James Watson Cronin (born 29.09.1931) is an American experimental physicist and a member of the National Academy of Sciences (1970). He was born in Chicago. He graduated from the university in Dallas (1951), and in 1955 he received a doctorate from the University of Chicago. In 1955 — 58 he worked at Brookhaven National Laboratory and in 1958—71 at Princeton University (professor from 1965). From 1971 he has been professor at the E. Fermi Institute for Nuclear Studies at the University of Chicago.
His research concerns nuclear physics and the physics of elementary particles. He studied the decay of hyperons and the production of muons, and measured the total cross sections for pion-proton scattering. He improved the spark chamber and was one of the first to carry out real experiments with it, using it as a particle detector and obtaining high-quality photographs of tracks. Together with V. Fitch he detected (1964) the violation of CP invariance in the decay of the neutral K-meson (Nobel Prize, 1980).