Everywhere I look it up the prediction of the charm quark is predicted by the electroweak interaction, which of course recovers the weak interaction with the spontaneous symmetry break and the Brout-Englert-Higgs mechanism.

My question is, did the new features of the electroweak theory allow for the prediction of the charm quark? Or it could have been predicted with the old features of the weak theory?

As far as I am concerned what the electroweak unification gave rise to was the existence of neutral currents, i.e. interactions mediated by the $Z^0$ boson. But the prediction of the charm was made to avoid neutral current interactions in semileptonic decays with $\Delta S \neq \Delta Q$ as well as $\Delta S = 2$, which hadn't been observed. These neutral currents could have been avoided in weak theory because neutral currents hadn't been observed, or in electroweak because decays with $\Delta S \neq \Delta Q$ hadn't been observed.

So again, was electroweak theory necessary for the prediction of the existence charm quark?

Thank you.


1 Answer 1


You might wish to move or repost your question to HSM where such issues are discussed.

Indeed, the Weinberg-Salam model did not figure at all in the historic GIM paper, 1970, not even as a reference. What is required for the suppression of FCNC is a "conventional mixing" of charged currents, reasonably well understood at the time.

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(Now, Glashow and Bjorken in 1964 had speculated on a 4th quark, but without making it necessary, as a component of an explanation of a physical "fact on the ground" like GIM.)

Subsequently, Gaillard, Lee, and Rosner, 1975 discussed all components going into their anticipation of the mass of the charmed quark, Gaillard & Lee 1974, but that was much later, after the discovery of neutral currents and hence the triumph of WS: so the latter prediction does reference Weinberg and Salam, but this is only because by that time 't Hooft had proven renormalizability of the model, so of course loop corrections should be mindful of and grateful for that...

  • $\begingroup$ neutrino oscillation effect between say $\nu_{\mu}$ and $\nu_{\tau}$ is not consequential here, right? $\endgroup$
    – MadMax
    Oct 28, 2019 at 14:57
  • $\begingroup$ Not that I can think of. What would you have in mind? $\endgroup$ Oct 28, 2019 at 18:32

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