Solar neutrino problem

The persistent deficit of detected solar electron neutrinos relative to solar-model predictions, resolved when experiments showed that neutrinos change flavor on their way to Earth.

Neutrino oscillation: The periodic change in a neutrino’s flavor as it propagates because flavor states are mixtures of mass states. Oscillations transform some solar electron neutrinos into flavors that early detectors could not see.
Raymond Davis Jr.: An American chemist and physicist who led the pioneering Homestake solar-neutrino experiment. His chlorine detector produced the first sustained measurement below solar-model expectations.
Solar neutrino: A neutrino produced by nuclear reactions in the Sun. The problem concerns the measured flux of these particles reaching Earth.
Neutrino astronomy: The study of astronomical sources through neutrinos that travel largely unimpeded from their origins. Solar measurements became an early demonstration that neutrinos can probe a star’s interior.
Neutrino mixing: The relationship between neutrino flavor states and neutrino mass states, described by a unitary mixing matrix. Mixing allows solar electron neutrinos to propagate as combinations of different mass states.
John N. Bahcall: An American astrophysicist who developed influential theoretical calculations of solar-neutrino fluxes. His predictions made the shortfall in Davis’s measurements quantitatively clear.
Solar model: A theoretical description of the Sun’s structure and evolution based on physical laws and measured properties. Solar models predicted the neutrino flux against which detector results were compared.
Neutrino mass ordering: The unresolved question of whether the third neutrino mass state is heavier or lighter than the other two. Solar matter effects constrain neutrino properties that contribute to determining the ordering.
Mikheyev–Smirnov–Wolfenstein effect: The alteration of neutrino flavor evolution by coherent forward scattering in matter. Matter inside the Sun strongly reshapes the flavor conversion of higher-energy solar neutrinos.
Homestake experiment: A radiochemical experiment that detected solar neutrinos using a tank of perchloroethylene in South Dakota. It established the original deficit by measuring only a fraction of the predicted electron-neutrino flux.

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