# Stephen Barr

Stephen Matthew Barr (born November 28, 1953) is an American physicist who is a professor emeritus of physics at the University of Delaware. A member of its Bartol Research Institute, 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." == Career == His notable work includes co-discovering the much studied flipped SU(5) 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.