Shortfall in the expected number of neutrino s produced by the Sun that are detected on Earth.
Long-running experiments begun in the 1960s found only one-third to two-thirds the number of neutrinos predicted by theory to arrive from the Sun, where they are emitted as a result of nuclear fusion in the solar core. The discrepancy implied either that the theory of solar energy production was wrong or that neutrinos transformed en route to Earth in a way that made some of them seem to vanish. By the early 2000s strong evidence had been gathered in support of the latter explanation
that neutrinos "oscillate" in flight among their different types, not all of which could be detected in the experiments. For this to occur, neutrinos must have at least a tiny mass, though the specific values remained to be determined.