Flavour structure of low-energy hadron-pair photoproduction

Abstract

We consider the process γγ→H₁H̄₂, where H₁ and H₂ are either mesons or baryons. The experimental findings for such quantities as the pp̄ and K_SK_S 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 K_SK_S 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 pp̄ and K⁺K^- data.