Heavy Majorana Neutrinos ate+e−Colliders

We investigate possibilities for detecting heavy Majorana neutrinos ( $N$'s) in $e^{+}{e}^{-}$ at LEP200 and future linear colliders. We concentrate on the processes where the pairs of intermediate heavy $N$'s produce a clear signal of total lepton number violation $(e^{+}{e}^{-}\to \mathrm{NN}\to W^{+}{l}^{-}{W}^{+}{l}^{\prime -})$. Such a signal is not possible if the heavy neutrinos are of Dirac nature. Our approach is general in the sense that the intermediate $N$'s can be either on shell or off shell. Discussion of the relative numerical importance of the $s$ and the $t+u$ channels of the $\mathrm{NN}$ production is also included.