This viewgraph shows the energy budget for the generic set of nuclear reactions that account for energy generation among main sequence stars, the burning of four protons to give an alpha particle, two positrons, and two neutrinos. The maximum amount of energy the star receives as thermal energy is about 25 MeV. Neutrinos may take away a significant fraction of this energy.

The quotation at the top of this viewgraph is the only motivation for doing solar neutrino experiments that Ray Davis and I presented in our back-to-back Phys. Rev. Lett. papers [Phys. Rev. Lett. 12, 300 (1964); Phys. Rev. Lett. 12, 303 (1964)] in which we argued that a chlorine experiment was feasible and desirable. It is ironic that solar neutrino experiments have generated the most interest among physicists who want to test the properties of neutrinos. This motivation was not considered by us in our original papers and did not become a major impetus for doing solar neutrino experiments until the ideas of solar neutrino oscillations, developed by Pontecorvo, Gribov, Wolfenstein, Mikheyev, and Smirnov, became popular. The astrophysical comparison between what was known in 1964 and what has been observed so far is based upon the discussion in: ``Solar Neutrinos: Where We Are, Where We Are Going,'' ApJ 467, 475 (1996), hep-ph/9512285.

The predictions of John Bahcall and his collaborators of neutrino capture rates in the ³⁷Cl experiment are shown as a function of the date of publication.

Fortunately, there are now five new funded solar neutrino experiments (Super-Kamiokande, SNO, GNO, BOREXINO, and ICARUS). Although five may seem like a large number of new experiments, I am very concerned that we do not have sufficient redundancy to test non-standard ideas in physics independent of the solar model predictions. Redundancy is also essential to test whether unrecognized systematic errors have crept into even the most careful measurements. There is only one experiment that is planned to measure the pure neutral current reaction (the SNO measurement of deuteron disintegration, which will have some redundancy within SNO itself) and there is only one experiment (BOREXINO) planned to measure directly the crucial flux from the beryllium line (although KamLAND also may ultimately be able to measure the beryllium line). There are no funded projects for measuring individual events from pp neutrinos. although there are several very promising possibilities. No experiments are planned with sufficient precision to measure the 1.3 keV predicted shift (relative to the laboratory value) of the average energy of the beryllium neutrino line. This shift is a direct measure of the central temperature of the sun (see Phys. Rev. Lett. 71, 2369, 1993, hep-ph/9309292).

We need more experiments, especially experiments sensitive to neutrinos with energies below 1 MeV and experiments sensitive to neutral currents. My views on this subject are spelled out in "Why Do Solar Neutrino Experiments Below 1 MeV," hep-ex/0106086.

New Solar Neutrino Experiments

This viewgraph is an updated version of Table 2 from the paper ``An Introduction to Solar Neutrino Research," in ``Physics of Leptons,'' Proceedings of the XXV SLAC Summer Institute on Particle Physics. August 4-15, 1997, SLAC R-528, ed. A. Breaux, J. Chan, L. DePorcel, and L. Dixon (National Technical Information Service, U.S. Department of Commerce, 1998), pp. 181-199, hep-ph/9711358.