Borexino Solar Neutrino Experiment

Princeton University, Princeton NJ

Investigators

Abstract

Summary of the Borexino Solar Neutrino Experiment Borexino is a large scintillation detector designed to observe low energy neutrinos produced by the Sun. It will contain 300 tons of liquid scintillator made of materials chosen for their low content of radioactive atoms and is located in the underground Gran Sasso laboratory in Italy. Measurements to be performed will test our understanding of how the Sun produces energy and our understanding of neutrinos. The Sun is thought to derive its energy from a series of nuclear fusion reactions that convert four hydrogen nuclei into a helium nucleus, with the release of a large amount of energy (26.7 MeV). The reactions occur in the dense hot core of the sun and the energy is transferred to charged particles, gamma rays, and neutrinos. However, only the neutrinos escape into space as messengers from the hot core. The chlorine detector of Ray Davis and colleagues was the first experiment designed for solar neutrinos as a test of the solar model. Surprisingly, it recorded only about 1/3 of the neutrinos predicted. Doubts about the experiment and the solar model, on which the expected neutrino rate is based, persisted. However, after several measurements with new detectors, it became clear that both the Davis experiment and the calculated neutrino rates were correct. More recently, the Sudbury Neutrino Observatory in Canada, using heavy water, demonstrated beyond doubt that the deficit of detected solar neutrinos was due to a fundamentally new process known as "neutrino oscillations". The neutrinos are produced in the sun as "electron-neutrinos", one of the three known neutrino states, and then oscillate to other neutrino states in their journey from the Sun to Earth. Since solar neutrino experiments detect mainly electron neutrinos, the observed rates were always smaller than expected. A process proposed by Mikheyev, Smirnov and Wolfenstein (MSF effect) would enhance oscillations by the interaction of the neutrinos with electrons in the solar core. What will Borexino contribute to the story? The first goal is to test the MSW theory. A transition in the nature of neutrino oscillations is expected to occur at a neutrino energy of approximately 2 MeV. Above the transition energy, neutrino oscillations are dominated by MSW oscillations, whereas below that energy, the matter effect is small and neutrinos should oscillate by "vacuum oscillations". Borexino will be the first experiment to directly detect neutrinos below the transition energy. The goal is to measure the rate of the 0.86 MeV 7Be neutrino and the 1.44 MeV pep neutrino, for which vacuum oscillations should dominate. The SNO and SuperK experiments detected high energy neutrinos, where MSW oscillations dominate. A significant change in the survival probability of the neutrinos above and below the transition energy should provide a unique test the MSW picture. A second goal of Borexino is to test whether all the energy produced by the Sun arises from the fusion reactions. This can be done by measuring the neutrino rates from all the nuclear reactions that occur in the Sun. From the observed neutrino rates, corrected for neutrino oscillations, one determines the total power produced by the Sun from nuclear reactions. This total power is then compared to the power determined from the radiant power, the photon luminosity. A difference would imply another source of energy, or possibly a non-equilibrium condition. Borexino also has applications beyond nuclear physics and astronomy. It is an ideal detector of geophysical anti-neutrinos (produced by natural radioactivity in the earth), and supernova neutrinos. Borexino will additionally provide a sensitive search for so-called sterile neutrinos and non-standard neutrino interactions. The detector was built with prior funding from the NSF and with funds from European science agencies. The goal of this proposal is to bring the detector into full operation within the next two years and to start data acquisition; significant parts of this effort will involve graduate students and postdoctoral research associates, in addition to the faculty members involved.