Arthur B. McDonald

Arthur B. McDonald is a Canadian physicist who shared the 2015 Nobel Prize in Physics for showing that solar neutrinos change flavor, proving neutrinos have mass.

Takaaki Kajita: A Japanese physicist who shared the 2015 Nobel Prize in Physics for showing that neutrinos oscillate. His atmospheric-neutrino findings independently established oscillation alongside McDonald’s solar-neutrino result.
Neutrino oscillation: The quantum-mechanical process in which a neutrino changes among electron, muon, and tau flavors as it travels. McDonald’s measurements showed that solar neutrinos arrive on Earth in altered flavor proportions.
Solar neutrino problem: The historical discrepancy between predicted and detected numbers of neutrinos produced by the Sun. McDonald’s observatory resolved the deficit by showing that detectors had missed non-electron flavors.
Neutrino mass: The nonzero rest mass carried by neutrinos, which is not specified by the original Standard Model of particle physics. Oscillation requires neutrino mass differences, making McDonald’s discovery evidence that neutrinos are massive.
Raymond Davis Jr.: An American physicist who led an early experiment that detected solar neutrinos in a deep mine. His unexpectedly low solar-neutrino count set the puzzle McDonald’s experiment helped resolve.
Sudbury Neutrino Observatory: An underground Canadian observatory that detected solar neutrinos using a heavy-water detector. Its flavor-sensitive reactions let McDonald’s team compare total neutrinos with electron neutrinos.
Homestake experiment: Raymond Davis Jr.’s pioneering experiment that measured solar neutrinos in a South Dakota gold mine. Its low detection rate began the solar-neutrino problem that McDonald’s work addressed.
Standard Model: The quantum field theory describing known elementary particles and three of the fundamental forces. Neutrino mass revealed a limitation of its original formulation.
John Bahcall: An American astrophysicist who developed influential theoretical predictions of solar-neutrino fluxes. His calculations made the discrepancy between predicted and detected solar neutrinos measurable.
Heavy water: Water in which ordinary hydrogen is replaced by deuterium, an isotope containing a proton and neutron. Deuterium enabled the observatory to detect solar neutrinos through both charged- and neutral-current reactions.

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