Model-Independent $Z'$ Limits from Electron-Electron Collisions
  Model independent constraints on the mass of an extra neutral gauge boson and its couplings to charged leptons are given for the $e^-e^-$ option of a future linear collider. Analytic exclusion limits are derived in the Born approximation. The results are compared with those of the $e^+e^-$ mode. The influence of radiative corrections is discussed. 
D. Choudhury, F. Cuypers, A. Leike