Probing the anomalous $γγγZ$ coupling at the LHC with proton tagging

TL;DR

This work assesses the LHC sensitivity to anomalous γγγZ couplings using photon-induced γZ production with intact forward protons. By employing forward proton detectors and EFT descriptions with dimension-eight operators, it demonstrates that proton tagging dramatically suppresses backgrounds and enables both leptonic and hadronic Z decays to be used. The study provides detailed event generation, background treatment, and selection strategies, yielding 5σ and 95% CL reach on the couplings around 2×10−13 GeV−4 at 300 fb−1 and improving toward 1–2×10−13 GeV−4 at HL-LHC, with the γZ channel offering discrimination against γγ final states. The results show a strong potential for probing New Physics scenarios, and the combination of γZ and γγ channels can constrain or reveal the nature of heavy states coupling to electroweak gauge bosons.

Abstract

The sensitivities to the anomalous quartic gauge boson coupling $γγγZ$ are estimated via $γZ$ production with intact protons in the forward region at the LHC. Proton tagging proves to be a powerful tool to suppress the background, which allows consideration of the hadronic decays of the $Z$ boson in addition to the leptonic ones. We discuss the discovery potential for an integrated luminosity of $300\,\mathrm{fb}^{-1}$ and $3000\,\mathrm{fb}^{-1}$. The sensitivity we obtain at $300\,\mathrm{fb}^{-1}$ goes beyond the one expected from LHC bounds on the $Z\rightarrow γγγ$ decay by about three orders of magnitude. The $γZ$ channel provides important discriminatory information with respect to the exclusive $γγ$ channel, as many particles beyond the Standard Model (such as a radion or Kaluza Klein gravitons) predict a signal in the latter but not the former.

Figures (6)

• Figure 1: Anomalous $\gamma Z$ production via photon fusion with intact protons in the final state.
• Figure 2: $\gamma Z$ mass distribution for the signal in the $\ell\bar{\ell}\gamma$ channel for two coupling values ($\zeta = 10^{-12}, 10^{-13}$ GeV$^{-4}$) for events within the $0.015<\xi<0.15$ proton detectors acceptance and the requirement on transverse momenta $p_{T \gamma},p_{T \ell\bar{\ell}}>100$ GeV. The main contribution to the background is the SM $Z\gamma$ production in association with protons arising from the pile-up. The plot assumes an integrated luminosity of $300\,\mathrm{fb}^{-1}$ and an average pile-up of $\mu=50$.
• Figure 3: Missing diproton mass $m_{pp} = \sqrt{\xi_1\xi_2 s}$ to central mass ratio distribution (Top) and rapidity difference distribution (Bottom) in the $\ell\bar{\ell}\gamma$ channel for signal and background within the acceptance $0.015<\xi_{1,2}<0.15$ considering two different coupling values after applying the requirements on the acceptance, $p_T$, invariant mass $m_{\gamma Z}$, $p_T$ ratios and angle separation according to Table \ref{['llbar_table']}. The width of the signal is due mainly to the $\xi_{1,2}$ resolution. The integrated luminosity is $300$ fb$^{-1}$ and the average pile-up is $\mu = 50$.
• Figure 4: $\gamma Z$ mass distribution for the signal in the $jj\gamma$ channel for two coupling values ($\zeta = 10^{-12}, 10^{-13}$ GeV$^{-4}$, $\tilde{\zeta} =0$) for events within the $0.015<\xi_{1,2}<0.15$ proton detectors acceptance and after the transverse momenta requirement as in Table \ref{['qqbar_table']}. The plot assumes an integrated luminosity of 300 fb$^{-1}$ and an average pile-up of $\mu = 50$.
• Figure 5: Missing diproton mass $m_{pp} = \sqrt{\xi_1\xi_2 s}$ to central mass ratio distribution (Top) and rapidity difference distribution (Bottom) in the $jj\gamma$ channel for the signal and background within the acceptance $0.015<\xi_{1,2}<0.15$ considering two different coupling values after applying the requirement on $p_T$, invariant mass $m_{Z\gamma}$, $p_T$ ratios and angle separation according to Table \ref{['qqbar_table']}. The integrated luminosity is $300$ fb$^{-1}$ and the average pile-up is $\mu = 50$. The signal width is due to a combined effect of the reconstructed jet energy low resolution ($\approx$ 15%) and the $\xi_{1,2}$ resolution from the proton detectors. The asymmetry on the $m_{pp}/m_{Z\gamma}$ distribution is due to the resolution on the jet energy.