Gauge invariance and amplitudes of two-photon processes

A method to derive the convenient representations for many two-photon amplitudes is suggested. It is based on the use of the gauge in which the photon propagator has only space components. The amplitudes obtained have no any strong numerical cancellations and, therefore, are very convenient for numerical evaluations. Our approach is illustrated by the consideration of the processes $e^+e^-\to e^+e^-e^+e^-$, $e^+e^-\rightarrow e^+e^-μ^+μ^-$, and $e^+e^-\to μ^+μ^-π^+π^-$. The method is extended on the case of polarized particles. The amplitudes obtained in this approach have been employed for extension of the event generator developed by F.A. Berends, P.H. Daverveldt, and R. Kleiss.

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