Dimension-six anomalous $tqγ$ couplings in $γγ$ collision at the LHC

TL;DR

The study probes flavor-changing neutral current couplings of the top quark to a photon through dimension-six operators in an effective Lagrangian, focusing on the process $pp\to p\gamma\gamma p\to p t\bar{q}p$ at $\sqrt{s}=14$ TeV using very forward detectors. It derives the $\gamma\gamma\to t\bar{q}$ amplitude, the decay width $\Gamma_{t\to q\gamma}$, and cross sections/kinematic distributions within the EFT, performing sensitivity analyses for $BR(t\to q\gamma)$ across multiple forward-detector acceptances. The results yield 95% CL bounds that improve current limits on $BR(t\to c\gamma)$ for several luminosities, and show that the top-spin asymmetry $A$ is highly sensitive to the ratio of chiral couplings $|\lambda^R|/|\lambda^L|$, enabling discrimination of operator structure. Overall, photon-photon fusion with forward detectors provides a clean, low-background avenue to constrain or discover new physics in top FCNCs and to distinguish between EFT operator realizations.

Abstract

We have investigated the flavor changing top quark physics on the dimension-six anomalous $tqγ$ ($q=u,c$) couplings through the process $pp\to pγγp\to p t\bar{q}p$ at the LHC by considering different forward detector acceptances. In this paper, we have taken into account and examine the effects of top quark decay. The sensitivity bounds on the anomalous couplings and, $t\to qγ$ branching ratio have been obtained at the $95\%$ confidence level for the effective lagrangian approach. Besides, we have investigated the effect of the anomalous couplings on single top quark spin asymmetry.

Figures (8)

• Figure 1: Shematic diagram for the reaction $pp\to p \gamma \gamma p \to pXp$.
• Figure 2: Three level Feynman diagrams for the subprocess $\gamma \gamma \to t\bar{q}$ ($q=u,c$) in the presence of the anomalous dimension-six $tq \gamma$ couplings.
• Figure 3: The cross sections of $pp \to p\gamma \gamma p \to p t\bar{q} p$ as a function of branching ratios of $t\to q\gamma$ ($BR(t \to q\gamma$)) for three forward detector acceptances: $0.0015< \xi < 0.5$, $0.0015< \xi < 0.15$ and $0.015< \xi <0.15$.
• Figure 4: Cross sections of $pp \to p\gamma \gamma p \to pt\bar{q}p$ as a function of the transverse momentum cut on the final state particles for $BR(t\to q\gamma)=0.0005$ and three forward detector acceptances: $0.0015< \xi <0.5$, $0.0015< \xi <0.15$, and $0.1< \xi <0.5$.
• Figure 5: Differential cross sections of $pp \to p\gamma \gamma p \to pt \bar{q} p$ as a function of the transverse momentum on the final state particles for $BR(t\to q\gamma)=0.0005$ and three forward detector acceptances: $0.0015< \xi <0.5$, $0.0015< \xi <0.15$, and $0.015< \xi <0.15$.