Single top production in $e^+ e^-$, $e^- e^-$, $\gamma e$ and $\gamma \gamma$ collisions

Single top quark cross section evaluations for the complete sets of tree-level diagrams in the \(e^+ e^-\), \(e^- e^-\), \(\gamma e\) and \(\gamma \gamma\) modes of the next linear collider with unpolarized and polarized beams are performed within the standard model and beyond. From a comparison of all possibilities we conclude that the process \(\gamma_+ e^-_{\mathrm{L}} \to e^- t \bar b\) is extremely favored due to large cross sections, there being no \(t \bar t\) background, the high degrees of beam polarization, and the exceptional sensitivities to \(V_{tb}\) and anomalous Wtb couplings. Similar reasons favor the process \(e^- e^- \to e^- \nu_e \bar t b\) for probing top quark properties despite a considerably lower cross section. Less favorable are processes like \(e^+ e^-, \gamma \gamma \to e^- \nu_e t \bar b\). Three processes were chosen to probe their sensitivities to anomalous Wtb couplings, with best bounds found for \(\gamma_+ e^-_{\mathrm{L}} \to e^- t \bar b\) and \(e^+_{\mathrm{R}} e^-_{\mathrm{R}} \to e^- \nu_e t \bar b\).