  The simplest extension of the SM to account for the observed neutrino masses
and mixings is the addition of at least two singlet fermions (or right-handed
neutrinos). If their masses lie at or below the GeV scale, such new fermions
would be produced in meson decays. Similarly, provided they are sufficiently
heavy, their decay channels may involve mesons in the final state. Although the
couplings between mesons and heavy neutrinos have been computed previously,
significant discrepancies can be found in the literature. The aim of this paper
is to clarify such discrepancies and provide consistent expressions for all
relevant effective operators involving mesons with masses up to 2 GeV.
Moreover, the effective Lagrangians obtained for both the Dirac and Majorana
scenarios are made publicly available as FeynRules models so that fully
differential event distributions can be easily simulated. As an application of
our setup, we numerically compute the expected sensitivity of the DUNE near
detector to these heavy neutral leptons.
