# Particle coupling predictions of the Standard Model

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?

• 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. – AccidentalFourierTransform May 24 '18 at 19:48