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For example, Wikipedia says:

The existence of the electroweak interactions was experimentally established in two stages, the first being the discovery of neutral currents in neutrino scattering by the Gargamelle collaboration in 1973, and the second in 1983 by the UA1 and the UA2 collaborations that involved the discovery of the W and Z gauge bosons in proton–antiproton collisions at the converted Super Proton Synchrotron.

I have also seen this claim repeated elsewhere. But I don't understand why the existence of the weak neutral current (and the Z boson that mediates it) is evidence that the weak interaction unifies with the electromagnetic interaction. Because I don't understand the history of electroweak theory, to me this just looks like evidence of the weak interaction having a broken SU(2) symmetry and the electromagnetic interaction having a separate U(1) symmetry rather than both interactions being unified under SU(2) x U(1). I did see in one source that it has something to do with renormalizability, but I don't understand the details.

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    $\begingroup$ This belongs to the History of Science and Mathematics, and not this one, I suspect. Electroweak theory is predicated on Glashow’s weak mixing. It is the essence of the theory. $\endgroup$
    – Cosmas Zachos
    Commented Apr 12, 2023 at 2:19
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    $\begingroup$ Agreed with @CosmasZachos Prior to the 1970s, weak interactions were understood to occur via the debunked "Fermi interaction", which in contrast with quantum electrodynamics, involved no gauge boson to mediate the interaction, but instead considered the weak interaction to be a "point like interaction". The standard model (1967 Salam and Weinberg) i.e., unification of weak and EM interactions, which solved the problems with existing theories and divergences at high energies etc., required the existence of weak currents. The rest is history. $\endgroup$
    – joseph h
    Commented Apr 12, 2023 at 2:32
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    $\begingroup$ That the neutral current was not a $W^0$, but a more massive (by some function of the mixing angle) $Z$ was a big factor. $\endgroup$
    – JEB
    Commented Apr 12, 2023 at 3:35

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I believe your question is really more suited to HSM, to which you might well transfer it. Galison 1994 should be good reading for you. Here, I'll parse out the strictly physics issues involved, touching upon an older answer.

The key point in the unified EW theory of the SM is that electromagnetism cannot be separate, as Glashow argued in 1961, a dozen years before the actual discovery of neutral currents he introduced. The U(1) of EM has to be enmeshed nontrivially with the $T_3$ of SU(2) and the U(1) of hypercharge in SU(2)×U(1). The technical details are in the linked answer: The chiral structure of the charged currents (matter coupling to $W^\pm$) dictates that if they fit into an SU(2), their commutator, the above $T_3$, is also chiral, much unlike electromagnetism... but still not independent of it! A conundrum...

Glashow dared extend the SU(2) minimally by appending a hypercharge U(1) to it, and twist its generator with $T_3$, thereby enmeshing, rather than unifying, the two, through a weak Gell-Mann—Nishijima formula, weak mixing: this is the very heart of the electroweak theory.

The other combination between the EM charge/curent, the neutral current, had to couple to a neutral gauge boson, Z, unobserved then. But the theory, unexpectedly twisted for the times, and "cockeyed" according to Feynman, on account of its (then) inelegant mixed chiral structure, was the basis of most researchers all but dismissing this "unified" theory at the time. Six years after that, Weinberg and Salam provided a minimal workable method for the Ws and the Z to pick up masses leaving the photon massless, utilizing the Englert-Brout-Higgs mechanism, and six years after that the predicted Z was discovered. At that time, 't Hooft and Veltman had shown such gauge theories to be renormalizable which mollified and reassured the theory community, unused to effective theories at the time, and satisfied their aptitude for concrete meaningful radiative calculations.

  • So, indeed, the discovery of the neutral current, and subsequently the discovery of the massive gauge bosons vindicated the best model of EW unification, nowadays risen to the full status of a correct theory.
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