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-eight operators in the case when a light Higgs is present and from dimension-six operators in the Higgsless case where electroweak symmetry is nonlinearly 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 -> gamma gamma)pp and pp(gamma gamma -> 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.