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I've been reading Lee and Yang's paper that looks into the possible of parity violation. You can find it here: A Question on Parity Conservation in Weak Interactions. And they say

This phenomenon belongs to the discussions of the last section. The following phenomena are then examined: allowed spectra, unique forbidden spectra, forbidden spectra with allowed shape, $\beta$-neutrino correlation, and $\beta$$\gamma$ correlation.

Could any anyone explain what forbidden spectra with allowed shape refers to? I couldn't find anything using that terminology when looking into this. Additionally they also claim that the mixing for parity in electromagnetic interactions $(\mathscr{F}^{2})$should be less than $(\frac{r}{\lambda})^{2}$ what are $r$ and $\lambda$?

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In answer to the second part of your question, I believe $(r/\lambda)^2$ refers to the ratio of the characteristic size of the system to the wavelength of the transition radiation. For an atom with typical values $r\approx 10^{-10}$ m and $\lambda \approx 10^{-7}$ m, the square of this ratio is on the order of $10^{-6}$, as they quote.

As to the first part of your question, I have somewhat of a roundabout answer: They explicitly say that the list of experiments you refer to are unable to provide evidence for parity violation in the weak interaction, and they explain what the specific requirements are for observable parity violation. So historically speaking, the decays of $^{60}$Co and pions are the important ones to know about.

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  • $\begingroup$ But I'm just unsure as to what "forbidden spectra with allowed shape" would be. Is it old terminology for second order forbidden transitions? $\endgroup$ – David G. Apr 4 '19 at 4:30
  • $\begingroup$ I'm not sure! Hopefully someone with more historical knowledge can help you out. $\endgroup$ – Will C Apr 4 '19 at 4:45
  • $\begingroup$ I think it might have something to do with Spectral line shape, though I cannot be sure. It seems weird that forbidden spectra with shape is distinguished from just ordinary forbidden spectra. $\endgroup$ – David G. Apr 5 '19 at 4:31

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