Solar neutrino problem
Why did every experiment from the 1960s through the 1990s detect only about one-third to one-half of the electron neutrinos from the Sun's fusion reactions that Standard Solar Model calculations predicted?
History and references
Settled in 2001 by Sudbury Neutrino Observatory (SNO) Collaboration.
Raymond Davis Jr.'s Homestake chlorine experiment, starting in 1968, consistently measured only about one-third of the solar neutrino flux predicted by John Bahcall's Standard Solar Model, a persistent discrepancy that lasted three decades and led to proposals ranging from flawed solar models to new neutrino physics. The Sudbury Neutrino Observatory resolved it definitively in 2001-2002 by detecting all three neutrino flavors and showing the total flux matched predictions once electron neutrinos oscillating into muon and tau neutrinos en route from the Sun were accounted for, confirming neutrino oscillation and nonzero neutrino mass. Davis and Koshiba shared the 2002 Nobel Prize in Physics for this line of work.
The Sun was sending exactly as many neutrinos as predicted all along; they were just changing costume mid-flight, and it took thirty years and a mine a mile underground in Canada to catch them at it.
Explanations that circulated
- That the Standard Solar Model's predicted neutrino flux was simply wrong — the deficit was instead due to previously unknown physics (neutrino oscillation), not an error in solar modeling
References
- Measurement of the Rate of nu_e + d -> p + p + e- Interactions Produced by 8B Solar Neutrinos at the Sudbury Neutrino Observatory, SNO Collaboration (Q. R. Ahmad et al.), Physical Review Letters, 2001
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