Probing Quartic Neutral Gauge Boson Couplings using diffractive photon fusion at the LHC

TL;DR

The paper investigates Quartic Neutral Gauge Boson Couplings (QNGCs) involving photons and Z bosons within an effective field theory framework, distinguishing between a light-Higgs scenario (leading dimension-$8$ operators) and a higgsless scenario (leading dimension-$6$ operators). It analyzes how QNGCs can arise from extra-dimensional graviton exchange and how they can be probed at the LHC via diffractive photon fusion channels $pp(\gamma\gamma\to \gamma\gamma)pp$ and $pp(\gamma\gamma\to ZZ)pp$, using forward proton detectors and the Equivalent Photon Approximation. The work derives the operator basis, examines high-energy behavior and unitarity constraints, computes proton-level cross sections with and without form factors, and assesses LHC sensitivity under several benchmark cases, finding substantial improvements over existing limits, especially for $\gamma\gamma ZZ$, while noting current extra-dimensional bounds on the graviton scale. The results underscore the potential of exclusive photon-induced processes as clean probes of higher-dimension operators and possible extra-dimensional physics at the LHC.

Abstract

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(γγ\to γγ)pp and pp(γγ\to ZZ)pp at the 14 TeV LHC. We find that the γγγγ-coupling can be probed most sensitively and values as small as 1/(1.8 TeV)^{4} can be measured. For the γγZZ-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.

Figures (6)

• Figure 1: The diffractive photon fusion processes $pp(\gamma\gamma \to \gamma\gamma)pp$ and $pp(\gamma\gamma \to ZZ)pp$. The outgoing protons can be detected by very forward detectors to be installed by ATLAS and CMS. In the figure above ${\cal O}_i$ represents operators contributing to Quartic Neutral Gauge Couplings (QNGCs).
• Figure 2: Here we plot $\left( ( {\rm Re}(b_l))^2+\beta \sum_{\epsilon_3,\epsilon_4} |a_l|^2 \right)$ in eq. \ref{['UB']} vs the photon-photon center of mass energy for $l=0$ with and without the form factor in eq. \ref{['ff']} taking $b_i/\Lambda^2=1$ TeV$^{-2}$ and $c_i/\Lambda^4=1$ TeV$^{-4}$. We take $\Lambda^{\gamma\gamma/ZZ}_f=\Lambda^{\gamma\gamma/ZZ}_{UB}$ in the form factor for all the different cases other than the lower solid line where we take $\Lambda^{\gamma\gamma/ZZ}_f=0.9~ \Lambda^{\gamma\gamma/ZZ}_{UB}$.
• Figure 3: The Luminosity function $dL/dW$ in eq. \ref{['lf']} taking $q^2_{max}=2$ GeV$^2$.
• Figure 4: The $pp(\gamma \gamma \rightarrow \gamma\gamma)pp$ cross section we obtain as a function of $W_{max}$ with and without a form factor. For the form factor we use in eq. \ref{['ff']} with $m=2,n=1$ and $\Lambda^{\gamma\gamma/ZZ}_f=\Lambda^{\gamma\gamma/ZZ}_{UB}$. We have taken $b_i/\Lambda^2=1$ TeV$^{-2}$ and $c_i/\Lambda^4=1$ TeV$^{-4}$in eq. \ref{['transform']}, the Higgs mass $m_h =120$ GeV and the proton-proton center of mass energy equal to 14 TeV.
• Figure 5: The $pp(\gamma \gamma \rightarrow Z Z)pp$ cross section we obtain as a function of $W_{max}$ with and without a form factor. For the the form factor we use in eq. \ref{['ff']} with $m=2,n=1$ and $\Lambda^{\gamma\gamma/ZZ}_f=\Lambda^{\gamma\gamma/ZZ}_{UB}$. We have taken $b_i/\Lambda^2=1$ TeV$^{-2}$ and $c_i/\Lambda^4=1$ TeV$^{-4}$ in eq. \ref{['transform']}, the Higgs mass $m_h =120$ GeV and the proton-proton center of mass energy equal to 14 TeV.