Solar neutrino events
Solar neutrino interactions are qualitatively similar to fully-contained events at higher energy: the neutrino interaction occurs inside the detector, and no particles exit. However the distinguishing feature of the solar neutrino data is its low energy. As a result, natural radioactivity in the detector itself produces events which appear indistinguishable from the sought-after signal. By using the known position of the Sun at the time of each possible solar neutrino event, however, a clear excess of low-energy contained events from the direction of the Sun is evident. Using this directional information, the background (appears in all directions) can be subtracted from the signal in the direction of the Sun to measure the number of neutrinos which truly originate from the Sun. This rate is only 36% of that expected from models of the reactions which cause the Sun to shine. Recent measurements of the Sun's dynamics (helio-seismology) appear to leave little room for error in these models, at least not at the level required to explain the data. Hence, electron neutrinos produced in the Sun are also disappearing en route to the Earth.