Probing Quartic Neutral Gauge Boson Couplings using diffractive photon   fusion at the LHC
  A complete list of operators contributing at the lowest order to Quartic Neutral Gauge Boson Couplings involving photons and Z-bosons, is presented. We show that, for the couplings we consider, the lowest order contribution is from dimension 8 operators in the case when a light Higgs is present and from dimension 6 operators in the higgsless case where electroweak symmetry is non-linearly realized. We also show that these operators are generated by exchange of the Kaluza-Klein partners of the graviton in extra-dimensional models. We then explore the possibility of probing these couplings in the diffractive photon fusion processes pp(\gamma\gamma \to \gamma\gamma)pp and pp(\gamma\gamma \to ZZ)pp at the 14 TeV LHC. We find that the \gamma \gamma \gamma \gamma-coupling can be probed most sensitively and values as small as 1/(1.8 TeV)^{4} can be measured. For the \gamma\gamma ZZ-coupling, values as small as 1/(850 GeV)^{4} and 1/(1.9 TeV)^2 can be probed in the light Higgs and higgsless cases respectively, which is an improvement by orders of magnitude over existing limits. 
Rick S. Gupta
hep-ph hep-ex