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How was the solar neutrino problem solved?

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Illustration showing the eight planets of the solar system in order from the Sun-Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune-depicted to scale by size, with labeled diameters in miles and kilometers. The Sun is partially visible on the left for comparison. — Solar system The planets (in comparative size) in order of distance from the Sun.

The solar neutrino problem was a long-standing discrepancy where the number of electron-neutrinos detected from the Sun was significantly lower than theoretical predictions.12

The problem was solved in 2002 by the Sudbury Neutrino Observatory (SNO) in Ontario, Canada. While earlier experiments could only detect electron-neutrinos, the SNO used a detector filled with heavy water to observe all three known flavors of neutrinos: electron, muon, and tau.132

The results demonstrated that while the count of electron-neutrinos remained lower than expected, the total number of all three neutrino types matched the predictions of solar models. This confirmed that electron-neutrinos undergo "oscillations," changing their flavor as they travel from the Sun. Because these oscillations can only occur if a particle has mass, the discovery proved that neutrinos possess a small, non-zero mass, overturning the previous assumption that they were massless.231