Nathan Isgur, a theoretical physicist who made landmark contributions to the physics of quarks in hadrons, died on 24 July 2001 in Williamsburg, Virginia. For the five years before his death, Nathan had been fighting multiple myeloma, a rare cancer of the bone marrow. Nathan was the chief scientist of the Thomas Jefferson National Accelerator Facility at Newport News, Virginia.
Nathan was born in South Houston, Texas, on 25 May 1947. He went to Caltech, intending to major in molecular biology. However, exposure to the Feynman Lectures on Physics , Feynman himself, and a poor memory for chemical names led Nathan to switch to physics, in which he obtained his BS in 1968. This was the era of the Vietnam War, and Nathan decided to go to the University of Toronto for a PhD in particle theory, which he received in 1974. His ability to travel freely was limited until 1977, when President Carter issued a pardon.
After obtaining his PhD, Nathan was forced to stay in Canada because of passport problems. Robin L. Armstrong, who chaired the physics department at Toronto, and other members of the department recognized Nathan’s outstanding talent and allowed him to stay as a postdoctoral fellow. In 1976, Nathan joined the faculty as an assistant professor. These circumstances helped one of us (Karl) start, in 1977, a long collaboration with Nathan on the physics of baryons in the quark model. It was natural to extend to excited baryons the ideas of quantum chromodynamics (QCD) previously applied to ground states; a few new ingredients were needed. Without any prior knowledge of excited baryons, and nominally the junior collaborator, Nathan had the insight, surefootedness, and drive that made a success of the enterprise. In addition to formula derivations, there was a lot of numerical work. For example, Nathan would diagonalize 5 × 5 matrices approximately, by hand, to check the computer output of his collaborator. The QCD-improved quark model for baryons was successful and remains the benchmark.
Simultaneous with this work on baryons, Nathan taught undergraduate and graduate courses—mentoring five to six graduate students at a time—advised experimentalists, organized seminars, and arranged picnics, among other activities. He was a superb teacher and lecturer at all levels. While on the faculty at Toronto, he mentored 14 PhD students; most of them now have faculty positions. The research with his students was also at the interface between particle physics and nuclei. The University of Toronto became a center of quark physics. Nathan helped organize memorable conferences in Toronto in 1980 and in the Yukon in 1984.
While on sabbatical and other leaves at Oxford University in the 1980s, Nathan collaborated with Jack Paton on flux-tube models for gluons in hadrons. Their model made predictions for new excited hadrons, which remain to be confirmed. In another notable collaboration at Oxford, with Chris Llewellyn Smith, the applicability of perturbative QCD to exclusive processes was discussed in detail and some results in the literature were criticized.
Nathan’s most celebrated work was with Mark Wise, one of his undergraduate students at Toronto who was by then at Caltech. Isgur and Wise studied semileptonic decays of mesons containing a charm or beauty quark; this research led to the discovery of heavy quark symmetry in QCD. This symmetry, which becomes exact in the limit of infinite quark mass, allows an economical description of many heavy meson decays. Two of their seminal papers received more than 1000 citations. Their discovery also led to the award of the American Physical Society’s J. J. Sakurai Prize for Theoretical Particle Physics to Nathan Isgur, Mark Wise, and Misha Voloshin (see Physics Today, Physics Today 0031-9228 54 4 2001 81 https://doi.org/10.1063/1.2405626 April 2001, page 81 ).
In 1990, Nathan moved from Toronto to Jefferson Lab to assume leadership of the theory group. He was attracted by the opportunities to build a group of theorists of his own choosing and to play a role in guiding the experimental program of the new facility.
Nathan recognized that Jefferson Lab was well suited to answer open questions relating QCD to nuclei. He focused the experimental program on these key issues by always asking, “What do we learn from this experiment?” Nathan was very effective in such discussions because of his ability to express basic ideas in simple ways. This same ability was a great asset in contacts with public, state, and congressional bodies. Because of these activities, Nathan was effectively the chief scientist at Jefferson Lab; this became his official title in 1996.
Nathan was also concerned with building ties with physics groups in the Southeast region of the US. Through joint appointments with local universities, he doubled the number of positions in the theory group; Jefferson Lab extended his approach to experimental appointments. Nathan also instituted a program of bridged positions, which allowed physics departments to recruit young nuclear physicists for future faculty positions. These programs resulted in more than 60 new physics faculty positions in the Southeast. Nathan devoted a great deal of effort to these programs and was very pleased when they were imitated elsewhere.
When Nathan was diagnosed with his illness, he realized there would not be enough time to record all of his ideas related to physics. In addition to his work as chief scientist and head of theory, Nathan began to publish at an accelerated rate, producing some 10 papers in refereed journals in the four years before his death and leaving about seven preprints in the process of publication. During his last two years at Jefferson Lab, he also established a lattice QCD collaboration with MIT. Nathan’s determination was heroic.
A great enthusiast for the outdoors and for baseball, Nathan never lost his Eagle Scout spirit of adventure. He was very competitive, even in endeavors such as hiking in the desert, climbing mountains, and taking canoe trips. This same spirit also showed up in research, in which a calculation was often a little competition with his collaborators (Nathan usually won). When physics controversies ensued, Nathan defended his position skillfully and with vigor.
Nathan was a Fellow of the Royal Society of Canada and received many honors both in Canada and in the US. He was very devoted to his wife and his two sons.
We lost a very special friend. Physics lost a great leader and teacher.
