It's widely noted that the Standard Model does not predict the existence of flavor or the three lepton families: We put those facts into the Lagrangian "by hand" for agreement with observations, not due to any a priori requirement of Quantum Field Theory or the SM. Less commonly noted appears to be the fact that the same manual process puts in which fields (particles) couple to which interactions. For instance, there seems to be no theoretical reason in QFT or the SM for quarks to participate in the weak or EM interactions. Is there one?

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    $\begingroup$ Misquoting Gell-Mann: That which is not forbidden is mandatory. In other words, if there is no reason not to include an interaction, you must in fact include it. The SM does precisely that. $\endgroup$ – AccidentalFourierTransform May 24 '18 at 19:48

As a gauge field theory, what's postulated are the gauge group, and the existence of fermions (and a scalar) transforming under specific representations of the gauge group. The gauge group and the representations have to be freely chosen, but the interactions then follow.

  • $\begingroup$ Yes, and the quarks, for example, are postulated as transforming under both an EW SU(2)L rep as well as under an SU(3) IRR, due to experimental evidence, not as a math consequence of the initial set of assumptions, no? $\endgroup$ – Jim Eshelman May 26 '18 at 1:26

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