The US physicist Val Fitch, who shared the 1980 Nobel Prize for Physics with James Cronin, died on 5 February at the age of 91. Fitch and Cronin were awarded the prize for the discovery in 1964 that subatomic particles called K-mesons violate a fundamental law in physics known as CP symmetry, allowing physicists to make an absolute distinction between matter and antimatter.

Three of the most important symmetry operations in physics are charge conjugation, C, in which the particles are replaced by their antiparticles; parity inversion, P, in which all three spatial co-ordinates are reversed; and time reversal, T. In experiments conducted on the Alternating Gradient Synchrotron at the Brookhaven National Laboratory in 1964, Fitch and Cronin showed that the decay of K-mesons violated the general conservation law for weak interactions known as CP symmetry.

Violating symmetry

Fitch and Cronin studied long-lived K-mesons, which decay into a variety of particles including three pions. However, the physicists found that, in 0.2% of the cases, long-lived neutral K-mesons actually decay into pairs of charged pions. If CP symmetry were conserved, a neutral K-meson could not decay into two pions, so the very existence of this decay demonstrated that the electroweak force does not obey CP symmetry. The Nobel prize was given to Fitch and Cronin “for the discovery of violations of fundamental symmetry principles in the decay of neutral K-mesons”.

Born on 10 March 1923, Fitch worked as a technician on the Manhattan atomic-bomb project during the Second World War at Los Alamos, New Mexico. In 1948 he then graduated from McGill University with a degree in electrical engineering, before completing a PhD at Columbia University in 1954. During his PhD, Fitch designed and built an experiment to measure gamma rays emitted from atoms in which an electron is replace by a muon. After obtaining his doctorate, Fitch moved to Princeton University, where he remained for the rest of his career. He also served as president of the American Physical Society from 1988 to 1989.