Heavy quark production in $\gamma\gamma$ collisions: a theoretical   reappraisal
  Heavy quark production in $\gamma\gamma$ collisions is reanalyzed. It is argued that evaluating the cross section $\sigma(\gamma\gamma\to Q\overline{Q})$ in a well-defined renormalization scheme requires the inclusion of direct photon contributions up to the order $\alpha^2\alpha_s^2$. The order $\alpha^2\alpha_s^2$ direct photon contributions are furthermore needed for factorization scale invariance of the sum of direct and resolved photon contributions. The importance of quantitative analysis of renormalization and factorization scale dependence of the approximation currently used for the evaluation of $\sigma(\gamma\gamma\to Q\overline{Q})$ is emphasized as the only way of estimating the theoretical uncertainty related to the ambiguity in choosing these scales. 
Jiri Chyla