Searching for Invisible and Almost Invisible Particles ate+e−Colliders

We explore the techniques, cross sections, and expected signal significance for detecting invisible and almost invisible particles at CERN $e^{+}{e}^{-}$ collider LEP2 and the NLC (next linear $e^{+}{e}^{-}$ collider) by means of a hard photon tag. Examples from supersymmetry include the lightest chargino and second lightest neutralino when their masses are nearly the same as that of the lightest neutralino [the LSP (lightest supersymmetric particle)], and invisibly decaying sneutrinos. The importance of particular features of the detectors is discussed.