Practical corollaries of transverse Ward-Green-Takahashi identities

Understand

The gauge principle is fundamental in formulating the Standard Model.

• Fermion--gauge-boson couplings are the inescapable consequence and the primary determining factor for observable phenomena.
• Vertices describing such couplings are simple in perturbation theory and yet the existence of strong-interaction bound-states guarantees that many phenomena within the Model are nonperturbative.
• It is therefore crucial to understand how dynamics dresses the vertices and thereby fundamentally alters the appearance of fermion--gauge-boson interactions.