Reconstructing a Heavy Neutral Lepton at the LHC

Heavy lepton singlets N slightly mixed with a standard neutrino ν _ℓ are usually searched for at the LHC in the trilepton plus $$ {p}_{\textrm{T}}^{\textrm{miss}} $$ p T 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 |V _ℓN |² for any mass m _N ≥ M _W .