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I have to prove that the amplitude of the process

[itex]\gamma \gamma \to W^+ W^- [/itex]

does not depend on the gauge we will choose, [itex]R_{\xi}[/itex].

So I use the most general expressions for the propagators and vertices. I find 5 diagrams. One that involves only the 4 fields and a vertex, 1 t and one u channel with a W boson as propagator and 1 t and 1 u where the propagator is a non-physical field [itex]\phi[/itex].

I just sum up the last 4, since the first one does not depend on the [itex]\xi[/itex] parameter but then I am stuck since nothing seems to make the horrible expressions in such a way that there will be no gauge dependence in the end..

Any help?

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# General Gauge Invariance Problem!

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