Page "Sudbury Neutrino Observatory" Paragraph 2

The first measurements of the number of solar neutrinos reaching the earth were taken in the 1960s, and all experiments prior to SNO observed a third to a half fewer neutrinos than were predicted by the Standard Solar Model.

As several experiments confirmed this deficit the effect became known as the solar neutrino problem.

Over several decades many ideas were put forward to try to explain the effect, one of which was the hypothesis of neutrino oscillations.

All of the solar neutrino detectors prior to SNO had been sensitive primarily or exclusively to electron neutrinos and yielded little to no information on muon neutrinos and tau neutrinos.