In the context of the Phenomenological Type I Seesaw, we investigate the LHC’s sensitivity to exclusive, mesonic decay modes of long-lived, light (Dirac and Majorana) heavy neutrinos N when they are produced in the decays of W bosons. We present a new framework that combines massless QCD to describe N ’s production up to NLO in QCD via weak bosons with a low-energy effective field theory to...