Searching for invisible and almost invisible particles at e(+)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.