Top quark FCNC in Randall-Sundrum models: post-LHC allowed rates and searches at $e^+e^-$ and $μ^+ μ^-$ colliders

TL;DR

The paper tackles top quark FCNCs within Randall-Sundrum models by mapping RS-induced $Ztc$ interactions onto SMEFT operators and recasting LHC Heavy Vector Triplet searches to bound RS resonances. It then assesses the reach of future $e^+e^-$ and $\mu^+\mu^-$ colliders in probing $Ztc$ and four-fermion top–charm FCNCs, using cut-based and machine-learning analyses of the $\ell^+\ell^- \to t c$ channel across Higgs-factory and multi-TeV energies. The key findings show HL-LHC can constrain ${\rm BR}(t\to Zc)$ down to the $\sim 10^{-6}$ level for $M_{KK}>5$ TeV, while a 240 GeV $e^+e^-$ Higgs factory and especially a 10 TeV $\mu^+\mu^-$ collider can substantially improve bounds on SMEFT operators—dipole and four-fermion operators benefiting most from higher energy. The work emphasizes the energy-dependent sensitivity patterns of different operator classes and highlights the complementary capabilities of future lepton colliders to explore flavor structure beyond HL-LHC.

Abstract

We present the sensitivity to Flavor Changing Neutral Currents (FCNC) in interactions involving the top quark at future $e^+e^-$ and $μ^+μ^-$ machines. We consider the $Ztc$ vertex as well as four-fermion contact interactions involving top and charm quarks. To incorporate limits from (HL-)LHC we consider FCNC from Randall-Sundrum models and we recast LHC searches for the resonances that at the microscopic level give rise to the FCNC effects. We determine the maximal strength of the effective FCNC couplings $Ztc$ coupling allowed by LHC. We find that the LHC currently improves on the limit set by previous machines, e.g. LEP indirect sensitivity to heavy vectors. Future improvements of direct searches at HL-LHC may reach a level equivalent to $BR(t\to c Z)\simeq 10^{-6}$. We explore the possibility to probe even smaller FCNC coupling strength using an $e^+e^-$ machine at center-of-mass energy suitable for a Higgs factory $E_{cm}\in$ [200,240] GeV or to probe contact interactions involving top and charm flavors at a high-energy muon collider at $E_{cm}=10$ TeV.

Figures (9)

• Figure 1: $95\%$ confidence level (C.L.) limits (blue) on the production of $W_{KK}$ decaying into $WH$ final states as a function of mass, based on the results of CMS:2022pjv. The orange lines represent the predictions from the HVT-B benchmark model Pappadopulo:2014qza with $g_V = 3$. The green bands indicate the expected cross sections for vector resonances production at the 13 TeV LHC in the RS model, computed using the branching fractions listed in \ref{['tab:branching_ratio_RS']}.
• Figure 2: BSM contribution to lepton annihilation into a $tc$ pair from a $Z$ boson FCNC coupling.
• Figure 3: Distribution of $m_{t, \textrm{recoil}}$ (left) and $m_{jj}$ (right) of signal and background events at the HTE factory. The darker and lighter shades correspond to 1% and 5% gaussian energy smearing for the jets and the leptons.
• Figure 4: Left: Output of the BDT in arbitrary units. Right: BDT score in the plane $m_{t,recoil}$, $m_{jj}$.
• Figure 5: Bounds on Wilson coefficients $\mathcal{C}_{\varphi u}^{(3+2)}$, $\mathcal{C}_{uZ}^{(23)}$ and $\mathcal{C}_{lq}^{-(1,3+2)}$ as function of $\sqrt{\hat{s}}$ using a cut based analysis and BDT.