UNDERSTANDING THE CROSS-SECTION FOR ISOLATED PROMPT PHOTON PRODUCTION

We describe the proper evaluation in perturbation theory of the cross section for prompt photon production with an isolation cut. It is convenient to separate the cross section into two components: a purely perturbative piece and a nonperturbative piece. If the opening angle (δ) of the photon isolation cone is not too small, the isolation cut on the nonperturbative fragmentation functions can be handled by proper choice of the fragmentation scale. For the pertutbative component, we show that as long as the energy resolution parameter (ε{lunate}) is a finite number, the isolated cross section is infrared finite. From an explicit next-to-leading order calculation, we find that the leading soft gluon correction for the isolated cross section is proportional to sin2( 1 2δ)( αs π)ln( 1 ε{lunate}) times the leading order contribution. If ε{lunate} is too small, the anomalous soft gluon contributio can be resummed. © 1990.