Abstract
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.
Highlights
Dark matter (DM) direct-detection experiments typically search for recoiling nuclei from DM-nucleus scattering events
Since a significant uncertainty in estimating the solar neutrino background is how much ionization is generated by low-energy nuclear recoils, ENR ≲ 1 keV, we present our results under different assumptions for the low-energy ionization efficiency
We find that a 1-electron threshold sets the best limit for almost all DM masses; we present the results for a 2-electron threshold in Appendix B, which may be easier to achieve in future experiments
Summary
Dark matter (DM) direct-detection experiments typically search for recoiling nuclei from DM-nucleus scattering events. Direct-detection experiments could measure the 8B spectrum and (depending on the ionization efficiencies in silicon, germanium, and xenon) probe lower energies than existing SNO measurements (SNO detects 8B neutrinos via CNS that break apart a deuteron via neutral-current, inelastic scattering; there was no spectral information [36]). Combining this with existing Borexino measurements of 8B neutrinos scattering elastically off electrons, which only probes the electron-neutrino component of the solar flux, the electronneutrino survival probability can be directly measured as a function of energy. In Appendix C, we briefly discuss searches for a neutrino magnetic moment
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