Probe Anomalous $tqγ$ couplings through Single Top Photoproduction at the LHC

TL;DR

The paper addresses constraints on anomalous $tqγ$ couplings in top quark FCNC interactions at the LHC using photon-induced photoproduction with forward proton detectors. It adopts a model-independent effective Lagrangian with coupling $κ_{tqγ}$ and employs the Equivalent Photon Approximation to model quasi-real photons in $\gamma p$ collisions, analyzing two channels: direct leading single-top photoproduction and single-top jet associated photoproduction. The authors implement the $tqγ$ vertices in FeynArts, compute signal and backgrounds with CT10 PDFs via BASES, apply realistic cuts and $b$-tagging efficiencies, and derive $95\%$ C.L. bounds on $κ_{tqγ}$ as a function of luminosity and detector acceptance. Their results show bounds tightening from the $\mathcal{O}(10^{-2})$ level at a few fb$^{-1}$ to the $\mathcal{O}(10^{-3})$ level at several hundred fb$^{-1}$, with corresponding $Br(t\rightarrow qγ)$ reaching the few$\times 10^{-6}$–$10^{-5}$ range, and highlight complementary sensitivity between the two channels along with practical pileup considerations for high-luminosity runs. Overall, the study demonstrates that forward-detector–assisted photon-induced processes at the LHC can substantially improve constraints on anomalous $tqγ$ couplings beyond current limits, while acknowledging potential challenges at high pileup.

Abstract

In this work we study the constraints on the anomalous $tqγ$ ($q=u$, $c$) couplings by photon-produced leading single top production and single top jet associated production through the main reaction $pp\rightarrow pγp\rightarrow pt\rightarrow pW(\rightarrow\ell ν_\ell) b+X$ and $pp\rightarrow pγp\rightarrow ptj\rightarrow pW(\rightarrow\ell ν_\ell) bj+X$ assuming a typical LHC multipurpose forward detectors in a model independent effective lagrangian approach. Our results show that: for the typical detector acceptance $0.0015<ξ_1<0.5$, $0.1<ξ_2<0.5$ and $0.0015<ξ_3<0.15$ with a luminosity of 2 $\rm{fb}^{-1}$, the lower bounds of $κ_{tqγ}$ through leading single top channel (single top jet channel) are 0.0096 (0.0115), 0.0162 (0.0152) and 0.0098 (0.0122), respectively, correspond to $\rm{Br}(t\rightarrow qγ)\sim 3\times 10^{-5}$. With a luminosity of 200 $\rm{fb}^{-1}$, the lower bounds of $κ_{tqγ}$ are 0.0031 (0.0034), 0.0051 (0.0047) and 0.0031(0.0038), respectively, correspond to $\rm{Br}(t\rightarrow qγ)\sim 4\times 10^{-6}$. We conclude that both channels can be used to detect such anomalous $tqγ$ couplings and the detection sensitivity on $κ_{tqγ}$ can be improved.

Figures (10)

• Figure 1: Partonic Feynman diagrams for $\rm \gamma q \rightarrow W(\rightarrow \ell\nu_\ell) + b$ with q=u, c. Black blobs represent the anomalous $\rm tq\gamma$ couplings parameterized by Eq.(\ref{['lagrangian']}).
• Figure 2: The transverse momentum distributions for the jet ($\rm p_T^{jet}$) of $\rm pp\rightarrow p\gamma p\rightarrow pW(\rightarrow\ell\nu_{\ell})j$ ($\rm \ell=e$, $\mu$) with basic cuts in Eq.(\ref{['basiccuts']}). The anomalous coupling is chosen to be $\rm \kappa_{tu\gamma}=\kappa_{tc\gamma}=\kappa=0.01(0.03)$. The forward detector acceptance is chosen to be $0.0015<\xi_1<0.5$. The b-tagging efficiency and the rejection factors for the light jets are taken into account.
• Figure 3: The transverse momentum distributions for the jet ($\rm p_T^{jet}$) of $\rm pp\rightarrow p\gamma p\rightarrow pW(\rightarrow\ell\nu_{\ell})j$ ($\rm \ell=e$, $\mu$) with basic cuts in Eq.(\ref{['basiccuts']}) plus invariant mass (W+jet) cut ($\rm 150~{\rm GeV}<M_{Wj}<200~{\rm GeV}$[left panel], $\rm 160~{\rm GeV}<M_{Wj}<180~{\rm GeV}$[right panel]). The anomalous coupling is chosen to be $\rm \kappa_{tu\gamma}=\kappa_{tc\gamma}=\kappa=0.01(0.03)$. The forward detector acceptance is chosen to be $0.0015<\xi_1<0.5$. The b-tagging efficiency and the rejection factors for the light jets are taken into account.
• Figure 4: The pseudorapidity distributions for the jet ($\rm y^{jet}$) of $\rm pp\rightarrow p\gamma p\rightarrow pW(\rightarrow\ell\nu_{\ell})j$ ($\rm \ell=e$, $\mu$) with basic cuts in Eq.(\ref{['basiccuts']}), $\rm p_T^{jet}>35\ GeV$ plus invariant mass (W+jet) cut ($\rm 160\ GeV<M_{Wj}<180~\ GeV$) [left panel] and the reconstruction of the top quark mass [right panel].
• Figure 5: The total cross sections of signal processes $\rm pp\rightarrow p\gamma p\rightarrow pW(\rightarrow\ell \nu_{\ell}) b$ as functions of the anomalous $\rm \kappa_{tq\gamma}$ coupling and three forward detector acceptance regions: $0.0015<\xi_1<0.5$, $0.1<\xi_2<0.5$ and $0.0015<\xi_3<0.15$.