2001

Solar Neutrino Problem

Raymond Davis, Jr. (1914–2006), Masatoshi Koshiba (b. 1926)

The so-called standard model of twentieth-century physics provides a theory that links matter and energy through the interactions of elementary particles and forces. Familiar elements of the standard model include the electron, a fundamental particle of charge, and the photon, a fundamental particle of light. The discovery of the neutron in 1933 led to the prediction and discovery of the neutrino in 1956, and the hypothesis that neutrinos are massless elementary particles that travel at the speed of light (like photons) and that are created in three “flavors” (called electron, muon, and tau neutrinos) depending on the process and environment. Neutrino astronomy became an active way to probe high-energy processes in inaccessible places, like the interior of the Sun.

Large neutrino detectors started coming online in the 1960s in order to detect elusive neutrino particles generated in the Sun or during high-energy cosmic events such as supernova explosions. The standard model predicted that fusion of hydrogen to helium inside the Sun should create electron neutrinos that these detectors could find. Indeed, the predicted solar neutrinos were discovered, but at a rate only about one-third of what the model predicted. The “missing” neutrinos created what particle physicists called the solar neutrino problem.

Finding the solution to the solar neutrino problem became a high priority for physicists, because if it could not be solved, then the standard model might not actually be correct. Theorists began to wonder if neutrinos could change flavors, or oscillate between their different types. New higher-resolution detector experiments were built in the 1990s to improve the fidelity of the measurements, and in 2001 the new data revealed a surprise: neutrinos are not massless particles. They appear to have very small masses and travel at just slightly below the speed of light. Most important, experiments showed that they can oscillate between electron, muon, and tau flavors, and that about two-thirds of the electron neutrinos created inside the Sun do eventually convert to other flavors.

Solving the solar neutrino problem led to the Nobel Prize in Physics for Raymond Davis, Jr. and Masatoshi Koshiba in 2002. More importantly, it led to critical revisions of the standard model so that it now accounts for oscillations by neutrons and other particles.

Sunspots photographed from the Swedish Solar Telescope at La Palma, Chile. A spectrum of electromagnetic radiation and a wide variety of elementary particles such as neutrinos are produced by nuclear-fusion reactions in the Sun. The scene here spans nearly five times the diameter of Earth.