Flavour and polarisation in heavy neutrino production at e+e− colliders

We analyse ℓWν production at ILC, paying special attention to the role of the final lepton flavour and beam polarisation in the search for a new heavy neutrino N. We show that a sizeable coupling to the electron VeN∼10−2 is necessary to have an observable signal in any of the channels, despite the fact that the signal may be more visible in muon or tau final states. The non-observation of a heavy neutrino at ILC will improve the present upper bound on its mixing with the electron by more than one order of magnitude, VeN⩽0.007 for mN between 200 and 400 GeV.