Heavy lepton singlets N slightly mixed with a standard neutrino νℓ are usually searched for at the LHC in the trilepton plus \( {p}_{\textrm{T}}^{\textrm{miss}} \) channel: pp → W + → ℓ+N with N → ℓ−W + → ℓ−ℓ′+ν. We show that, although the longitudinal momentum of the final ν escapes detection, the mass of the heavy lepton can be reconstructed. While this possibility has not been considered in recent LHC searches, we find that the search for a mass peak could improve the current collider bounds on the mixing |VℓN|2 for any mass mN ≥ MW.
