QCD corrections to charged Higgs-mediated b --> c$\tau\nu$decay

We calculate the O(\alpha_s) corrections to the charged Higgs mediated inclusive semi-tauonic B decay. By working in the Landau gauge, we demonstrate how to obtain the charged Higgs contributions (both direct and interference terms) from the known QCD corrections to t \to b H^+. Combining our results with O(1/m_b^2) corrections to the spectator model and comparing the theoretical prediction with the recent experimental measurements, we find a model-independent 2\sigma upper bound on the ratio of Higgs vacuum expectation values, \tan\beta<0.52(m_H/1 GeV).