German-American physicist who, with Willis E. Lamb, Jr. (Lamb, Willis Eugene, Jr.), was awarded the Nobel Prize for Physics in 1955 for his accurate determination that the magnetic moment of the electron is greater than its theoretical value, thus leading to reconsideration of and innovations in quantum electrodynamics.

Kusch was brought to the United States in 1912 and became a citizen in 1922. In 1937, at Columbia University, he worked with the physicist Isidor I. Rabi (Rabi, Isidor Isaac) on studies of the effects of magnetic fields on beams of atoms. He spent the wartime years in research on radar and returned to Columbia in 1946 as professor of physics, a position he held until 1972. Among other posts held by Kusch at Columbia were department chairman (1949–52, 1960–63), director of the radiation laboratory (1952–60), and academic vice president and provost (1969–72). In 1972 he took a position as professor at the University of Texas, Dallas, where he remained until his retirement in 1982.

In 1947, through precise atomic beam studies, Kusch demonstrated that the magnetic properties of the electron were not in agreement with existing theories. Subsequently, he made accurate measurements of the magnetic moment of the electron and its behaviour in hydrogen. In work characterized by great accuracy and reliability, he measured numerous atomic, molecular, and nuclear properties by radio-frequency beam techniques.