate that asearch for an annual modulation signal in upcoming xenon experiments (XENON1T,XENONnT, LZ) could substantially improve the above bounds even in the presenceof large backgrounds. We also emphasize that in simple benchmark models ofsub-GeV dark matter, the dark matter-electron scattering rate can be as high asone event every ten (two) seconds in the XENON1T (XENONnT or LZ) experiments,without being in conflict with any other known experimental bounds. While thereare several sources of backgrounds that can produce single- or few-electronevents, a large event rate can be consistent with a dark matter signal andshould not be simply written off as purely a detector curiosity. This factmotivates a detailed analysis of the ionization-only ("S2-only") data, takinginto account the expected annual modulation spectrum of the signal rate, aswell as the DM-induced electron-recoil spectra, which are another powerfuldiscriminant between signal and background.
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