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?