The Wu experiment was a particle and 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 conservation of parity, which was previously established in the electromagnetic and strong interactions, also applied to weak interactions. If parity conservation were universal, particle decays governed by the weak interaction would behave similarly to particle decays involving the other interactions. A parity transformation negates the coordinates in a theory. If the predictions of the theory are not altered, it is said to "conserve parity". A parity transformation creates a mirror image. In mirror images objects spinning clockwise appear to spin counterclockwise. The Wu experiment created an atomic system with spin, then compared weak-interaction particle decay for spins clockwise and anti-clockwise.

The experiment established that conservation of parity was violated by the weak interaction, thus providing a way to operationally define left and right. This result was not expected by the physics community, which had previously regarded parity as a symmetry that applied to all forces of nature. 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. While not awarded the Nobel Prize, Chien-Shiung Wu's role in the discovery was mentioned in the Nobel Prize acceptance speech of Yang and Lee, but she was not honored until 1978, when she was awarded the first Wolf Prize.

The modern concept of parity was developed in 1927 when Eugene Wigner formalized the principle of the conservation of parity (P-conservation). The motivation for this theoretical principle was empirical observations by Otto Laporte for iron and Henry Norris Russell for titanium. They noted that certain transitions between atomic energy levels are not observed in experimental spectra, a concept now called a selection rule. Wigner showed that the atoms have two kinds of levels. Those of positive parity have the same functional form if the spatial coordinates are reversed (x → −x, y → −y, and z → −z). Those of negative parity change the sign of this form under this reversal of coordinates. Transitions between levels of the same parity are not observed because any emitted photon will have negative parity. The products of the parities before and after the transition have to be equal to "conserve" parity.

This principle was widely accepted by physicists because it simply reflected the intuitive idea that our world and its mirror image would behave in the same way, with the only difference being that left and right would be reversed (for example, a clock that spins clockwise would spin counterclockwise if a mirrored version of it were built). P-conservation was experimentally verified in the electromagnetic and strong interactions. However, during the mid-1950s, two different decays of seemingly identical kaon particles were observed. The "τ" kaon decayed into three pions, but the "θ" kaon decayed into two pions. The pion was known to have odd parity (–1): unless the relative motion of the products was unusual, decay into three pions predicted odd "τ" kaon parity [(–1)³ = –1], but decay into two pions predicted even "θ" kaon parity [(–1)² = 1].

As more data was accumulated on the properties of the decay products, it became clear that every property of these particles was identical except for their parity. It seemed possible that these were the same particle except for this violation of parity. This was known as the τ–θ puzzle.

Theoretical physicists Tsung-Dao Lee and Chen-Ning Yang did a literature review on the question of parity conservation in all fundamental interactions. They concluded that in the case of the weak interaction, experimental data neither confirmed nor refuted P-conservation. This information was missing for two reasons: first, the weak interaction is revealed in beta decay and the whole process must be studied for parity conservation and second, the beta decay involves a neutrino whose mass cannot be measured directly, preventing some approaches to parity checks. In the summer of 1956 Lee and Yang approached Chien-Shiung Wu, who was an expert on beta decay spectroscopy, with various ideas for experiments. They settled on the idea of testing the directional properties of beta decay in cobalt-60. Wu understood the potential for a breakthrough experiment and began work in earnest at the end of May 1956, cancelling a planned trip to Geneva and the Far East with her husband, wanting to beat the rest of the physics community to the punch. Most physicists, such as close friend Wolfgang Pauli, thought it was impossible and even expressed skepticism regarding the Yang–Lee proposal.

The key problems that needed to be solved for a successful experiment was creating and maintaining extreme cryogenic temperature while measuring beta decay from nuclei with spin. Wu had to contact Henry Boorse and Mark W. Zemansky, who had extensive experience in low-temperature physics, to perform her experiment. Boorse and Zemansky suggested that Wu contacted Ernest Ambler, of the National Bureau of Standards. Ambler arranged for the experiment to be carried out in 1956 at the NBS low-temperature laboratories. After several months of work to overcome technical difficulties, in December 1956 Wu's team observed an asymmetry that indicated parity violation.

Lee and Yang, who prompted the Wu experiment, were awarded the Nobel Prize in Physics in 1957, shortly after the experiment was performed. Wu's role in the discovery was mentioned in the prize acceptance speech. Many were outraged that she had been overlooked for the prize, from her close friend Wolfgang Pauli, to Lee and Yang, with 1988 Nobel Laureate Jack Steinberger labeling it as the biggest mistake in the Nobel committee's history. Wu did not publicly discuss her feelings about the prize, but in a letter she wrote to Steinberger, she said, "Although I did not do research just for the prize, it still hurts me a lot that my work was overlooked for certain reasons." She was not honored until 1978, when she was awarded the inaugural Wolf Prize.