For three decades, physicists had a problem with the Sun. They knew, from extraordinarily careful theory, how many neutrinos a star like the Sun should produce in fusing hydrogen to helium. They knew the rate to within roughly 20%. And every time anyone actually measured solar neutrinos, the answer came back wrong — far too low. The Sun, in the simplest reading of the data, was emitting only about a third as many neutrinos as theory predicted.
This was the solar neutrino problem. It was discovered in 1968, lived through hundreds of papers, several Nobel Prizes, and three generations of experiments. When it was finally solved in 2001, the solution turned out to be something stranger than anyone proposing the experiment had expected: the neutrinos were there. They had simply been changing identity on the way to Earth.
What the Sun should do
Stars work by fusing protons. In the Sun, the dominant reaction chain converts four hydrogen nuclei into one helium-4 nucleus, releasing energy as photons, kinetic energy, and — at several steps along the way — electron neutrinos. The number and energy spectrum of those neutrinos is fixed by nuclear physics and by how hot and dense the Sun’s core is.
Starting in the 1960s, the astrophysicist John Bahcall built increasingly detailed standard solar models that predicted, with steadily improving precision, how many neutrinos of each energy should arrive at Earth per second per square centimeter. Bahcall was famously confident in those numbers, and famously combative about defending them.
Davis goes underground
The experimentalist who decided to test Bahcall’s predictions was Raymond Davis Jr. Davis built one of the most peculiar telescopes in the history of astronomy: 615 tons of liquid perchloroethylene — ordinary dry-cleaning fluid — installed deep inside the Homestake gold mine in Lead, South Dakota. The mine depth (around 1,500 meters underground) shielded the detector from cosmic rays. The chlorine atoms in the perchloroethylene were the targets.
When a solar electron neutrino with enough energy interacts with a chlorine-37 nucleus, the chlorine turns into argon-37. Argon-37 is radioactive, with a 35-day half-life. By periodically purging his tank, extracting the trace argon, and counting the decays, Davis could effectively count how many solar neutrinos had passed through his detector during the run.
The math was unforgiving. A typical run lasted weeks. Davis would extract perhaps two dozen argon-37 atoms from his 615-ton tank. Every step had to be controlled to the level of single atoms. He pulled it off.
The number that wouldn’t budge
Davis published his first results in 1968. He was seeing roughly a third of the neutrinos Bahcall’s models predicted. Both men were certain their work was right. So one of them had to be wrong.
For thirty years, that question stayed open. Possibilities included:
- Bahcall’s solar models had something fundamentally wrong about the Sun’s core
- Davis’s chemistry was missing argon atoms somewhere
- Some calculation of the nuclear cross-sections was off
- The neutrinos were doing something exotic on the way to Earth
Davis kept running. Other experiments came online in the late 1980s and 1990s — Kamiokande in Japan, GALLEX in Italy, SAGE in Russia — each using different techniques and sensitive to different parts of the solar neutrino spectrum. They all saw a shortfall, though to varying degrees. The story refused to resolve.
One hypothesis, suggested by Bruno Pontecorvo and others as early as the late 1960s, was that the missing solar neutrinos had simply oscillated into other flavors during the eight-minute trip from the Sun to Earth. Davis’s chlorine detector was sensitive only to electron neutrinos. If two-thirds of the solar electron neutrinos had become muon or tau neutrinos by the time they arrived, the chlorine tank would never know.
For decades, this remained one hypothesis among many.
SNO closes the case
The decisive experiment was the Sudbury Neutrino Observatory (SNO) in Ontario, Canada. Built two kilometers underground in a former nickel mine, SNO held 1,000 tons of heavy water — water in which the ordinary hydrogen had been replaced with deuterium. Heavy water lets a detector see neutrinos in two complementary ways.
The first reaction was sensitive only to electron neutrinos, like Davis’s chlorine. The second reaction, called the neutral-current channel, was sensitive to all three flavors equally — every neutrino flavor would produce a signal at the same rate.
The result, announced in 2001 and 2002, was clean and unambiguous. The electron-neutrino flux measured by SNO matched what the chlorine and water experiments had been seeing — only about a third of what Bahcall predicted. But the total flux, summed over all three flavors, matched Bahcall’s prediction almost exactly.
The conclusion was inescapable. The solar neutrinos were not missing. Two-thirds of them had oscillated into other flavors on the trip to Earth.
Aftermath
The solar neutrino problem turned out to be one of the most productive “anomalies” in modern physics. It forced the construction of half a dozen distinct experiments. It produced the first robust evidence (combined with Super-Kamiokande’s atmospheric result) that neutrinos oscillate, and therefore that they have mass. It rewrote a chunk of the Standard Model.
It also won Nobel Prizes. Davis, with Masatoshi Koshiba and Riccardo Giacconi, shared the 2002 Prize in Physics for the detection of cosmic neutrinos. SNO’s leader Arthur McDonald shared the 2015 Prize with Super-Kamiokande’s Takaaki Kajita for the discovery of neutrino oscillation. John Bahcall, whose models had been right all along, was nominated repeatedly but never won; he died in 2005.
The lesson — that a stubborn experimental discrepancy can carry world-shaking physics inside it — is one of the durable themes of 20th-century science.
For the broader history of neutrino physics, see our full timeline. For the mechanism that resolved the problem, see How neutrino oscillation works.
Further reading
Primary sources
- Davis, Harmer & Hoffman, “Search for Neutrinos from the Sun”, Phys. Rev. Lett. 20:1205 (1968) — the original Homestake result
- Cleveland et al., “Measurement of the Solar Electron Neutrino Flux with the Homestake Chlorine Detector”, Astrophys. J. 496:505 (1998) — Davis’s final compilation
- Bahcall & Davis, “Solar Neutrinos: A Scientific Puzzle”, Science 191:264 (1976) — joint theory+experiment retrospective
Background and context
- Raymond Davis Jr.’s 2002 Nobel Lecture
- Wikipedia: Solar neutrino problem
- John Bahcall’s archived homepage at the Institute for Advanced Study — preserved by the IAS after his death, full of his solar-neutrino papers and lectures
Frequently asked
What was the solar neutrino problem?
Beginning in 1968, Raymond Davis's chlorine experiment at the Homestake gold mine measured only about a third of the solar electron-neutrino flux predicted by John Bahcall's standard solar model. The deficit persisted across multiple experiments using different techniques for three decades, becoming one of the longest-running puzzles in particle physics.
When was it solved?
Definitively in 2001, by the Sudbury Neutrino Observatory (SNO). SNO's heavy-water target let it measure both the electron-neutrino flux and the total all-flavor flux. The total matched Bahcall's prediction; the electron-neutrino component was only about a third of it. The 'missing' neutrinos had simply oscillated into other flavors during the journey from the Sun.
What did the resolution prove?
Two things simultaneously. First, that John Bahcall's standard solar model was correct — the Sun is producing as many neutrinos as predicted. Second, that neutrinos have non-zero, non-degenerate masses, since oscillation requires this. The resolution validated both decades of solar-model work and the oscillation hypothesis.
Who won Nobel Prizes for it?
Raymond Davis and Masatoshi Koshiba shared the 2002 Nobel Prize for the experimental detection of cosmic neutrinos. Takaaki Kajita and Art McDonald shared the 2015 Nobel Prize for the discovery of neutrino oscillation. John Bahcall died in 2005 and was not eligible for either prize, despite his foundational theoretical work.
Why did it take 30 years to solve?
Because distinguishing the two possible explanations — wrong solar model versus neutrino oscillation — required an experiment that could measure both the electron-neutrino flux and the total flux of all neutrino flavors. SNO's heavy-water target was the first to provide this capability. Building it took decades.
Cite this article 5 formats
APA
Neutrino Times Editorial Team. (2025, June 8). The solar neutrino problem: 30 years of missing particles from the Sun. Neutrino Times. https://neutrino-times.com/articles/solar-neutrino-problem-davis-homestake/
Chicago
Neutrino Times Editorial Team. "The solar neutrino problem: 30 years of missing particles from the Sun." Neutrino Times, June 8, 2025. https://neutrino-times.com/articles/solar-neutrino-problem-davis-homestake/.
MLA
Neutrino Times Editorial Team. "The solar neutrino problem: 30 years of missing particles from the Sun." Neutrino Times, 8 Jun. 2025, https://neutrino-times.com/articles/solar-neutrino-problem-davis-homestake/.
BibTeX
@misc{neutrino-times-solar-neutrino-problem-davis-homestake,
author = {Neutrino Times Editorial Team},
title = {The solar neutrino problem: 30 years of missing particles from the Sun},
howpublished = {Neutrino Times},
year = {2025},
month = {jun},
url = {https://neutrino-times.com/articles/solar-neutrino-problem-davis-homestake/},
note = {Accessed: 2025-06-08}
} RIS
TY - GEN TI - The solar neutrino problem: 30 years of missing particles from the Sun AU - Neutrino Times Editorial Team PY - 2025 DA - 2025-06-08 PB - Neutrino Times UR - https://neutrino-times.com/articles/solar-neutrino-problem-davis-homestake/ ER -