Stupid question on the qed renormalization

[paolorossi]

I did not understand one thing: imagine we have calculated a cross section relates to a process, for example compton scattering. The parameters (charge, mass, ...) that come into play in cross section are "bare parameters"? Then after the renormalization of the theory, getting the "bare parameters" in terms of "physical parameters", we must re-express the cross section calculated in terms of "physical parameters"?

 

[Einj]

It depends on what order are you calculating the cross section. If you are working on tree level then the "bare" parameters coincide with the renormalized one (at that particular order of perturbation theory).
On the other hand, if you want to work at higher orders then some inifinities arise. This means that Green functions are infinite. So you have to redefine you theory with new "renormalized" parameters in order to obtain finite Gree functions. So, in higher orders the observables (like differential cross section), which are contructed over Green functions, are directly created via renormalized finite parameters.

 

[paolorossi]

this is evident if we consider processes of second order perturbation, in fact, being the differences between bare and renormalized parameters of the second order, for example for charge

e[SUB]phys[/SUB] = e[SUB]bare[/SUB] + O(e[SUB]bare[/SUB] [SUP]2[/SUP])

these be to undermine the terms of the development of higher-order... now beginning to understand, thanks

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but what happens to subsequent orders?