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How did neutrino oscillations help explain the solar neutrino problem?

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laser-activated fusion — nuclear fusion Interior of the U.S. Department of Energy's National Ignition Facility (NIF), located at Lawrence Livermore National Laboratory, Livermore, California. The NIF target chamber uses a high-energy laser to heat fusion fuel to temperatures sufficient for thermonuclear ignition. The facility is used for basic science, fusion energy research, and nuclear weapons testing.

The solar neutrino problem was a long-standing discrepancy in astrophysics where the number of electron-neutrinos detected from the Sun was significantly lower than predicted by solar models.12

For years, scientists debated whether this was due to an incomplete understanding of solar nuclear reactions or a fundamental property of the neutrino itself. The latter hypothesis suggested that neutrinos could change their "flavor"—oscillating between electron, muon, and tau types—as they traveled from the solar core.123

This theory was confirmed in 2002 by the Sudbury Neutrino Observatory (SNO). By using heavy water to detect all three neutrino flavors, researchers demonstrated that while the count of electron-neutrinos remained low, the total number of all neutrino types matched the theoretical predictions. This proved that electron-neutrinos had oscillated into muon or tau neutrinos during their journey. Because these oscillations are only physically possible if neutrinos possess a non-zero mass, this discovery resolved the solar neutrino problem and fundamentally altered our understanding of particle physics.213