  The extension of the standard model with new high-scale weakly coupled
physics involving right-handed neutrinos in an effective field theory framework
(SMNEFT) allows for a systematic study of heavy neutrinos phenomenology in
current and future experiments. We exploit the outstanding angular resolution
in future lepton colliders to study the sensitivity of forward-backward
asymmetries to discover the possible single production of heavy Majorana
neutrinos via $e^{+}e^{-} \to N \nu$, followed by a purely leptonic decay $N
\to \mu^{-} \mu^{+} \nu$ or a semi-leptonic decay $N \to \mu^{-} \mathrm{j}
\mathrm{j} $, for masses $m_N > 50$ GeV. In this regime, we consider the $N$
production and decays to be dominated by scalar and vectorial four-fermion
$d=6$ single $N_R$ operators. This is an alternative analysis to searches using
displaced vertices and fat jets, in a higher mass regime, where the $N$ is
short-lived but can be found by the angular distribution of its decay products.
We find that a forward-backward asymmetry between the final muons in the pure
leptonic decay mode provides a sensitivity up to 12$\sigma$ for $m_N=100$ GeV,
for effective couplings $\alpha=0.2$ and new physics scale $\Lambda=1$ TeV. In
the case of the semi-leptonic decay, we can compare the final muon and higher
$p_T$ jet flight directions, again finding up to 12$\sigma$ sensitivity to the
effective signals.
