New prospects for real time spectroscopy of low-energy electron neutrinos from the sun

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