Dalitz-Yennie-type equivalent-photon spectrum forγγcollisions in electron storage rings

Abstract

It is shown that the Dalitz-Yennie version of the equivalent-photon approximation, originally established for electroproduction in the target rest frame, becomes inappropriate, to some extent, for application to $\gamma \gamma$ collision experiments in electron storage rings, when the outgoing electrons are tagged at finite angles. For this type of experiment, a new Dalitz-Yennie-type formula is given, the use of which is, however, restricted in principle to single-tagging measurements. In the configurations thus considered, quantitative differences between the original and the new Dalitz-Yennie-type formula may become significant; a numerical comparison shows indeed that the new formula definitely provides a better approximation, with respect to an "effective-photon spectrum" derived from the exact calculation for various processes, than the former one.