Testing gauge-invariant perturbation theory

Understand

Gauge-invariant perturbation theory for theories with a Brout-Englert-Higgs effect, as developed by Fr\"ohlich, Morchio and Strocchi, starts out from physical, exactly gauge-invariant quantities as initial and final states.

• These are composite operators, and can thus be considered as bound states.
• In case of the standard model, this reduces almost entirely to conventional perturbation theory.
• This explains the success of conventional perturbation theory for the standard model.