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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