  In this work, we study the lepton number violating Bc meson decays via one
intermediate on-shell heavy neutrino. The specific studied process is
$B_{c}^{+} \to \mu^{+} \ N \to \mu^{+} \mu^{+} \tau^{-} \nu$ which could allow
distinguishing the nature of the heavy neutrino nature (Dirac or Majorana) by
studying the tau lepton energy spectrum in the LHCb experiment. The result
suggests that this signature could be observed in the collected data during the
HL-LHCb lifetime.
