On the inclusive determination of Vub from the lepton invariant mass spectrum

Bauer, Ligeti and Luke have recently proposed a new method for measuring|V_{ub}| in inclusive semileptonic B decays, using a cut on the leptoninvariant mass to discriminate against b->c transitions. We investigate thestructure of the heavy-quark expansion for this case and show that to allorders the magnitude of the leading perturbative and nonperturbativecorrections is controlled by a hadronic scale mu_c=O(m_c) depending on theminimal value of q^2. These corrections can be analyzed using a modifiedversion of the heavy-quark expansion (``hybrid expansion''). We find that thetheoretical uncertainty in the extraction of |V_{ub}| is a factor 2.5 largerthan previously estimated, which allows for a determination with 10% accuracy.Comment: 19 pages, 8 figure