Sensitivity to anomalous quartic gauge couplings in photon-photon interactions at the LHC

TL;DR

The work investigates anomalous quartic gauge couplings in exclusive $\\gamma\\gamma$ interactions at the LHC, exploiting clean two-photon final states and forward proton tagging to probe $$ gauge boson couplings $a_0^W/a_C^W$ and $a_0^Z/a_C^Z$ via dimension-6 effective operators. A full simulation pipeline yields strong 95% CL limits on these couplings from leptonic WW and ZZ channels, with 1–10 fb$^{-1}$ of data showing orders of magnitude improvement over LEP. The analysis carefully addresses unitarity constraints: without suppression, high-$W_{\\gamma\\gamma}$ regions threaten unitarity, motivating dipole form factors and/or kinematic tagging to keep the theory well-behaved, while maintaining substantial sensitivity. The results suggest substantial potential to constrain or reveal new physics in the electroweak sector up to a few TeV scales, and propose concrete avenues for improvement, including semi-leptonic channels and differential distribution studies.

Abstract

The exclusive two-photon production at the LHC of pairs of W and Z bosons provides a novel and unique test-ground for the electroweak gauge boson sector. In particular it offers, thanks to high gamma-gamma center-of-mass energies, large and direct sensitivity to the anomalous quartic gauge couplings otherwise very difficult to investigate at the LHC. An initial analysis has been performed assuming leptonic decays and generic acceptance cuts. Simulation of a simple counting experiment has shown for the integrated luminosity of 10 fb-1 at least four thousand times larger sensitivity to the genuine quartic couplings, a_0^W, a_0^Z, a_C^W and a_C^Z, than those obtained at LEP. The impact of the unitarity constraints on the estimated limits has been studied using the dipole form-factors. Finally, differential distributions of the decay leptons have been provided to illustrate the potential for further improvements of the sensitivities.

Figures (11)

• Figure 1: Elastic luminosity spectrum of photon-photon collisions at the LHC assuming the maximal photon virtuality $Q_{max}^2=2~\textrm{GeV}^2$ (solid line). The luminosity spectrum assuming the photon tagging range $20~\textrm{GeV}~<~E_{\gamma}~<~900~\textrm{GeV}$ is also shown (dashed line).
• Figure 2: Relative elastic $\gamma\gamma$ luminosity for photon collisions at the center-of-mass energy above $W_0$, obtained using the EPA for proton collisions at $\sqrt{s}=14~TeV$, and assuming the maximal photon virtuality $Q_{max}^2=2~\textrm{GeV}^2$.
• Figure 3: Profiles of the 95$\%$ CL upper limits of the cross section after the acceptance cuts for $pp (\gamma\gamma\rightarrow W^+W^-\rightarrow$$l^+l^-\nu\bar{\nu}) pp$ and $pp (\gamma\gamma\rightarrow ZZ\rightarrow$$l^+l^- j j) pp$ as a function of relevant anomalous couplings $\mathrm{a_0^W/ \Lambda^2}$, $\mathrm{a_{\mathrm{c}}^W/ \Lambda^2}$, $\mathrm{a_0^Z/ \Lambda^2}$ and $\mathrm{a_{\mathrm{c}}^Z/ \Lambda^2}$. The contours are shown assuming two values of the integrated $pp$ luminosity.
• Figure 4: Cross sections of $pp (\gamma\gamma\rightarrow W^+W^-\rightarrow$$l^+l^-\nu\bar{\nu}) pp$ and $pp (\gamma\gamma\rightarrow ZZ\rightarrow$$l^+l^- j j) pp$ after the acceptance cuts are shown as a function of the genuine anomalous quartic vector boson couplings $\mathrm{a_0^W / \Lambda^2}$ and $\mathrm{a_0^Z/ \Lambda^2}$ (for $\mathrm{a_C^W/ \Lambda^2}=\mathrm{a_C^Z/ \Lambda^2=0}$), together with the upper cross section limits at CL=95$\%$ (horizontal lines).
• Figure 5: Unitarity limit for the W neutral anomalous coupling $a_0^W/\Lambda^2$ calculated according to Eq. \ref{['eq.unitarity-bound']} for J=0 (full line), and assuming that the left hand side of Eq. \ref{['eq.unitarity-bound']} is smaller than $(0.4)^2$ (dashed line). The arrow indicates the obtained limit at 95% CL from Tab. \ref{['tab.1st-limits']} for $10$ fb$^{-1}$.