Probing the Single Production of First-Generation Singlet Vector-like Leptons at Future $e^{+}{e}^{-}$ Colliders

We study the single production of first-generation weak-isosinglet vector-like leptons (VLLs) at future $e^{+}{e}^{-}$ colliders, considering the channels $e^{+}{e}^{-}\to e^{\pm }{E}^{\mp }$ with $E^{\pm }\to W^{\pm }{\nu }_e$ and $E^{\pm }\to e^{\pm }Z$. For the heavy VLL masses under consideration, the decay products of the highly boosted $W$ and $Z$ bosons merge into a single fat-jet, providing a powerful handle for signal identification and background suppression. A comprehensive Monte Carlo simulation is carried out at $\sqrt{s}=1$ TeV at the International Linear Collider (ILC) and 1.5 TeV at the Compact Linear Collider (CLIC). The $2\sigma$ exclusion and $5\sigma$ discovery reaches are determined as functions of the integrated luminosity and the mixing parameter $\sin {\theta }_L$ for representative benchmark masses. Our results show that future $e^{+}{e}^{-}$ colliders can effectively probe the first-generation singlet VLL scenario through these channels. The 1 TeV ILC can probe masses up to 900 GeV, while the 1.5 TeV CLIC extends the reach to 1400 GeV, surpassing existing limits from hadron colliders and complementing constraints from electroweak precision measurements.