Fred Reines

Frederick Reines and the Neutrino

Jonas Schultz and Hank Sobel

The award of the 1995 Nobel Prize in Physics to Frederick Reines provides the ultimate recognition of an extraordinary discovery and an exceptional scientific career in pursuit of fundamental knowledge.

Perhaps no scientist in history has been so intimately associated with the discovery of an elementary particle and the subsequent thorough investigation of its fundamental properties as has Fred Reines. Starting with the discovery of the neutrino, with his colleague Clyde Cowan in the mid-1950's, Reines has devoted the major part of his outstandingly productive career to the understanding of the neutrino's properties and interactions. This imposing volume of work has led to the vast enrichment of our knowledge of the role of the neutrino not only in the context of elementary particle physics, but in astrophysical processes as well. Indeed, one of the most recent achievements in Reines's long list of neutrino "firsts" was the co-discovery of neutrinos emitted from Supernova SN1987A by the IMB (Irvine-Michigan-Brookhaven) Collaboration, demonstrating conclusively for the first time the theoretically postulated role of the neutrino in stellar collapse.

The association of Fred Reines and the neutrino began in the early 1950's, when Reines and Cowan set out to perform the first detection. About twenty years before, the neutrino had been postulated by Wolfgang Pauli as a means of preserving fundamental conservation laws in radioactive decays of nuclei. Enrico Fermi suggested the name "neutrino" (i.e. "little neutral one"), distinguishing it from the neutron, and formulated a successful theory of weak nuclear processes with the neutrino as a central participant. However, the extraordinarily weak coupling of the neutrino to matter made the prospect of actually detecting this elusive particle too remote for most physicists to contemplate. There the matter stood until Reines and Cowan, starting at a reactor in Hanford, Washington and later moving to the new Savannah River Plant reactor in South Carolina, performed their definitive and ground-breaking experimental detection. This feat had two consequences: i) resolving and clarifying the highly unsatisfactory situation of a fundamental particle needed for the consistency of theory, but virtually unobservable; and ii) demonstrating the possibility and practicality of doing "neutrino physics," thus opening the door to the use of neutrinos as a sensitive probe of particle physics. Indeed, several years after the completion of the seminal work of Reines and Cowan, neutrinos were beginning to be used regularly to investigate the weak interactions, the structure of protons and neutrons and the properties of their constituent quarks. These investigations have used accelerator-produced neutrinos, as well as the reactor and cosmic ray neutrinos which Reines continued successfully to use.

Reines's studies produced a host of fundamental findings and a number of "firsts." These include: the first detection of neutrinos produced in the atmosphere; the study of muons induced by neutrino interactions underground; the first observation of the scattering of electron antineutrinos with electrons; the detection of the weak neutral current interactions of electron antineutrinos with deuterons; investigations searching for neutrino oscillations (the possibility of neutrino transformations from one type to another); and, as indicated above, the first detection of neutrinos from a supernova. These studies have had, and continue to have, most important consequences for exploring and revealing aspects of electro-weak theory. Furthermore, if neutrino oscillations do indeed occur, then the neutrino must have non-zero mass, which gives rise to extraordinarily profound cosmological implications. The Reines group's neutrino oscillation studies continued at the Savannah River Plant reactor until quite recently.

In addition to the neutrino studies mentioned above, Reines and his co-workers have pursued, for almost four decades, a related program of experiments to test some of the fundamental conservation laws of nature, including conservation of lepton number (which would be violated in the decay of an electron or neutrino, or in the change of lepton type), and conservation of baryon number, which would be manifested in the decay of the proton, as predicted in Grand Unified Theories of elementary particles. A series of increasingly sensitive tests and detection techniques were devised to investigate the validity of these conservations laws. This work, deriving directly from Reines's vision and foresight, demonstrated the feasibility and led to the development of large-scale detectors, which have produced stringent limits on the violations of these laws. Indeed, starting with his earliest studies of neutrinos and conservation laws, Reines has led the development and pioneered the use of many new techniques, including the large-scale use of liquid scintillator and water Cerenkov detectors. The IMB experiment (of which Reines was initially co-spokesman and later the sole spokesman) used an 8,000 ton water Cerenkov detector in a salt mine near Cleveland, Ohio, to set the best limits on the lifetime of the proton, thus significantly constraining allowed particle theories.

The IMB detector was also used to study neutrino physics, employing primarily neutrinos produced by interactions of cosmic rays in the atmosphere, and it was its impressive size and neutrino detection capability which led to the historic detection of the burst from the supernova SN1987A, and what is generally regarded as the birth of the new field of neutrino astronomy. Nothing could be more fitting than that the program and effort which culminated in the observation of neutrinos from SN1987A was led by Fred Reines. And there could be no more fitting tribute than the 1995 Nobel Prize to recognize the extraordinary association of Fred Reines and the neutrino.