Radiative production of invisible charginos in photon photon collision
  If in a supersymmetric model, the lightest chargino is nearly degenerate with the lightest neutralino, the former can decay into the latter alongwith a soft pion (or a lepton-neutrino pair). Near degeneracy of the chargino and neutralino masses can cause the other decay products (the pion or the lepton) to be almost invisible. Photon-photon colliders offer a possibility of clean detection of such an event through a hard photon tag. 
Debajyoti Choudhury, Biswarup Mukhopadhyaya, Subhendu Rakshit, Anindya   Datta