This Month in Physics History

December 27, 1956: Fall of Parity Conservation

Symmetries have long played a crucial role in physics. Since 1925, physicists had assumed that our world is indistinguishable from its mirror image - a notion known as parity conservation - and prevailing scientific theory reflected that assumption. Until a series of pivotal experiments at the National Bureau of Standards in 1956 (now the National Institute of Standards and Technology), parity conservation enjoyed exalted status among the most fundamental laws of physics, including conservation of energy, momentum and electric charge. But as with relativity, Nature once again demonstrated that it is not always obliged to follow the rules of "common sense".

Parity conservation implies that Nature is symmetrical and makes no distinction between right- and left-handed rotations, or between opposite sides of a subatomic particle. For example, two similar radioactive particles spinning in opposite directions about a vertical axis should emit their decay products with the same intensity upwards and downwards. Yet although there were many experiments that established parity conservation in strong interactions, the assumption had never been experimentally verified for weak interactions. Indeed, when the weak force was first postulated to explain disintegration of elementary particles, it seemed inconceivable that parity would not hold there as well.

All that changed in the 1950s, when high-energy physicists began observing phenomena that could not be explained by existing theories, most notably the decays of K mesons emitted in the collision of a high-energy proton with an atomic nucleus. The K meson appeared in two distinct versions, decaying into either two or three pi mesons, (which necessarily had opposite parity), although in all other characteristics they seemed identical. In June of 1956, theoretical physicists Chen Ning Yang and Tsung Dao Lee submitted a short paper to the Physical Review raising the question of whether parity is conserved in weak interactions, and suggesting several experiments to decide the issue.

Lee and Yang's paper did not immediately spark more than passing curiosity among physicists when it appeared in October 1956. Freeman Dyson later admitted that while he thought the paper was interesting, "I had not the imagination to say, 'By golly, if this is true, it opens up a whole new branch of physics!' And I think other physicists, with very few exceptions, at that time were as unimaginative as I." Richard Feynman pronounced the notion of parity violation "unlikely, but possible, and a very exciting possibility," but later made a $50 bet with a friend that parity would not be violated.

One of the simplest proposed experiments involved measuring the directional intensity of beta radiation from cobalt-60 nuclei oriented with a strong magnetic field so that their spins aligned in the same direction. Parity conservation demands that the emitted beta rays be equally distributed between the two poles. If more beta particles emerged from one pole than the other, it would be possible to distinguish the mirror image nuclei from their counterparts, which would be tantamount to parity violation.

Representation of parity not being conserved. The ellipsoid on the left represents a large number of cobalt nuclei, all with their spins in the same direction, all emitting beta rays. On the right this process is seen in a mirror. The direction of spin is reversed, while the direction in which most beta rays are emitted remains unchanged. The mirror world is thus distinguishable from the real world. The parity-transformed world is not identical with the real world; parity is not conserved. [Photos from http://physics.nist.gov/GenInt/Parity/cover.html]

Between Christmas of 1956 and New Year's, NBS scientists set about performing beta decay experiments. The team was led by Columbia Professor C. S. Wu. Professor Wu had been born in China in 1912, had received her PhD from the University of California in 1940, and had worked on the Manhattan Project during World War II. In 1975 she would serve as the first woman president of the APS.

When the results were in, the NBS team arrived at a startling conclusion: the emission of beta particles is greater in the direction opposite to that of the nuclear spin. Thus, since the beta emission distribution is not identical to the mirror image of the spinning cobalt-60 nucleus, parity was unequivocally shown not to be conserved. Leon Lederman, who at the time worked with Columbia University's cyclotron, performed an independent test of parity with that equipment, involving the decay of pi and mu mesons, and also obtained distinct evidence for parity violation.

In short, Nature is a semi-ambidextrous southpaw. And Feynman lost his bet. The result shattered a fundamental concept of nuclear physics that had been universally accepted for 30 years, thus clearing the way for a reconsideration of physical theories and leading to new, far-reaching discoveries - most notably a better understanding of the characteristics of elementary particles, and a more unified theory of the fundamental forces.

Further Reading:
S. Weinberg, Reviews of Modern Physics, 52, 515 (1980); A. Salam, p. 525; S.L. Glashow, p. 539.

See also:

• http://physics.nist.gov/GenInt/Parity/cover.html
• http://ccreweb.org/documents/parity/parity.html