Why "mediated by photons"? Why do we say that the electromagnetic interaction is mediated by photons (or to use http://en.wikipedia.org/wiki/Force_carrier" [Broken] phrase, that the photon is the force carrier for the interaction)? (I understand "mediated" … it's "photons" I object to.) Take the simplest-order Feynman diagram for Compton scattering: an "H"-shaped diagram in which a real electron interacts with a real photon by "exchanging" a virtual electron. Here, the interaction is obviously mediated by the virtual electron. More generally, simple counting (per vertex) shows that in any large Feynman diagram, the number of virtual electrons must be roughly twice the number of virtual photons. Perhaps we should simply say that the photon is the vector (or boson?) field associated with the interaction, while the electron is the spinor (or fermion?) field associated with the interaction, but they both mediate it? As Meir Achuz says, every gauge interaction has a gauge particle, which must be a vector particle … … but where does the Higgs scalar boson come into this?