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 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 dark matter 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 (silicon and germanium) and xenon targets, considering several possibilities for the unmeasured ionization efficiency at low energies. For dark-matter-electron-scattering searches, neutrinos start being an important background for exposures larger than ∼1–10 kg-years in silicon and germanium, and for exposures larger than ∼0.1–1 kg-year in xenon. For the absorption of bosonic dark matter (dark photons and axion-like particles) by electrons, neutrinos are most relevant for masses below ∼1 keV and again slightly more important in xenon. Treating the neutrinos as a signal, we find that the CNS of B8 neutrinos can be observed with ∼2σ significance with exposures of ∼2, 7, and 20 kg-years in xenon, germanium, and silicon, respectively, assuming there are no other backgrounds. We give an example for how this would constrain nonstandard 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 silicon or germanium (∼1000 kg-years in xenon), and observing CNO neutrinos would require an order of magnitude more exposure. Only silicon could potentially detect Be7 neutrinos. These measurements would allow for a direct measurement of the electron-neutrino survival probability over a wide energy range.