Abstract
In the 1950s the physics community was confronted with the θ − τ puzzle. There were two elementary particles the θ and the τ that on one set of criteria, mass and lifetime, seemed to be the same particle. On a second set of criteria, spin and parity, they were different. The puzzle resisted solution using then accepted physics. T. D. Lee and C. N. Yang proposed a radical solution, parity was not conserved in the weak interactions. Their hypothesis was thought to be sufficiently plausible to merit further investigation. Lee and Yang proposed several experimental tests of their hypothesis, one of which was to investigate any possible asymmetry in the β decay of oriented nuclei. This experiment was performed by C. S. Wu and her collaborators and clearly demonstrated parity nonconservation. (The results were reported in February 1957 and Lee and Yang were awarded the Nobel Prize in Physics later that year.) The Wu result was sufficient to justify acceptance of parity nonconservation, but because it did not measure the asymmetry parameter with sufficient precision it also justified further experimental pursuit.
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Notes
- 1.
For the proof of the theorem see Gibson and Pollard (1976, pp. 119–127, 160–162).
- 2.
For more details and the theoretical support for these parity determinations (including the role of the L1 + L2 superscript) see Gibson and Pollard (1976, pp. 159–160).
- 3.
For the current status of the puzzle see Bellantoni (2016).
- 4.
This reference contains a reprint of a 1983 lecture by Wu.
- 5.
See Wu (2008, pp. 55–56) for just how difficult this turned out to be.
- 6.
David Christen (Oak Ridge National Laboratory) informs us that the current product of choice used to secure wayward components when conducting experiments near absolute zero is waxed dental floss. It behaves exactly as you’d want.
- 7.
Note however that we have simplified insofar as we have made use of only two dimensions.
- 8.
It is important to keep in mind that this analysis does not assume that the β emissions are themselves polarized, only that the Co60 source is. It was only later determined that β emissions are in fact longitudinally polarized which is by itself a counterexample to parity conservation. This because in the case of a longitudinally polarized beam of β emissions, parity is violated in very much the same way that it was violated in the case of the Wu experiment, namely, that spin direction remains intact after the parity transformation while all else changes.
- 9.
A handy and frequently used way of stating the lack of congruence is that the pseudo-scalar (S ⋅ p) changes in value, where the vector S represents spin and p the momentum of the system.
- 10.
We note here that while Wu does not explicitly state that in this case the horizontal field was turned on, we think that is implicit in the argument. This is indicated by the fact that the third systematic check dealt with the case of where there was in effect no cooling but there was a vertical polarizing field. Hence, it’s natural that the fourth systematic check would deal with the case where there was cooling (produced by the horizontal field) but no polarization produced by a vertical field.
- 11.
For some background information on the Friedman and Telegdi experiment and the publication of its results see Goudsmit (1971).
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Laymon, R., Franklin, A. (2022). The Wu Experiment. In: Case Studies in Experimental Physics. Synthesis Lectures on Engineering, Science, and Technology. Springer, Cham. https://doi.org/10.1007/978-3-031-12608-6_3
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