Decay of $B^pm to tau^pm +$ missing momentum and direct measurement of the mixing parameter $U_{tau N}$

We derive the decay widths for the leptonic decays of heavy charged pseudoscalars to massive sterile neutrinos, $M^{pm} to ell^{pm} + N$, within the frameworks involving the Standard Model and two-Higgs doublets (type II). We then apply the result to $B^pm to tau^pm +$ missing momentum of the Belle/BaBar experimental results, in order to measure directly the relevant parameter space, including the mixing parameter $U_{tau N}$.