Hadron production in photon-photon collisions

Abstract

We analyze deep-inelastic photon-photon collisions via the two-photon mechanism in electron-positron (-electron) colliding beams in a form especially suitable for experimental analysis. It is shown that by a helicity analysis similar to that used in electroproduction experiments, we can separate five of the eight structure functions describing the process ${\ensuremath{\gamma}}^{*}+{\ensuremath{\gamma}}^{*}\ensuremath{\rightarrow}\mathrm{hadrons}$. The helicity cross sections for this process and for the process with one real photon (inelastic electron-photon scattering) are related to structure functions, and are evaluated using quark light-cone algebra. There are anomalous contributions to the structure functions for the inelastic electron-photon scattering which arise both in parton as well as generalized vector-meson-dominance models. This suggests a connection between these two types of models for photon-photon scattering. Further, we use vector-meson dominance to construct a sum rule for ${\ensuremath{\sigma}}_{\ensuremath{\gamma}\ensuremath{\gamma}\ensuremath{\rightarrow}\mathrm{hadrons}}$ from which it is estimated that roughly 20% of the cross section should be built up from higher-mass vector states. Using a spectral representation for the total transverse cross section, and the "aligned-jet" vector-dominance model we achieve a connection, via a "correspondence principle," with the parton model for the hadron multiplicities in photon-photon collisions. We also comment on inclusive pion multiplicities and the approach to scaling for photon-photon processes in the light-cone algebra.