New Prospects for Real-Time Spectroscopy of Low Energy Electron Neutrinos from the Sun

Flavor-specific real-time spectroscopy of solar electron neutrinos $\left({\nu }_e\right)$ from the $p+p$, ${}^7\mathrm{Be}{+e}^{-}$, and CNO reactions in the Sun may become practical for the first time via a newly discovered class of ${\nu }_e$ reactions. Highly specific signatures of ${\nu }_e$ capture by the targets ${}^{176}$Yb, ${}^{160}$Gd, or ${}^{82}$Se can discriminate against a background $\sim {10}^7$ times larger than the ${\nu }_e$ signals. A high-quality liquid scintillator containing up to $\sim 15\mathrm{wt}\%$ Yb has been developed for observing these ${\nu }_e$ reactions. Experiments based on the new approach can, in principle, resolve basic puzzles set by current solar neutrino results and the underlying question of ${\nu }_e$-flavor conversion.