Anomalous WW $γ$ coupling in photon-induced processes using forward detectors at the LHC

TL;DR

This work proposes probing the Standard Model through photon-induced WW production at the LHC by tagging intact protons in forward detectors and analyzing WW decays in the central detectors. It provides a precise SM prediction for the two-photon process, with a cross section of $\sigma_{WW}=95.6\,\mathrm{fb}$, and demonstrates how forward proton tagging enables a clean measurement and stringent constraints on the WWγ triple gauge vertex. The study introduces an effective Lagrangian with anomalous parameters $\Delta\kappa^{\gamma}$ and $\lambda^{\gamma}$, explores form-factor regularization, and shows that the LHC with forward detectors can improve existing limits by up to two orders of magnitude depending on luminosity and detector acceptance, particularly constraining $\lambda^{\gamma}$ at high $W_{\gamma\gamma}$. The results highlight the forward-detector program as a powerful, complementary approach to inelastic LHC measurements, enabling precise tests of gauge-boson self-interactions and sensitivity to new TeV-scale physics while exploiting a clean experimental signature.

Abstract

We present a new method to test the Standard Model expectations at the LHC using photon-induced $WW$ production. Both $W$ decay in the main ATLAS or CMS detectors while scattered protons are measured in forward detectors. The sensitivity to anomalous $WWγ$ triple gauge coupling can be improved by more than a factor 5 or 30 compared to the present LEP or Tevatron sensitivity respectively.

Figures (9)

• Figure 1: S-channel diagram, one of the three Feynman diagrams of the $W$ pair production in two-photon processes. The others not shown correspond to a u-channel diagram and a diagram with direct coupling $\gamma\gamma WW$. $p'_i$ are protons that do not break up but continue to travel down the beam pipe at small angles.
• Figure 2: Relative effective $\gamma\gamma$ luminosity in $pp$ collisions at $14\,\hbox{TeV}$ as a function of the two-photon invariant mass. The maximal virtualities of the emitted photons are set to $Q^2_{max}=4\,\,\hbox{GeV}^2$. The dashed curve shows the photon spectrum within the ATLAS or CMS forward detector acceptance.
• Figure 3: $\xi$ dependence of the cross section for photon (black line) and pomeron induced events (shaded region). The former shows a steep $\xi$ dependence while the pomeron background is suppressed by tagging the protons within the acceptance $0.0015<\xi<0.15$.
• Figure 4: The full two-photon $pp\rightarrow pWWp$ production cross section as a function of the $WW\gamma$ anomalous coupling parameters $\Delta\kappa^{\gamma}$ and $\lambda^{\gamma}$ for $\Lambda=\infty$, i.e. with no form factor assumed. The SM value corresponding to $\Delta\kappa^{\gamma}=\lambda^{\gamma}=0$ is $\sigma_{SM}=95.6\,\hbox{fb}$.
• Figure 5: $\xi$ dependence of the two-photon $WW$ cross section for different values of $\Delta\kappa^{\gamma}$ (left) and $\lambda^{\gamma}$ (right) (SM values are 0). For $\lambda^{\gamma}$, the cross section is enhanced at high $\xi$ which is at the edge of the forward detector acceptance ($\xi=0.15$). On the contrary, varying $\Delta\kappa^{\gamma}$ in the interesting range $(-0.05<\Delta\kappa^{\gamma}<0.05)$ changes mainly the normalization and not the shape of the $\xi$ distribution.