The Solar Neutrino Problem

Our understanding of energy production in the Sun is that it comes mainly from the proton-proton cycle. There are three reaction paths for proton-proton fusion which lead to the production of alpha particles, each of which liberates neutrinos. None of the other particles involved can penetrate out of the sun to be directly observed, so considerable effort has been devoted to trying to detect the solar neutrinos. In 1964, S. N. Bahcall predicted a solar neutrino flux of 5 x 10⁶ neutrinos/cm²s from solar modeling.

An early experiment consisted of a huge tank of perchloroethylene buried deep in the earth (the solar neutrino telescope). The neutrinos detected were only about a third of those expected from the best models of the Sun's interior. Since we have accurate measurements of the amount of energy released by the Sun, a factor of three change in the rate of the main production reactions is hard to explain. More recent experiments at Super Kamiokande, the SAGE and GALLEX detectors, and the Sudbury Neutrino Observatory all get about half the expected neutrino flux, so the neutrino deficiency persists.

Of the possibilities explored, some involve changes in the neutrinos themselves before they reach the detectors. The possibility that the electron neutrinos change "flavor" and were thus not detected by the current experiments will be explored with the new Sudbury Neutrino Observatory. Recent experiments at the Super Kamiokande neutrino detector in Japan have found evidence supporting this "neutrino oscillation".

Turner reports on the status of the solar neutrino flux in the December 2001 issue of Physics Today. The experimenters at the Sudbury Neutrino Observatory reported on 1000 electron neutrino events in June of 2001 and implied a flux of (1.75 +/- 0.14) x 10⁶ neutrinos/cm²s, about 35% of Bahcall's predicted flux. But correlation with the electron-scattering results at Super Kamiokande led the SNO team to calculate a flux of (5.44 +/- 1) x 10⁶ neutrinos/cm²s for all three types of neutrinos, a figure which agrees well with the Bahcall calculation.

The work which is current as of this writing is that of the Sudbury Neutrino Observatory as reported in the Scientific American article by McDonald, Klein and Wark. The fact that the detector is heavy water, providing data on neutrino scattering off deuterium, has moved us closer to the solution of the solar neutrino problem. This detector is sensitive to neutral current interactions mediated by the Z⁰ boson. These interactions are equally sensitive to all three generations of neutrinos and the early data indicates that the total of all generations of neutrinos is in agreement with Bahcall's predicted flux. This equality is taken as evidence of neutrino oscillation, and can further be taken as evidence that the neutrino's mass in non-zero.