Reconstructing a Heavy Neutral Lepton at the LHC

Heavy lepton singlets $N$ slightly mixed with a standard neutrino $\nu_\ell$ are usually searched at the LHC in the trilepton plus $p_{\rm T}^{\rm miss}$ channel: $pp \to W^+ \to \ell^+ N$ with $N\to \ell^- W^+ \to \ell^- \ell'^+ \nu$. We show that for $m_N>m_W$ the mass of the heavy lepton can be reconstructed. For $m_N\approx 90$-$130$ GeV and the current luminosity (138 fb$^{-1}$ at 13 TeV), we estimate that this reconstruction would set the bound on $|V_{\ell N}|^2$ at $4\times 10^{-4}$, versus the limit around $2\times 10^{-3}$ obtained in a recent CMS analysis or $|V_{\ell N}|^2<10^{-3}$ from flavor and electroweak precision data.