William Allen Zajc /ˈzaɪts/ is a U.S. physicist and the I.I. Rabi Professor of Physics at Columbia University in New York, USA, where he has worked since 1987.
Born in Barstow, California on November 14, 1953, and raised in Brookfield, Wisconsin, he received his bachelor's degree from the California Institute of Technology in 1975. He went on to the doctoral program in physics at the University of California, Berkeley, where, as his thesis topic, he became the first person to use Hanbury-Brown Twiss correlations to measure the size of the interacting region between two colliding heavy ions.
From 1982–86 he was first a post-doctoral fellow and then a professor at the University of Pennsylvania. In 1987 he accepted a professorship at Columbia University, where he has remained as a professor ever since. He has been a scientific leader in the field of heavy ion physics since early in his career, and he has performed extensive service for the broader nuclear physics community in the U.S. William A. Zajc was named a Fellow of the American Physical Society in 1997 and a Fellow of the AAAS in 2012.
Since the 1980s, his research has focused on experiments performed at Brookhaven National Laboratory (BNL) on Long Island, New York, first at the Alternating Gradient Synchrotron (AGS) and now at the Relativistic Heavy Ion Collider (RHIC). He was co-spokesperson of the AGS E859 experiment, which investigated strangeness production in heavy ion collisions, and later spokesperson of the PHENIX experiment at RHIC from 1997 to 2006. PHENIX is a multinational collaboration with over 500 scientists from more than a dozen countries and is one of the two large experiments at RHIC. PHENIX, along with three other RHIC experiments, determined that the relativistic heavy ion collisions at RHIC were successful in creating the quark–gluon plasma (QGP), a state of matter believed to have existed approximately 10 microseconds after the Big Bang. The RHIC experiments also discovered that this matter is in fact strongly interacting and nearly a perfect fluid. Since stepping down after nine years of dedicated service as spokesperson of PHENIX, he continues his research with the experiment, further characterizing the hot, dense matter formed in the collisions.
Most recently Zajc has taught Physics C1601 and C1602, the introductory physics courses for undergraduates majoring in the sciences and engineering. He has served as Chair of the Columbia University Physics Department since 2009.