For their innovative research in astrophysics leading to a revolution in understanding the properties of the elusive neutrino, the lightest known particle with mass.
John Bahcall, theorist, and Raymond Davis, Jr., experimentalist, are the scientists most responsible for the field of solar neutrino physics and neutrino astronomy. While contributions to nuclear physics and astrophysics are numerous and varied, this award honors their contribution to fundamental physics-the probable determination that the neutrino has a nonzero rest mass. Bahcall's calculations and Davis's experiments have proved that the sun is definitely powered by nuclear fusion reaction, and that electron neutrinos oscillate into many "flavors" on their way from the sun to the earth.
Dr. Bahcall pioneered the development of neutrino astrophysics in the early 1960s, when he theorized that neutrinos, because they interact so weakly with matter, provide a unique opportunity to look deep inside the Sun and test our understanding of how stars shine. He was the first person to correctly calculate, in 1963, the rate of neutrino capture by 37C1. This result was crucial to the chlorine experiment Dr. Davis was planning. He was the first person, in 1964, to propose that the source of neutrinos could be located by the detection of the recoil electrons in an electron-neutrino scattering experiment, thus allowing one to ascertain whether the neutrinos came from the Sun. This effect was finally detected in 1989. Over the years, Bahcall constructed increasingly sophisticated theoretical models of the sun using the best available nuclear reaction rates and other input physics to determine the expected number of neutrinos that should be observed from the sun. His models consider a host of effects and constitute the gold standard for solar models. They played a critical role in persuading experimenters and funding agencies to invest considerable time and money in neutrino detector experiments.
Dr. Davis was the experimentalist who, working with Bahcall's results, first showed that the earth-measured neutrino output from the sun was considerably less than had been anticipated by standard nuclear physics theory. His radiochemical chlorine detector in the Homestake mine was an heroic experiment and the first to directly detect neutrinos from the sun. Over several decades, the puzzle of why he was seeing only about 40% of the flux expected from Bahcall's calculations challenged many in the physics community. No obvious explanations were forthcoming. Dr. Davis stood by his data, however, as he progressively refined his techniques. His constancy forced the physics community to do more complete experiments.
The achievements of Bahcall and Davis are truly singular and their contributions completely entangled. Their path to success was a lonely one for the first twenty years or so. It was not until the 1980s that the combination of Bahcall's persistence with his calculations and Davis's elegant and heroic experiment that others were convinced to initiate a new generation of solar-neutrino-physics experiments-Kamiokande, SuperKamiokande, GALLEX, SAGE, and most recently Sudbury Neutrino Observatory. With its ability to detect all neutrino flavors, the Sudbury Neutrino Observatory has demonstrated convincingly that the Bahcall solar neutrino flux is correct, that the Davis experiment is correct, and that electron neutrinos produced by the sun are oscillating into another flavor. Data from the new Sudbury Neutrino Observatory, together with data from the SuperKamiokande, have confirmed Dr. Davis's data. We now have definite and deeper insight into neutrino physics: what had been only a theoretical possibility (neutrino oscillation among flavors) must now be accepted as established reality. The implications for theories of particle physics are immense.
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