The top threshold effect in the $\gamma\gamma$ production at the LHC
  We compute the top quark threshold contributions to the $\gamma\gamma$ production at the LHC. They appear when the invariant mass of the photon pair, $M_{\gamma\gamma}$ just exceeds two times the mass of the top quark and induce some feature in the $M_{\gamma\gamma}$ distribution. We determine the magnitude of this threshold effect and characterize this feature with a simple empirical fitting function to show that it is possible to observe this effect at the LHC in future. We also explore some possible improvements that may enhance its significance. 
Shashikant R. Dugad, Pankaj Jain, Subhadip Mitra, Prasenjit Sanyal and   Ravindra K. Verma
hep-ph