Solar Neutrinos as a Signal and Background in Direct-Detection Experiments Searching for Sub-GeV Dark Matter With Electron Recoils

Direct-detection experiments sensitive to low-energy electron recoils from sub-GeV dark matter (DM) interactions will also be sensitive to solar neutrinos via coherent neutrino-nucleus scattering (CNS), since the recoiling nucleus can produce a small ionization signal. Solar neutrinos constitute both an interesting signal in their own right and a potential background to a DM search that cannot be controlled or reduced by improved shielding, material purification and handling, or improved detector design. We explore these two possibilities in detail for semiconductor (Si and Ge) and Xe targets, considering several possibilities for the unmeasured ionization efficiency at low energies. For DM-electron-scattering searches, neutrinos start being an important background for exposures larger than ~1-10 kg-years in Si and Ge, and for exposures larger than ~0.1-1 kg-year in Xe. For the absorption of bosonic DM (dark photons and axion-like particles) by electrons, neutrinos are most relevant for masses below ~1 keV and again slightly more important in Xe. Treating the neutrinos as a signal, we find that the CNS of B-8 neutrinos can be observed with ~2 sigma significance with exposures of ~2, 7, and 20 kg-years in Xe, Ge, and Si, respectively, assuming there are no other backgrounds. We give an example for how this would constrain non-standard neutrino interactions. Neutrino components at lower energy can only be detected if the ionization efficiency is sufficiently large. In this case, observing pep neutrinos via CNS requires exposures ~10-100 kg-years in Si or Ge (~1000 kg-years in Xe), and observing CNO neutrinos would require an order of magnitude more exposure. Only Si could potentially detect Be-7 neutrinos. These measurements would allow for a direct measurement of the electron-neutrino survival probability over a wide energy range.