Preface to 2002 edition

Reading original scientific papers provides some of the intense pleasure that comes from witnessing the birth of an idea. In addition, the first papers in a subject are often among the clearest. We have selected therefore a representative collection of papers that we believe to be important in the development of solar neutrino research and which reveal the tortuous path by which the surprising scientific results have unfolded. We hope that the reader will find our selection of papers revealing and informative; we are certain that making their acquaintance will be fun and stimulating.

The motivation for studying solar neutrinos was originally to provide an experimental answer to the question: How does the sun shine? This question goes back to the 19th century struggle to reconcile the age of the sun inferred from the theory of gravitational contraction with the much longer time scales required by geological arguments. In the first half of the 20th century, the problem was solved in principle by the recognition that the amount of mass by which four hydrogen atoms are heavier than a helium atom corresponds to a large amount of energy. In the 1930's, the basic nuclear reactions responsible for this process were worked out in some detail by Hans Bethe and C. F. Weizsäcker, following upon the pioneering work of Rutherford, Atkinson and Houtermans, and Gamow among others. In the 1940's and 1950's, a strong experimental foundation was provided by many individuals who were inspired by the work of W. A. Fowler and A. G. W. Cameron to measure the properties of the nuclear fusion reactions that transform hydrogen to helium in the stars. For the reader's convenience, the Table lists some of the most important solar fusion reactions and neutrino sources. (See p. xxiii.)

We chose to begin our selection of papers in the year 1964, thirty years ago, when it first became apparent that a practical experiment to detect solar neutrinos was possible. Of course, no subject in science has a discrete beginning. In the later half of the 1950's, anti-neutrinos were detected by Reines and Cowan at the Savannah River reactor and the possibility that higher-energy neutrinos might be produced in the sun was made evident by the discovery of the larger-than-expected cross section for the ³He (, ) ⁷Be reaction by Holmgren and Johnston. These two experimental developments made it reasonable to think about detecting solar neutrinos. We have included in this collection a small number of papers written before 1964 that were essential to the development of the subject.

The four currently-operating solar neutrino experiments, described in the following pages, have brought to a successful conclusion the century-long effort to understand how the sun shines. Solar neutrinos have been detected with about the energies and the fluxes expected on the basis of theoretical models of the sun. Thousands of engineers and scientists, astronomers, chemists, and physicists, have worked together to make this dream-like result possible. The principal challenge now is to determine whether solar neutrinos are telling us something new about physics (a possibility that was not even mentioned in the two 1964 Phys. Rev. Lett. papers, Paper A.1 of Section I and Paper A.2 of Section II, in which it was argued that a chlorine solar neutrino experiment was feasible and important).

All of the editors participated in the selection of the papers in all of the areas included here. However, each of us has had the primary responsibility and the final choice in the area in which that person worked most extensively: Standard Model Expectations (Bahcall), Solar Neutrino Experiments (Davis), Nuclear Fusion Reactions (Parker), Physics Beyond the Standard Model (Smirnov), and Helioseismology (Ulrich).

We believe that a different group of experts would have selected most of the same papers that we have, but we acknowledge the unavoidable omission of many equally important works and apologize to our colleagues whose papers are not reprinted in those volume. We have tried to compensate for these omissions by giving brief introductions to each of the sections, introductions that in most cases include references to other key papers and reviews. We have also reprinted selections from a number of recent articles and reviews in order that the reader can locate relevant material on the forefront of the subject. The original papers themselves contain references to other crucial papers.

In a number of cases, we have reprinted only part of an original paper. In this way, we can give the reader a taste of a wider variety of important ideas and results. We hope that this broader exposure to the development of the field will compensate for the discontinuities that are inherent in partial reproductions. If this tactic is successful, some readers will be stimulated to read in their entirety articles that are only partially reprinted here.

Finally, for an informal, narrative history of this subject, the reader can consult the article ``An Account of the Development of the Solar Neutrino Problem'' by Bahcall and Davis (in Essays in Nuclear Astrophysics (1982) edited by Barnes, Clayton, and Schramm or reprinted in Neutrino Astrophysics (1989) by Bahcall).

Papers that are reprinted in this volume are referred to in the introductions by paper number, category letter, and area designation. Thus paper 1.A.I refers to the first paper in category A of the section I.

We have come a long way in the past thirty years. However, solar neutrino research is just beginning. Most of the quantities that need to be measured have not yet been determined. It will be fascinating to see to what extent the past, as represented in this volume, will be a good guide to the insights that the new experiments and theories will reveal.

We dedicate this volume to some of the principal pioneers in this field whose work and counsel has inspired and informed us: H. A. Bethe, A. G. W. Cameron, H. Chen, W. A. Fowler, V. Gavrin, T. Kirsten, M. Koshiba, B. Pontecorvo, F. Reines, Y. Totsuka, L. Wolfenstein, and G. Zatsepin.

J. N. Bahcall, R. Davis Jr., P. D. Parker, A. Smirnov, and R. K. Ulrich (August, 1994)