Sensitivity to Single Vector-Like Bottom Quark Production at 3 TeV CLIC
We study the sensitivity of the Compact Linear Collider at $\sqrt{s}=3~\mathrm{TeV}$ to the single electroweak production of a vector-like bottom quark in a type-II Two-Higgs-Doublet Model extended by vector-like quarks. We consider the process $e^+e^-\to B\bar b+\bar B b$, followed by $B\to\phi b$ with $\phi=H,A$ and $\phi\to t\bar t$. We focus on the semileptonic final state with one charged lepton, missing transverse momentum, four $b$-tagged jets, and two light jets. This channel directly probes the interplay between the extended quark and Higgs sectors. After applying theoretical constraints, electroweak precision constraints, Higgs-sector bounds, and current collider limits, we perform detector-level cut-based and multivariate analyses. The cut-based analysis remains below discovery sensitivity at $5~\mathrm{ab}^{-1}$, while a multilayer perceptron and an XGBoost classifier provide substantially improved signal-background separation. For viable scenarios with $m_B\simeq1.3$--$1.5~\mathrm{TeV}$, the expected significance remains above $5\sigma$ even for a $15\%$ uncertainty on the background normalization.