Hadronic processes and electromagnetic corrections

The inclusion of electromagnetism in a low energy effective theory is worth further study in view of the present high precision experiments (muon g − 2, π0 → γγ, τ decays, etc.). In particular in many applications of chiral perturbation theory, one has to purify physical matrix elements from electromagnetic effects. The theoretical problems that I want to point out here are following: the splitting of a pure QCD and a pure electromagnetic part in a hadronic process is model dependent: is it possible to parametrise in a clear way this splitting? What kind of information (scale dependence, gauge dependence,..) is actually included in the parameters of the low energy effective theory? I will attempt to answer these questions introducing a possible convention to perform the splitting between strong and electromagnetic parts in some examples. Talk given at SIGHAD2003, Pisa (Italy) October 8-10,2003.