Flavour structure of low-energy hadron-pair photoproduction

Abstract

We consider the process γγ→H1H2, where H1 and H2 are either mesons or baryons. The experimental findings for such quantities as the pp and KSKS differential cross sections, in the energy range currently probed, are found often to be in disparity with the scaling behaviour expected from hard constituent scattering. We discuss the long-distance pole–resonance contribution in understanding the origin of these phenomena, as well as the amplitude relations governing the short-distance contribution, which we model as a scaling contribution. When considering the latter, we argue that the difference found for the KSKS and the K+K- integrated cross sections can be attributed to the s-channel isovector component. This corresponds to the ρω→a subprocess in VMD (vector-meson-dominance) language. The ratio of the two cross sections is enhanced by the suppression of the φ component, and it is hence constrained. We give similar constraints to a number of other hadron-pair production channels. After writing down the scaling and pole–resonance contributions respectively, the direct summation of the two contributions is found to reproduce some salient features of the pp and K+K- data.