Top quark FCNC couplings at future circular hadron electron colliders

A study of single top quark production via flavor changing neutral current interactions at $tq\gamma$ vertices is performed at the future circular hadron electron collider. The signal cross sections for the processes $e^{-}p\to e^{-}{W}^{\pm }q+X$ and $e^{-}p\to e^{-}{W}^{\pm }bq+X$ in the collision of an electron beam with energy $E_e=60\mathrm{GeV}$ and a proton beam with energy $E_p=50\mathrm{TeV}$ are calculated. In the analysis, the invariant mass distributions of three jets reconstructing top quark mass, requiring one b-tagged jet and two other jets reconstructing the $W$ mass are used to count signal and background events after all selection cuts. The upper limits on the anomalous flavor changing neutral current $tq\gamma$ couplings are found to be ${\lambda }_q<0.01$ at the future circular hadron electron collider for $L_{\mathrm{int}}=100{\mathrm{fb}}^{-1}$ with the fast simulation of detector effects. Signal significance depending on the couplings ${\lambda }_q$ is analyzed and an enhanced sensitivity is found to the branching ratio $\mathrm{BR}(t\to q\gamma )$ at the future circular hadron electron collider when compared to the current experimental results.

Figure 1

Feynman diagrams for single top quark production through FCNC vertices and the top quark decays via charged current. The first two diagrams correspond to subprocess $e^{-}q\to e^{-}{W}^{+}b$, while the others correspond to $e^{-}g\to e^{-}q{W}^{+}b$ which contributes to the signal process.

Figure 2

The distribution of the jet size in signal events and also in the important backgrounds; $B_{tt}$: $e^{-}p\to e^{-}t\overline{t}+X$, $B_{bjj}$: $e^{-}{q}_b\to e^{-}{q}_b{j}_2+X$, with $q_b=b$ or $\overline{b}$ and $j_2=q\overline{q}$ or $gg$.

Figure 3

Distributions of reconstructed invariant mass of top quark plots for signal and relevant backgrounds with different anomalous FCNC couplings. The lower part of each plot shows the relative ratio of ($S+B$) and $B$.

Figure 4

On the left, the statistical significance depending on integrated luminosity for different anomalous FCNC couplings ($\lambda$). On the right, the integrated luminosity versus anomalous FCNC couplings at $3\sigma$ and $5\sigma$ significance.