The Solar Neutrino Problem

For more than twenty years, the Homestake Solar Neutrino Experiment in the Homestake Gold Mine in South Dakota has been attempting to measure neutrino fluxes from space; in particular, this experiment has been gathering information on solar neutrino fluxes. The results of this experiment have been checked against predictions made by standard solar models and it has been discovered that only one-third of the expected solar neutrino flux has been detected. This "Where are the missing neutrinos?" question is known as the Solar Neutrino Problem.

And it is not just the Homestake experiment that is detecting a shortage of neutrinos. Several other experiments, including Kamiokande II, GALLEX, and SAGE, have noticed a definite neutrino shortfall.