  Some of the outstanding questions of particle physics today concern the
neutrino sector, in particular whether there are more neutrinos than those
already known and whether they are Dirac or Majorana particles.There are
different ways to explore these issues. In this article we describe
neutrino-mediated decays of charged pseudoscalar mesons such as $\pi^{\pm}$,
$K^{\pm}$ and $B^{\pm}$, in scenarios where extra neutrinos are heavy and can
be on their mass shell. We discuss semileptonic and leptonic decays of such
kinds. We investigate possible ways of using these decays in order to
distinguish between the Dirac and Majorana character of neutrinos. Further, we
argue that there are significant possibilities of detecting CP violation in
such decays when there are at least two almost degenerate Majorana neutrinos
involved. This latter type of scenario fits well into the known neutrino
minimal standard model ($\nu$MSM) which could simultaneously explain the Dark
Matter and Baryon Asymmetry of the Universe.
