We here discuss the problem of back-factorization in γγ collision processes; i. e., how to extract, from the experimental measurement of a reaction e e → e e X' the information on the physically interesting process γγ → X in the form of only one term or very few terms (cross section, polarization terms, structure functions). Back-factorization procedures are basically associated with the equivalent-photon approximation.
In part I, we define the principles of back-factorization for the reactions considered, and we discuss its application to experiments with finite-angle tagging of both outgoing electrons. Using a helicity formalism, we show that, under certain conditions - which appear to be quite stringent, as is proven by our numerical checks -, the completely differential cross section of e e e e X may be expressed as a five-term formula; after integration over azimuthal angles, it becomes a one-term formula, i. e. the double equivalent-photon approximation. In addition to double-tagging, measurements based on single-tagging and on double anti-tagging are also discussed from the point of view of back-factorization.
In part II, we treat the special configuration to be considered for a determination of the structure functions of the photon. Using the naive quark model without higher-order corrections, we are studying analytically and checking numerically the conditions to be satisfied in order to obtain a one-term formula for extraction of the information on the photon's quark content from the data measured.
KeywordsStructure Function Differential Cross Section Azimuthal Angle Virtual Photon Polarization Term
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