The most recent in the American Physical Society's series of Physical Review Focus Landmark stories is about the experimental discovery of broken parity symmetry in a range of physical systems. That discovery was a precursor to later discovery of the breaking of both charge and parity, work which was recognized with the Nobel Prize in Physics this year.
A physical process has parity symmetry if it looks the same in a mirror. That isn't to say any particular event looks identical but the process overall should. So objects spinning clockwise should occur as frequently as those spinning anticlockwise. Otherwise you would see that the real world has a preference for, say, clockwise objects and the mirror world would have a preference for anticlockwise objects. That is an example of broken symmetry.
Tsung-Dao Lee of Columbia University and Chen-Ning Yang of Brookhaven National Laboratory proposed some experiments that might provide evidence of parity violation. (See the Landmark story for more details of the experiment.)
Chien-Shiung Wu of Columbia University, and who had provided guidance to Lee and Yang experiment, found parity violation in an experiment with low temperature cobalt crystals undergoing radioactive decay. Those decays were found to emit preferentially in one direction of the spin axis over the opposite. (A mirror image would appear to prefer the opposite direction because spins reverse between clockwise and anticlockwise in a mirror.)
Wu shared her remarkable result with Leon Lederman at Columbia's Nevis Cyclotron Laboratory, who, during a hectic weekend in the lab, brought in Richard Garwin and graduate student Marcel Weinrich to assist in looking for comparable results in a system of decaying muons (generated by the cyclotron's pion source). Wu, Lederman, Garwin, and their colleagues published their results back-to-back. Lee and Yang were awarded the Nobel Prize in Physics that year for their prediction.
The Landmark essay has a set of links to the original papers published in Physical Review in 1956 and 1957.
