The Solar Neutrino
A story about the missing neutrinos and the sun that was not broken.
The prediction
John Bahcall calculated the solar neutrino flux in 1964. He took the standard solar model and computed how many electron neutrinos the sun should produce per second. The answer was approximately six times ten to the tenth per square centimeter at Earth's distance. This was a prediction of nuclear physics, refined by solar astrophysics. Change the solar model in physically reasonable ways and the flux changes by at most ten percent.
The experiment
Raymond Davis Jr. built a detector: a tank of 615 tons of cleaning fluid placed 1,480 meters underground in the Homestake gold mine. Neutrinos interact so weakly that you need a lot of target atoms. The detection reaction was simple — an electron neutrino hits a chlorine-37 nucleus, transforms it into argon-37, and emits an electron. Argon-37 is radioactive with a 35-day half-life. Every few weeks, Davis flushed the tank with helium gas, extracted the argon, and counted its decays.
The result
Davis detected approximately one neutrino per day. The prediction was about one and a half to two per day. By a factor of three. The discrepancy was real. It was not experimental error. Davis spent thirty years on it.
The crisis
The solar model was trusted. The nuclear physics was understood. The experiment was carefully done. So either the sun was wrong — the standard solar model incorrect — or the neutrinos were wrong — they were changing flavor in ways the Standard Model did not account for.
Either option was radical. Rejecting the sun was like rejecting the best-understood star in the universe. Breaking the Standard Model was like breaking the foundation of particle physics.
The resolution
For twenty-five years, the discrepancy hung over both fields. Then SNO in Canada used heavy water to detect all three flavors of neutrino, not just electron neutrinos. Their 2001 result was definitive: the total neutrino flux matched Bahcall's prediction. The electron neutrino flux was one third. The other two-thirds had changed flavor.
The sun was fine. The neutrinos had told the truth in a language the detectors could not speak.
The coda
Borexino measured CNO neutrinos directly in 2020. Their flux was consistent with the standard solar model. The sun is still fine. The neutrinos are still changing. The solar neutrino problem is solved. The neutrino mass problem is not. The sun keeps burning. The detectors keep waiting.