Single Dilepton Production at $e^+e^-$, $eγ$ and $γγ$ Colliders

We calculate the cross sections for the single production of doubly-charged dileptons, both scalar and vector, at $e^+e^-$, $e\gamma$ and $\gamma\gamma$ colliders at $\sqrt{s}=500$ GeV and 1 TeV. The $e\gamma$ mode is by far the most promising -- dileptons whose coupling is as weak as $O(10^{-4})\alpha_{em}$ can be observed, for masses virtually up to the kinematic limit. Dileptons of mass up to $\sqrt{s}$ can also be seen in $e^+e^-$ and $\gamma\gamma$ colliders, for couplings of order $\alpha_{em}$. In all three colliders, most of the cross section comes from events in which the only particles detected are $e^-e^-$, the decay products of the dilepton, yielding an unmistakeable experimental signature.