[talanum1 (OP)]

What term do I add to the Lagrangian corresponding to Weak Isospin?

[Karamoonf]

I would like to know about this as well.

[rapheal21]

Neither actually. There was in fact a "proto-field" with 4 components that, once coupled to the Higgs field, became more natural considered as being understood in terms of 4 DIFFERENT components, with one of those components being the massless photon and the others being the mass-having W and Z bosons. So the photon is the massless portion that is "left over" once the original field and its interaction with the Higgs field is rebranded as new fields. Without the Higgs field there would therefore be no photon and only that proto-field.