In the eight years since the first edition of this collection was prepared, the field of solar neutrinos has become widely recognized as one of the most active frontiers of particle physics. The “solar neutrino problem” has been replaced by the “solar neutrino opportunity.”

The adiabatic transformation from problem to opportunity has been driven by three important developments: *) the precise confirmation of solar model parameters by helioseismological measurements; *) the precise measurement by the Super-Kamiokande collaboration of the average rate, the time-dependences, and the energy spectrum of ⁸B neutrino interactions with ordinary water; and *) the dramatic demonstration by the SNO collaboration that the electron neutrino flux from ⁸B solar neutrinos, measured in heavy water, is less than the combined interactions of electron, muon, and tau neutrinos in ordinary water.

This volume reproduces not only the original papers, collected in the first edition, which created and clarified the solar neutrino problem, but also includes three additional papers that led to the transformation of the subject. The first of these new papers (March, 1998), entitled “How uncertain are solar neutrino predictions?,” showed that helioseismological measurements confirm the standard solar model predictions to a high precision (0.1% for the solar interior sound speeds.) This agreement between standard solar model predictions and helioseismological measurements demonstrated that the long-standing discrepancies between solar model predictions and solar neutrino measurements could not be explained away as inadequacies in the standard solar model.

In the second paper (June 2001), entitled “Solar ⁸B and hep Neutrino Measurements from 1258 Days of Super-Kamiokande data,” the Super-Kamiokande collaboration reported the results of more than 18,000 neutrino-electron scattering events, increasing the number of previously reported solar neutrino events by more than an order of magnitude. These results provided precise information about the the total scattering rate, the recoil electron energy spectrum, and the day-night and other temporal dependences.

The third paper (August 2001), entitled “Measurement of the Rate of _e + d p + p + e^- Interactions Produced by ⁸B Solar Neutrinos at the Sudbury Neutrino Observatory,” describes the results of an epochal measurement of the electron-neutrino flux from ⁸B solar neutrinos. In this paper, the SNO collaboration showed that the _e flux that is measured in their heavy water detector is less than the combined flux of _e plus (detected with less efficiency) + that is measured in the Super-Kamiokande ordinary water detector. Thus _e are converted to + on the way to the Earth from the center of the Sun. In addition, the comparison of the SNO and Super-Kamiokande rates implied that the flux of active ⁸B solar neutrinos is in excellent agreement with the prediction of the standard solar model (given in the first of the three new papers and in many previous papers in this series going back in time almost four decades).

Thus the combined results of the Super-Kamiokande and SNO measurements show that: 1) new physics is required to described the propagation of solar neutrinos, and 2) the standard solar model prediction is verified to high accuracy, provided that the electron neutrino is only coupled significantly to the two known active neutrinos to + . This is an extraordinary achievement, which is built upon the contributions of thousands of previous workers whose research is described, directly or indirectly, in the original papers reprinted in this volume.

What are the challenges and the opportunities of the next decade of solar neutrino research? In the realm of particle physics, new experiments (with KamLAND and BOREXINO, and new measurements with SNO and ICARUS) are just beginning to come into operation that will help determine accurately—together with the previous solar neutrino data—the propagation characteristics (including masses and mixing angles) of solar neutrinos. One of the principal challenges will be to establish or constrain tightly the role of sterile neutrinos. In the real of astrophysics, the combined results of all of the solar neutrino experiments will provide more accurate values for the fluxes of ⁸B and ⁷Be solar neutrinos, in order to test and refine standard solar model predictions. Further precision laboratory measurements of the nuclear fusion reactions that produce ⁸B and ⁷Be isotopes in the Sun are essential in order to interpret the solar neutrino measurements with their full power.

The focus of solar neutrino research will shift gradually over the next decade from the relatively high energy ⁸B neutrinos (maximum energy greater than 14 MeV) to the low energy, ⁷Be, p-p, pep, and CNO neutrinos (energies less than or of order 1 MeV). For the standard solar model prediction, more than 98% of the solar neutrino flux lies below 1 MeV. The p-p neutrinos alone are predicted to carry about 91% of the flux, whereas the great science that has been done so far on the ⁸B neutrinos corresponds to only about 0.01% of the total predicted solar neutrino flux.

The currently favored neutrino oscillation solutions exhibit revealing energy dependencies below 1 MeV. Naturally, all of the currently favored oscillation solutions give similar predictions in the region about 5 MeV, where the Kamiokande, Super-Kamiokande, and SNO data are best. Future measurements of the low-energy total fluxes, time dependences, energy spectra, flavor composition, and sterile component (if any) will contribute in an important way to understanding the physics of neutrinos.

The standard solar model predictions¹ for the dominant low energy ⁷Be and p-p neutrinos are relatively accurate (± 1% for the p-p neutrinos and ± 10% for the ⁷Be neutrinos). Low energy solar neutrino experiments will test these fundamental predictions of the theory of stellar evolution. Improved measurements of the gallium solar neutrino capture rate with the GNO and SAGE experiments will provide important constraints on the combined capture rate of all of the _e solar neutrinos above 0.23 MeV, but these radiochemical experiments must be supported by additional experiments that measure the neutrinos from individual neutrino branches.

Redundant experiments measuring the same physics or astrophysics properties in different ways are essential to be sure that we have converged on the correct answer. The implications of the experimental results are too important for physics and for astronomy to depend upon single experiments.

J. N. Bahcall, R. Davis Jr., P. D. Parker, A. Smirnov, and R. K. Ulrich (March, 2002)

¹ “Solar Models: current epoch and time dependences, neutrinos, and helioseismological properties” (John N. Bahcall, M. H. Pinsonneault, and Sarbani Basu), ApJ, 555, 990-1012 (2001 July 10).