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The History of the Formulation of the Notion of the Nonconservation
of Parity in Nuclear Weak Force Phenomena and its Empirical Verification

The word parity has general meaning having to do with the interchange of right and left, but in particle physics parity has a more specific technical meaning. A particle has a wave function which is a solution of Schroedinger's equation. The squared magnitude of that wave function is the probability density function for the particle.

Suppose f(x, y, z) is the wave function over all points (x, y, z) of space. Consider what happens to f under the transformation x→−x, y→−y and z→−z. If f→f then the particle is said to have even parity represented as a quantum number of +1; if f→−f then the particle has odd parity and this is represented as a quantum number of −1.

The parity of a system of particles is the product of the parities of the particles. If the parity of a system of particles after interaction is the same as the parity of the system before interaction then parity is said to be conserved.

In examining the question of the conservation of parity in particle interaction some physicists noted the interesting case of the θ (theta} and τ (tau) meson particles. They are seemingly identical in terms of physical properties, but they have different modes of decay.

θ → π+ + π0
 
τ → π+ + π+ + π

(Note that charge is preserved.)

Thus the θ particle decays into two pions, one charged and one uncharged. The parity of one pion is −1 but the parity of two pions is +1=(−1)(−1). The τ particle decays into three pions so the parity of the decay product is −1=(−1)(−1)(−1). If the θ and τ particles have the same parity then parity conservation is violated in one of the two decay processes.

Annual conferences in high energy physics had been organized in Rochester, New York to bring theoretical and experimentalist physicists to share and debate ideas. At the 1956 such conference the θ-τ meson puzzle was on most of the conferees' mind.


Richard Feynman

Richard Feynman and Martin Block were room-mates at the conferences and debated the issues vociferously until the early morning hours. In the course of one of those debates Block said that maybe parity is not conserved. Feynman was about to disparage that idea when he stopped in mid-sentence and said, "Well, maybe you have something there."


Chen Ning "Frank" Yang

The last day of the conference was devoted to participants sharing their conclusions on the θ-τ puzzle. Frank Yang gave an introductory review. After several talks had been given, Robert Oppenheimer, the chairman of the session, was ready to close the session when several prominent physicists chose to make statements. Murray Gell Mann presented a list of approaches to the problem which he had considered, but without designating his choice. Richard Feynman brought up Block's suggestion in the form of the θ and τ mesons being the same particle but with no definite parity. Frank Yang told the meeting that he had looked into several aspects of the nonconservation of parity without reaching a conclusion.


Tsung Dao Lee

After the conference Frank Yang moved to the Brookhaven National Accelerator Laboratory at Upton on Long Island, New York. His long term friend and collaborator T.D. Lee was nearby at Columbia University. They met twice a week and formulated the θ-τ issue in the form that parity might be conserved in the nuclear strong force interactions involved in the creation of the θ and τ particles but not conserved in the weak force interactions involved in their decay.


Chien Shiung Wu

The international expert in beta decay at that time was Chien-Shiung Wu and she was at Columbia. T.D. Lee went to see her to see if she knew of any experiment that tested the question of the conservation of parity in weak force interactions. She did not know of any but lent Lee a near-thousand page compilation of the results of experiments entitled Beta- and Gamma-Ray Spectroscopy.

Yang and Lee went through that work over the course of two weeks and concluded that the conservation of parity in weak force interactions had never been tested. They subsequently wrote an article calling for the testing of the conservation of parity in weak force interactions. It was published in Physical Review in 1956.

Chien-Shiung Wu saw immediately the significance of the Yang-Lee proposals. She began designing the experiment and soliciting the collaborators necessary to carry it out. The experimental design was simple in principle. Beta emitters were to be aligned and the emission of electrons was to be checked for asymmetry in the angular distribution of ejected electrons.

Cobalt 60 was to be the beta emitter. Madame Wu knew from her previous work with beta emitters that the source had to be very thin to keep the observed distribution of electrons being distorted by the thickness of the beta source material. The beta sources had to be fastened to a crystalline material that gives off flashes of light when struck by an electron. The flashes of light would be amplified by a photomultiplier for recording.

The only way to get the Co-60 nuclei aligned was cooling them to within 0.001 K° of Absolute Zero. The facilities at Columbia could not achieve this so Wu went to the National Bureau of Standards in Washington, D.C. to seek collaborators. She gained these and set about setting up the experiment.

It was not easy technically. They could achieve the low temperature required for only about fifteen minutes at a time. One scientist slept next to the equipment to be available at any time to keep things operating.

The thin strip of Co-60 for a beta source had to be fastened to the crystal structure that gave off a spark of light when struck by an electron. They first tried fastening the beta emitter to the crystals with glue but found that came apart at the low temperature as which they had to work. They tied the Co-60 to the crystals with a nylon thread.

Two months of operation of the experiment revealed an asymmetry in the distribution of ejected electrons. At 2 AM on January 9th of 1957 the group concluded that they had enough results to establish that parity is not conserved in nuclear weak force reactions. The success of the experiment came as a great surprise to Wolfgang Pauli and Richard Feynman. They had been willing to bet money that the experiment would find that parity is preserved. Feynman actually lost $50 on his bet.

Yang and Lee were awarded the Nobel Prize in Physics in 1957. Unfortunately success destroyed their friendship. Nearly everyone feels Wu should have shared that prize, but it would have been difficult for the Nobel committee to award the prize only to the head of a team.

When all is said and done the result was less Universe-shaking than was at first thought. It appears that electrons have a definite direction of spin in the direction in which they are ejected (helicity). It is only in this sense that the Universe has a definite left-handedness.

The success of Yang, Lee and Wu speaks highly of the strength of the Chinese culture such that these three emigré were able to assimilate and excel in an alien culture.


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