I know that symmetry breaking in the Electroweak theory does a great job of explaining the existence and masses of the W and Z bosons, but I'm a little confused about the role of the fourth particle required by the theory. Some sources say that the fourth particle is just the photon, and that it remains massless as a gauge boson should (unlike the W and Z bosons). Other sources indicate that the fourth particle is the boson of the Higgs field itself, giving everyone's favorite latest discovered particle. So which scenario is it?
What do you mean by the fourth particle? If by fourth particle you mean the fourth gauge boson of the electroweak gauge symmetry, it is the photon ( After symmetry breaking and mixing between the neutral gauge bosons). However, the photon was not predicted by electroweak theory of course, it is just accounted for. If by fourth particle you mean the fourth particle predicted by the theory, it is the Higgs boson. But the higgs boson is not a gauge boson as the W and Z, it has spin 0 and not spin 1. In addition, its can have mass regardless of electroweak symmetry breaking. Hope this helps
It helps a little. If nothing else, it shows I need to be more careful with definitions. But as for the difference between them, wasn't the theory about the gauge symmetry and the consequences thereof? Pardon the slight ignorance, but the book I'm reading is a little fuzzy about the difference between the two cases.
The theory has both the gauge symmetry and the higgs mechanism for breaking the symmetry as its ingredients
So just to clarify, the fourth particle in regards to the gauge symmetry is the photon, but the fourth particle that is involved in the symmetry breaking is the Higgs boson (with the other three fields involved in the breaking giving the W's and Z their masses)?
There is just one field/particle involved in the breaking, the Higgs. The other four particles are there as a result of the gauge symmetry, not its breaking. The breaking (due to the Higgs) gives mass to the W and Z.
To break the symmetry a complex doublet is introduced. This scalar field is required with the right charges under su2 and u1_y to provide the yukawa interactions. A complex doublet has 4 degrees of freedom, 3 become helicity zero modes of the ~su2 gauge bosons. So the other thing left over from this scalar field is the Higgs.