The solar neutrino problem - an update

The B-8 solar neutrino flux that has recently been measured by Super-Kamiokande is consistent with the Ar-37 production rate in Cl-37 at Homestake. The gallium solar neutrino experiments, GALLEX and SAGE, continue to observe Ge-71 production rates in Ga-71 that are consistent with the minimal signal expected from the solar luminosity. The observed B-8 solar neutrino flux is in good agreement with that predicted by the standard solar model of Dar and Shaviv with nuclear reaction rates that are supported by recent measurements of nuclear fusion cross sections at low energies. But, the signals measured by Super-Kamiokande, SAGE and GALLEX leave no room for the contribution from the expected Be-7 solar neutrino flux. This apparent suppression of the Be-7 solar neutrino flux can be explained by neutrino oscillations and the Mikheyev-Smirnov-Wolfenstein effect, although neither a flavor change, nor a terrestrial variation, nor a spectral distortion of the B-8 solar neutrino flux has been observed. Detailed helioseismology data from SOHO and GONG confirm the standard solar model description of the solar core, but helioseismology is insensitive to the fate of Be-7 in the sun and the production rates of Be-7, CNO and B-8 neutrinos. Thus, helioseismology and the solar neutrino problem do not provide conclusive evidence for neutrino properties beyond the standard electroweak model. The solar neutrino problem may still be an astrophysical problem. The deviations of the experimental results from those predicted by the standard solar models may reflect the approximate nature of our knowledge of nuclear reaction rates and radiation transport in dense stellar plasmas and the approximate nature of solar models. Only future observations of spectral distortions, or terrestrial modulation or flavor change of solar neutrinos in solar neutrino experiments, such as Super-Kamiokande, SNO, Borexino and HELLAZ will be able to establish that neutrino properties beyond the minimal standard electroweak model are responsible for the solar neutrino problem. (C) 1999 Elsevier Science B.V. All rights reserved.