he worked on elementary particle theory with a primary interest in quantum field theory and symmetry. The technical contributions for which he is recognized include a precise definition of G-parity with Res Jost, and his treatment of SU(6) symmetry breaking. He is primarily associated with two concepts that directly contributed to major breakthroughs in his field. The first was the idea of "associated production" to explain the puzzling properties of strange particles. His ideas and those of Murray Gell-Mann resulted in the idea of a quantum number called strangeness. The second concept was Pais's and Gell-Mann's theory regarding the composition of the neutral kaons, proposing that the observed states were admixtures of particles and antiparticles, having different lifetimes; this was experimentally confirmed in the following year by Lederman and collaborators. In 1956, Pais became a naturalized citizen of the United States. Pais was elected to the National Academy of Sciences in 1962. In 1963 Pais accepted a position at Rockefeller University to head the theoretical physics group while Rockefeller was in transition from being a medical institute to a university. He finished his career there as the Detlev W. Bronk professor emeritus. In 1972, Pais was elected to the American Academy of Arts and Sciences. In 1979, Pais was awarded the J. Robert Oppenheimer Memorial Prize. In 1984, Pais was elected to the American Philosophical Society. Science historian In the late 1970s Pais became interested in documenting the history of modern physics. He felt he was in a unique position to do so, having known many of the key people and with his knowledge of the language, culture, and science. Pais was perhaps best known for his biography of Albert Einstein, 'Subtle is the Lord ...' : The Science and the Life of Albert Einstein, and its companion volume, Einstein Lived Here (Clarendon Press/Oxford University Press, 1994). "Subtle is the Lord—" won the 1983 U.S. National Book Award in Science. His Inward Bound: Of matter and forces in the physical world (Clarendon Press/Oxford University Press, 1988) describes the events in physics during the preceding 100 years, and tries to explain why they happened as they did. In 1991 he published Niels Bohr's Times: In physics, philosophy, and polity (Clarendon Press/Oxford University Press, 1991) which describes the life and scientific contributions of Bohr. In 1995 he teamed with Laurie M. Brown and Sir Brian Pippard to compile a three-volume reference collection