The Standard Solar Model (or SSM) predicts most aspects of our Sun's behaviour (the relationship between mass, temperature, luminousity, colour, magnetic field, etc.) to great accuracy, but has one major problem; the number of neutrinos we actually observe emenating from the Sun is approximately 1/3 that which we expect from the SSM. This is what is known as the Solar Neutrino Problem, or SNP.

The most widely accepted theory to explain this discrepancy is the existence of neutrino oscillations, by which electron neutrinos (the type emitted by the Sun, and which can be observed by most neutrino observatories) will turn into other types of neutrinos (which cannot be detected by conventional methods) as they travel. The Sudbury Neutrino Observatory is a project which hopes to prove or disprove this theory, utilizing its ability to detect neutrinos of mu and tau types.

Besides the energy released as photons (visible light and other electromagnetic radiation) from a fusion reaction, there are also many neutrinos. When these are measured on earth from fusion and collider experiments, the measurements fall within theoretical bounds showing theory to be correct. Neutrino flux from the sun, which uses the same reactions as have been tested here on earth, has been measured to be one to two thirds what it should be. This is a significant difference from theory, and has caused a lot of confusion among physicists -- and science fiction writers :-) -- since it was noticed in 1967.

Neutrinos come in three flavors, electron, muon, and tau. They are differentiated by the particle that is generated when they interact with other matter (such as an atomic nucleus) -- they release either electrons, muons or tau particles. Electron and muon neutrinos can be measured with instruments like the Super-Kamiokande and the Sudbury Neutrino Observatory, whereas tau neutrinos can only be found through the use of high-density emulsions to capture the released tau.

From observations made by the Super-Kamiokande, the reason for the missing neutrinos can only be neutrino oscillation. Super-Kamiokande measures this by taking readings on both sides of the earth, and comparing the difference. Since a negligible amount of neutrinos are absorbed by the earth, the difference describes the percentage of neutrinos that have oscillated into an undetectable form. Thus, the sun releases neutrinos in the proportion it should, but some of them oscillate into undetectable tau neutrinos before they reach earth, throwing off our measurements.

Besides being interesting because they explain the deficit of solar neutrinos, verified neutrino oscillations also mean verified neutrino mass. When suggested by Wolfgang Pauli in 1930 as an explanation for missing energy in beta decay, neutrinos were thought to have zero mass. Now, however, we see that neutrinos oscillate, and thus must have some mass due to quantum wave theory. Since neutrinos vastly outnumber every other kind of particle in the universe, their mass will be a determining factor in whether or not the universe eventually undergoes a Big Crunch. In fact, it has been shown that if neutrino mass was over 100 eV (where electrons mass something like 511,000 eV), the universe would've already collapsed. Also, neutrino mass is important because it pokes holes in the current electroweak theory of particle interaction, pointing to need of a better way of conceptualizing physics.

On June 18 2001 researchers working at the Sudbury Neutrino Observatory released their first results, announcing that they have obtained new evidence confirming the hypothesis that the sun does emit electron neutrinos as predicted by the Standard Model, but they transform into other varieties of neutrino as they travel.
The detector only registers an average of 10 events per day, so it's taken since November 1999 to get statisticaly significant results.
The fact that the transformation occurs at all shows that the Standard Model is either wrong, or at best incomplete. These observations prove that neutrinos have mass, and also provide an upper limit on what that mass can be. It was long speculated that neutrinos might make up a large fraction of the 'missing mass' of the universe, but upper energy/mass boundary is too low to allow them to make up more than a tiny fraction of the total mass of the universe.

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