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

TL;DR

This paper assesses the potential to detect top quark flavor-changing neutral currents (FCNC) at future electron-positron and muon colliders, building on LHC constraints and exploring new sensitivity levels for rare top decays.

Contribution

It provides a comprehensive analysis of FCNC sensitivity at future colliders, incorporating LHC limits and proposing new search strategies for rare top interactions.

Findings

LHC currently sets the strongest FCNC limits among colliders.

Future colliders could probe FCNC couplings down to BR(t→cZ)~10^{-6}.

High-energy muon colliders can explore contact interactions involving top and charm quarks.

Abstract

We present the sensitivity to Flavor Changing Neutral Currents (FCNC) in interactions involving the top quark at future $e^+e^-$ and $\mu^+\mu^-$ 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.