Abstract:

Borexino is a real time solar neutrino detector that has been taking solar neutrino data at the Gran Sasso laboratory in Italy since 2007. The detector measures solar neutrinos by detection of recoil electrons from neutrino-electron elastic scattering in 300 tons of liquid scintillator. A breakthrough in low background methods by Borexino enabled the first direct measurement of the flux of mono-energetic 862 keV 7Be neutrinos, followed by measurement of the 1.44 MeV pep neutrinos and 8B neutrinos. Development of methods for further reduction of background began in 2010 toward direct measurement of pp and CNO neutrinos. Current status and prospects will be presented. The measurement of solar neutrinos over a wide energy range tests the MSW theoretical prediction of a transition from vacuum to matter effect neutrino oscillations. The measurements also provide the most complete study of the nuclear fusion reactions that power the Sun, confirming the pp cycle as the primary source of energy, with the CNO cycle playing a small role. Future data may clarify the still puzzling solar metallicity problem. The methods developed for Borexino are yielding unprecedented low backgrounds that may benefit other rare event experiments, such as searches for dark matter and neutrinoless double beta decay.