On the Inclusive Determination of Vub from the Lepton Invariant Mass Spectrum

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Bauer, Ligeti and Luke have recently proposed a new method for measuring |Vub| in inclusive semileptonic B decays, using a cut on the lepton invariant mass to discriminate against b->c transitions. We investigate the structure of the heavy-quark expansion for this case and show that to all orders the magnitude of the leading perturbative and nonperturbative corrections is controlled by a hadronic scale muc=O(mc) depending on the minimal value of q². These corrections can be analyzed using a modified version of the heavy-quark expansion (``hybrid expansion''). We find that the theoretical uncertainty in the extraction of |Vub| is a factor 2.5 larger than previously estimated, which allows for a determination with 10% accuracy.