Transition to perturbative QCD in two-photon collisions
  We propose that the different angular distributions in two-photon collisions observed at low and high center-of-mass energies $W_{\gamma\gamma}$ indicate the transition from nonperturbative to perturbative QCD. We calculate the differential cross sections of $\gamma \gamma\to\pi \pi$, $KK$ in the angle $\theta$ of one of the final-state mesons using QCD sum rules and the perturbative QCD approach based on $k_T$ factorization theorem. Our predictions from sum rules (perturbative QCD) decrease (increase) with $|\cos\theta|$, consistent with the Belle data of $\gamma \gamma\to K^+K^-$ for $W_{\gamma\gamma}\approx 1.5$-1.7 (2.2-2.4) GeV. 
Ron-Chou Hsieh, Hsiang-nan Li