|Cary Grant and Madame Wu, circa 1966|
Wolfgang Pauli didn't believe parity was not conserved, saying before the results were announced:
"I do not believe that the lord is weak left-hander, and I am ready to bet a high sum that the experiments will give symmetrical results."
Afterwards, Pauli, who would die in 1958, admitted:
"Now that the first shock is over, I begin to collect myself. Yes, it was very dramatic. On Monday, the twenty first, 8 p.m. I was supposed to give a lecture on the neutrino theory. At 5.pm. I received three experimental papers [reports on the first three tests of parity] ... I am shocked not so much by the fact that the Lord prefers the left hand as by the fact that he still appears to be left-handed symmetric when he expresses himself strongly. In short, the actual problem now seems to be the question: Why are strong interactions right-and-left symmetric?"
paraphrased from Martin Gardner's book: " The New Ambidextrous Universe: Symmetry and Asymmetry from Mirror Reflections to Superstrings", Third Ed., Chapter 22: The Fall of Parity
Chien-Shiung Wu (simplified Chinese: 吴健雄; traditional Chinese: 吳健雄; pinyin: Wú Jiànxíong, May 31, 1912 – February 16, 1997) was a Chinese-American physicist with expertise in the techniques of experimental physics and radioactivity. Wu worked on theManhattan Project (she helped to develop the process for separating uranium metal into the U-235 and U-238 isotopes by gaseous diffusion). She later performed experiments that contradicted the "Law of Conservation of Parity" and which confirmed the theories of colleagues. Her honorary nicknames include the "First Lady of Physics", the "Chinese Marie Curie", and "Madame Wu".
Although parity is conserved in electromagnetism, strong interactions and gravity, it turns out to be violated in weak interactions. The Standard Model incorporates parity violation by expressing the weak interaction as a chiral gauge interaction. Only the left-handed components of particles and right-handed components of antiparticles participate in weak interactions in the Standard Model. This implies that parity is not a symmetry of our universe, unless a hidden mirror sector exists in which parity is violated in the opposite way.
It was suggested several times and in different contexts that parity might not be conserved, but in the absence of compelling evidence these suggestions were not taken seriously. A careful review by theoretical physicists Tsung Dao Lee and Chen Ning Yang went further, showing that while parity conservation had been verified in decays by the strong orelectromagnetic interactions, it was untested in the weak interaction. They proposed several possible direct experimental tests. They were almost ignored, but Lee was able to convince his Columbia colleague Chien-Shiung Wu to try it. She needed special cryogenic facilities and expertise, so the experiment was done at the National Bureau of Standards.
In 1956-1957 Wu, E. Ambler, R. W. Hayward, D. D. Hoppes, and R. P. Hudson found a clear violation of parity conservation in the beta decay of cobalt-60. As the experiment was winding down, with double-checking in progress, Wu informed her colleagues at Columbia of their positive results. Three of them, R. L. Garwin, Leon Lederman, and R. Weinrich modified an existing cyclotron experiment, and they immediately verified the parity violation. They delayed publication of their results until after Wu's group was ready, and the two papers appeared back to back in the same physics journal.
After the fact, it was noted that an obscure 1928 experiment had in effect reported parity violation in weak decays, but since the appropriate concepts had not yet been developed, those results had no impact. The discovery of parity violation immediately explained the outstanding τ-θ puzzle in the physics of kaons.
In 2010, it was reported that physicists working with the Relativistic Heavy Ion Collider (RHIC) had created a short-lived parity symmetry-breaking bubble in quark-gluon plasmas. An experiment conducted by several physicists including Yale's Donner Professor of Physics as part of the STAR experiment, which has been smashing atoms together since 2000, showed a variation in the law of parity itself.
Intrinsic parity of hadrons
To every particle one can assign an intrinsic parity as long as nature preserves parity. Although weak interactions do not, one can still assign a parity to any hadron by examining thestrong interaction reaction that produces it, or through decays not involving the weak interaction, such as rho meson decay to pions.