  We investigate the possible contribution of Majorana neutrinos to $B$ meson
decays in an effective interaction formalism, in the mass range $0.5$ GeV
$<m_N<5 $ GeV. We study the decay of the $B^-$ meson via $B^- \to \mu^- \mu^-
\pi^+$ at LHCb, which is a signal for leptonic number violation and the
presence of Majorana neutrinos, and put bounds on different new physics
contributions, characterized by their Dirac-Lorentz structure. We also study
the bounds imposed by the radiative $B$ decay ($B^- \rightarrow \mu^- \nu
\gamma$) results from Belle. The obtained bounds are more restrictive than
previous values found for dimension 6 four-fermion contact vectorial and scalar
Majorana neutrino interactions in the context of the Left-Right symmetric model
for higher Majorana masses at the LHC, showing that the direct calculation of
the effective $N$ interactions contribution to different processes can help to
put more stringent bounds to different UV-complete models parameterized by an
effective Lagrangian.
