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

TL;DR

This study evaluates the potential to detect rare top quark flavor-changing neutral current interactions involving the Z boson at future high-energy colliders, using detailed simulations of signal and background processes.

Contribution

It provides the first comprehensive analysis of $tqZ$ FCNC couplings at HL-LHC, HE-LHC, and FCC-hh, including full simulation and sensitivity limits for these colliders.

Findings

Sets upper limits on $t o qZ$ branching ratios at different colliders.

Demonstrates increased sensitivity at higher collider energies and luminosities.

Provides benchmark sensitivities for future collider experiments.

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$^{-1}$.