Single Leptoquark Production at $e^{+}{e}^{-}$ and $\gamma \gamma$ Colliders

We consider single production of leptoquarks (LQ’s) at $e^{+}{e}^{-}$ and $\gamma \gamma$ colliders, for two values of the centre-of-mass energy, $\sqrt{s}=500$ GeV and 1 TeV. We find that LQ’s which couple within the first generation are observable for LQ masses almost up to the kinematic limit, both at $e^{+}{e}^{-}$ and $\gamma \gamma$ colliders, for the LQ coupling strength equal to ${\alpha }_{em}$. The cross sections for single production of $2^{nd}$- and $3^{rd}$-generation LQ’s at $e^{+}{e}^{-}$ colliders are too small to be observable. In $\gamma \gamma$ collisions, on the other hand, $2^{nd}$-generation LQ’s with masses much larger than $\sqrt{s}\mathrm{/}2$ can be detected. However, $3^{rd}$-generation LQ’s can be seen at $\gamma \gamma$ colliders only for masses at most $\sim \sqrt{s}\mathrm{/}2$, making their observation more probable via the pair production mechanism.