Wu experiment

The Wu experiment was a nuclear physics experiment conducted in 1956 by the Chinese American physicist Chien-Shiung Wu in collaboration with the Low Temperature Group of the US National Bureau of Standards. The experiment's purpose was to establish whether or not conservation of parity (P-conservation), which was previously established in the electromagnetic and strong interactions, also applied to weak interactions. If P-conservation were true, a mirrored version of the world (where left is right and right is left) would behave as the mirror image of the current world. If P-conservation were violated, then it would be possible to distinguish between a mirrored version of the world and the mirror image of the current world.

The experiment established that conservation of parity was violated (P-violation) by the weak interaction. This result was not expected by the physics community, which had previously regarded parity as a conserved quantity. Tsung-Dao Lee and Chen-Ning Yang, the theoretical physicists who originated the idea of parity nonconservation and proposed the experiment, received the 1957 Nobel Prize in physics for this result.

In 1927, Eugene Wigner formalized the principle of the conservation of parity (P-conservation), the idea that the current world and one built like its mirror image would behave in the same way, with the only difference that left and right would be reversed (for example, a clock which spins clockwise would spin counterclockwise if you built a mirrored version of it).

This principle was widely accepted by physicists, and P-conservation was experimentally verified in the electromagnetic and strong interactions. However, during the mid-1950s, certain decays involving kaons could not be explained by existing theories in which P-conservation was assumed to be true. There seemed to be two types of kaons, one which decayed into two pions, and the other which decayed into three pions. This was known as the τ–θ puzzle.

...
Wikipedia