Stephen M. Barr is an American author and professor in the Department of Physics and Astronomy at the University of Delaware, and a member of its Bartol Research Institute.
He is also President of the Society of Catholic Scientists. Barr does research in theoretical particle physics and cosmology. In 2011, he was elected Fellow of the American Physical Society, the citation reading “for original contributions to grand unified theories, CP violation, and baryogenesis.” His notable work includes co-discovering the much studied Flipped SU scheme of unification, identifying the Barr-Zee diagram as an important source of electric dipole moment for basic particles such as the electron and neutron in many theories, and proposing the so-called Nelson-Barr mechanism as a solution to the Strong CP problem. He is the author of the article on Grand Unified Theories for the Encyclopedia of Physics.
He obtained his Ph.D. in physics from Princeton University in 1978. Princeton awarded him the Charlotte Elizabeth Proctor Fellowship “for distinguished research”. He went on to do research at the University of Pennsylvania as a post-doctoral fellow (1978–80), the University of Washington as a Research Assistant Professor (1980-5), and Brookhaven National Laboratory as an Associate Scientist (1985-7), before joining the faculty of the University of Delaware in 1987. He was elected Director of the Bartol Research Institute of the University of Delaware in 2011.
