  In this paper, we present the differential decay distributions for $\bar B
\to D^{(*)} \ell \bar{X}$ decays with a massive right-handed neutrino in the
low-energy effective field theory framework and show how the massive effects of
the RH neutrinos can explain the positive value of the difference in
forward-backward asymmetries, $\Delta A_{\text{FB}}\equiv
A_{\text{FB}}^\mu-A_{\text{FB}}^e$, tentatively inferred from Belle data. We
also make predictions for $q^2$ dependent angular observables to motivate
future measurements.
