solar neutrino problem
The Sun shines because deep in its core hydrogen is fused into helium, and that nuclear furnace must, by simple bookkeeping, pour out a flood of electron neutrinos. In the late 1960s an experiment buried deep in a mine first managed to count some of these solar neutrinos. The trouble was the count came up far too low — only about a third of what the well-understood physics of the Sun predicted. Where had the other two-thirds gone?
This shortfall, which stubbornly persisted across many experiments for three decades, became known as the solar neutrino problem. For a long time nobody could tell whether the fault lay with our model of the Sun, with the experiments, or with the neutrinos themselves. Detectors of the time were mostly sensitive only to electron neutrinos, so they could not tell whether the missing ones had truly vanished or had merely turned into a flavor the detector was blind to.
The resolution, around 2001 to 2002, was one of the great moments of modern physics. The Sudbury Neutrino Observatory in Canada used heavy water to count neutrinos two different ways — one sensitive only to electron neutrinos, the other to all three flavors together. The all-flavor total matched the Sun's prediction perfectly, while the electron-only count was low. The neutrinos had not disappeared at all: on the journey from the Sun's core they had changed flavor. The solar neutrino problem was the smoking gun for neutrino oscillation, and with it the proof that neutrinos have mass.
Imagine a factory ships out 100 red balls, but the receiving dock counts only 33. Did 67 vanish, or did some turn blue and green on the way? Sudbury's trick was to count balls of every colour at once — and found all 100 had arrived, just no longer all red.
Counting all flavors at once showed the neutrinos were transformed, not lost.
The Sun emits only electron neutrinos at birth; flavor change in space (and especially inside the dense Sun, an effect called the MSW mechanism) is what lowered the electron-flavor count seen on Earth.