The observed flux of neutrinos detected at Earth from the solar core is considerably less than that predicted by current models of solar nuclear fusion and our understanding of neutrinos themselves.

The Standard Solar Model is the working hypothesis of how the Sun produces its luminosity through controlled thermonuclear fusion of hydrogen into helium in its core. This model also explains what we see in other stars. According to the standard solar model, lots of neutrinos are produced in the nuclear reactions in the Sun's core, and since they interact weakly with matter, they should travel unimpeded from the core to Earth.

The current understanding of the deficit of solar neutrinos detected by experiments on Earth is related to the fact that current neutrino detectors are sensitive only to electron neutrinos. Pontecorvo in 1967 proposed that neutrinos might oscillate, or change, flavors if a mass difference existed between the three varieties of neutrinos. The theory of how such oscillations might alter the flavor of a neutrino passing through matter along its path has been worked out by Mikheyev, Smirnov and Wolfenstein (1985) and is now referred to as the "MSW effect". It is likely that the electron neutrinos produced in the reactions in the Sun's core are altered as they travel to Earth and thus the number of them that we detect does not measure the true number emitted.

Because neutrinos interact only weakly with ordinary matter, their detection is very difficult, and current neutrino "telescopes" are quite crude. Two of the most important current experiments are taking place at the Super-Kamiokande site and the Sudbury Neutrino Observatory.

Click here to the external link at the Super-Kamiokande experiment in Japan, the home page of the US Collaboration on Super-K, or the site at UC-Irvine. On June 5, 1998, the Super-K collaboration annoucned the discovery of evidence for neutrino mass.

Click here to the external link at the The Sudbury Neutrino Observatory which has lots of information about neutrinos, the solar neutrino problem and the SNO Detector itself. The first results were announced in June 2001.

In 2002, Raymond Davis and Masatoshi Koshiba were awarded the Nobel prize in physics for their independent work on measuring the solar neutrino flux.

Read what then-President Clinton had to say about neutrino research

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