CNO Solar Neutrinos with Borexino - A Quest For Ultra-Low Background

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Solar neutrino research has already discovered massive neutrinos and flavor mixing and is moving into an era where precision measurements can provide a unique probe of the structure and composition of the Sun. The carbon-nitrogen-oxygen (CNO) cycle is one of the fusion reactions by which stars convert hydrogen to helium, releasing electron-neutrinos. The CNO neutrino flux depends on the abundance of carbon in the core of the Sun. A measurement of the CNO flux would be the first direct measurement of the solar metallicity (the abundance of elements other than hydrogen or helium) in the core of the Sun. It could test the fundamental assumption of homogeneity in the Sun and provide crucial information regarding opacities in the solar interior that are important for helioseismology. Such studies could thus have a great impact on our understanding of the Sun, stellar astrophysics, the link between planetary and stellar formation, as well as basic neutrino physics. Borexino is a liquid scintillator detector, located at the Gran Sasso National Laboratory in Italy, with a unique sensitivity to solar neutrinos.The Borexino research program has involved many undergraduate and graduate students. Mentoring and training these younger researchers from Princeton and other schools is a high priority. The group is actively promoting women in science through Princeton Women in Physics.This award provides funding for the Princeton U. group to continue to improve the sensitivity and to assist in the operation of the Borexino detector. Borexino's sensitivity to solar neutrinos is due primarily to the extremely low background made possible after reducing backgrounds in the liquid scintillator by re-purification. The methods and equipment for purification operations were provided by Princeton, as was the staff that managed the operations. Further reduction of the scintillator background is possible with modest improvements of the purification system and with the additional scintillator purification operations planned here. With these changes, Borexino will be in a good position to make the first measurement of CNO neutrinos. Future data with lower background will also result in improvements of the earlier Borexino measurements of pp, pep, Be-7, and B-8 neutrinos. The future measurement of all solar neutrinos will map out with higher sensitivity the vacuum-to-matter-effect dependence of neutrino oscillations on the neutrino energy, as well as search for non-standard interactions.