Reconstructing a Heavy Neutral Lepton at the LHC

Heavy lepton singlets $N$ slightly mixed with a standard neutrino ${\nu }_{\mathrm{\ell }}$ are usually searched for at the LHC in the trilepton plus $p_{\mathrm{T}}^{\mathrm{m}\mathrm{i}\mathrm{s}\mathrm{s}}$ channel: $pp\to W^{+}\to {\mathrm{\ell }}^{+}N$ with $N\to {\mathrm{\ell }}^{-}{W}^{+}\to {\mathrm{\ell }}^{-}{\mathrm{\ell }}^{\mathrm{\prime }+}\nu$. We show that, although the longitudinal momentum of the final $\nu$ 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 systematically improve the current collider bounds on the mixing $\mathrm{\mid }{V}_{\mathrm{\ell }N}{\mathrm{\mid }}^2$ for any mass $m_N\ge M_W$.