Phys. Rev. Lett. 108, 051302 (2012)

A star the size of the Sun is predominantly powered by the proton–proton (pp) fusion reaction, which releases a substantial flux of neutrinos. Borexino — a 278-ton liquid-scintillator detector of electron neutrinos, located at Italy's Gran Sasso laboratory —has now achieved sufficient sensitivity in a particular energy range for Gianpaolo Bellini et al. to claim the first detection of electron neutrinos from the so-called pep reaction of the pp chain.

The fusion of two protons and an electron (pep) to produce deuterium is a rare but possible step in the elaborate pp chain that produces light elements such as lithium and beryllium in the Sun. The pep reaction also releases an electron neutrino of a specific energy (1.44 MeV). The challenge to detect the relatively small flux of these neutrinos has been met in large part by the efforts of Bellini et al. to suppress the main background around that energy: beta emission from carbon-11 atoms, which are produced from carbon-12 nuclei in Borexino's organic scintillator through interactions with cosmic muons. The authors have also managed to put the tightest constraint yet on the neutrino flux from the carbon–nitrogen–oxygen cycle for fusion in stars.