Two-gluon exchange contribution to elastic $\gamma \gamma \to \gamma   \gamma$ scattering and production of two-photons in ultraperipheral   ultrarelativistic heavy ion and proton-proton collisions
  We discuss the two-gluon exchange contribution (formally three-loops) to elastic photon-photon scattering in the high-energy approximation. The elastic $\gamma\gamma \to \gamma \gamma$ amplitude is given in the impact-factor representation for all helicity configurations and finite quark masses. We discuss the importance of including the charm quark, which contribution, due to interference, can enhance the cross section considerably. We investigate the contribution to the $\gamma \gamma \to \gamma \gamma$ amplitude from the soft region, by studying its dependence on nonperturbative gluon mass. Helicity-flip contributions are shown to be much smaller than helicity-conserving ones. We identify region(s) of phase space where the two-gluon exchange contribution becomes important ingredient compared to box and nonperturbative VDM-Regge mechanisms considered in the literature. Consequences for the $A A \to A A \gamma \gamma$ reaction are discussed. Several differential distributions are shown. A feasibility study to observe the effect of two-gluon exchange is presented. We perform a similar analysis for the $p p \to p p \gamma \gamma$ reaction. Only by imposing severe cuts on $M_{\gamma \gamma}$ and a narrow window on photon transverse momenta the two gluon contribution becomes comparable to the box contribution but the corresponding cross section is rather small. 
Mariola Klusek-Gawenda, Wolfgang Schaefer and Antoni Szczurek
hep-ph hep-ex nucl-th