Probing tqZ anomalous couplings in the trilepton signal at the HL-LHC, HE-LHC and FCC-hh

Abstract

We investigate the prospects for discovering the Flavour Changing Neutral Current (FCNC) $tqZ$ couplings via two production processes yielding trilepton signals: top quark pair production $pp\to t\bar{t}$ with one top decaying to the $Z$ boson and one light jet and the anomalous single top plus $Z$ boson production process $pp\to tZ$. We study these channels at various successors of the Large Hadron Collider~(LHC), i.e., the approved High-Luminosity LHC (HL-LHC) as well as the proposed High-Energy LHC~(HE-LHC) and Future Circular Collider in hadron-hadron mode (FCC-hh). We perform a full simulation for the signals and the relevant Standard Model (SM) backgrounds and obtain limits on the Branching Ratios (BRs) of $t\to qZ(q=u,c)$, eventually yielding a trilepton final state through the decay modes $t\to b{W}^{+}\to b{\ell }^{+}{\nu }_{\ell }$ and $Z\to {\ell }^{+}{\ell }^{-}$. The upper limits on these FCNC BRs at 95\% Confidence Level (CL) are obtained at the HL-LHC with $\sqrt{s}=14$ TeV and 3 ab${}^{-1}$, at the HE-LHC with $\sqrt{s}=27$ TeV and 15 ab${}^{-1}$ as well as at the FCC-hh with $\sqrt{s}=100$ TeV and 30 ab${}^{-1}$.

Comments: 21 pages, 6 figures, 8 tables, to appear in Chinese Physics C