gamma+gamma --> pi+pi, K+K : leading term QCD vs handbag model

Abstract

The "handbag" model was proposed as an alternative, at the present day energies, to the leading term QCD predictions for some hard exclusive processes. The recent precise data from the Belle Collaboration on the large angle cross sections $γγ--> ππ, KK $ allow a check of these two approaches to be performed. It is shown that the handbag model fails to describe the data from Belle, while the leading term QCD predictions are in reasonable agreement with these data

Figures (4)

• Figure 1: Two typical Feynman diagrams for the leading term hard contributions to $\gamma\gamma\rightarrow {\overline M}M$ , the broken line is the hard gluon exchange.
• Figure 2: The cross sections $d\sigma/d\cos\theta$ , integrated over the c.m. angular region $|\cos\theta|<0.6\,$, together with a $W^{-6}$ dependence line Belle1 : a) $\,\,\gamma \gamma\rightarrow \pi^+\pi^-\,,\,\,$ b) $\,\, \gamma\gamma\rightarrow K^+K^-\,,$ c) the cross section ratio, the solid line is the result of the fit for the data above $3\,{\rm GeV} \,,\,\,$ the errors indicated by short ticks are statistical only.
• Figure 3: a) the overall picture of the handbag contribution, b) the lowest order Feynman diagram for the light cone sum rule
• Figure 4: The measured energy dependence of $\sigma_o(K_SK_S)$Belle2. The solid line is $\,\sim W^{-k},\,\, k\simeq 10$.