Majorana vs. Dirac sterile neutrinos lighter than MW at the LHC

We propose to study the leptonic decays W ± → e ± e ± µ ∓ ν and W ± → µ ± µ ± e ∓ ν at the LHC to discover sterile neutrinos with masses below M W , and discriminate their Majorana or Dirac character. These decays are induced by a sterile neutrino N that goes on mass shell in the intermediate state. We find that, even though the final (anti-)neutrino goes undetected and thus lepton number is unchecked, one can distinguish between the Majorana vs. Dirac character of the intermediate sterile neutrino by comparing the production of e ± e ± µ ∓ vs. µ ± µ ± e ∓ , provided the N - e and N - µ mixings are different enough. Alternatively, one can also distinguish the Majorana vs. Dirac character by studying the energy spectra of the opposite charge lepton, a method that works even if the N - e and N - µ mixings are equal.