We present a phenomenological study of single production of a vector-like top partner $T$ at a high-energy $e^+e^-$ collider, focusing on a mass-agnostic strategy centered on the recoil-mass observable. Using a CLIC-like setup at $\sqrt{s}=3~\text{TeV}$ and $\mathcal{L}=5~\text{ab}^{-1}$, we design a compact selection that remains robust across $m_T$ hypotheses by avoiding narrow mass windows and instead fitting (or windowing) the recoil-mass spectrum against a reconstructed system $X$. Standard-Model backgrounds—dominated by $t\bar t$—are simulated and normalized to the target luminosity. We report baseline sensitivities at a representative coupling $\kappa=0.2$ with ISR and beam polarization turned off, and then assess realistic running conditions by enabling ISR and studying electron/positron beam polarizations. Finally, we map the expected discovery significance $\mathcal{Z}$ in the $(m_T,\kappa)$ plane. The results illustrate that a recoil-mass–based, model-agnostic selection can deliver competitive sensitivity to single-$T$ production at lepton colliders and offers complementary coverage to hadron-collider searches.

hmvd2002@gmail.com

Higinio Manuel Valle Delgado