Neutrino floor at ultralow threshold

By lowering their energy threshold, direct dark matter searches can reach the neutrino floor with experimental technology that is now in development. The ${}^7\mathrm{Be}$ flux can be detected with $\sim 10\mathrm{eV}$ nuclear recoil energy threshold and 50 kg/yr exposure. The $pep$ flux can be detected with $\sim 3$ ton/yr exposure, and the first detection of the CNO flux is possible with similar exposure. The $pp$ flux can be detected with threshold of $\sim \mathrm{eV}$ and only $\sim \mathrm{kg}/\mathrm{yr}$ exposure. These can be the first pure neutral current measurements of the low-energy solar neutrino flux. Measuring this flux is important for low mass dark matter searches and for understanding the solar interior.

Figure 1

Integrated number of events above a nuclear recoil threshold energy from coherent scattering due to solar neutrinos in a (left panel) Ge and (right panel) Si detector. The black solid curve is the sum of all components. The vertical dashed lines indicate the energy regions over which a particular flux component is dominant.

Figure 2

Fractional flux uncertainty ($\mathrm{\Delta }f$) on the $pp$, ${}^7\mathrm{Be}$, $pep$, and CNO components as a function of threshold nuclear recoil energy. The top row is for Ge, and the bottom row is for Si. For the $pp$, ${}^7\mathrm{Be}$, and $pep$ panels, the Borexino sensitivity is indicated. In the ${}^7\mathrm{Be}$, $pep$, and CNO panels, energy regions where the $pp$ signal dominates is shaded light grey. In the $pep$ and CNO panels, energy regions where the $pp$ signal dominates is shaded light grey, and energy regions where the ${}^7\mathrm{Be}$ signal dominates is shaded dark grey. Note the difference in energy ranges between the panels, and that the different curves in each panel correspond to different exposures.