Comment on form-factor shape and extraction of |Vub| from B→πlν

We point out that current experimental data for partial B→πlν branching fractions reduce the theoretical input required for a precise extraction of |Vub| to the form-factor normalization at a single value of the pion energy. We show that the heavy-quark expansion provides a bound on the form-factor shape that is orders of magnitude more stringent than conventional unitarity bounds. We find |Vub|=(3.7±0.2±0.1)×[0.8/F+(16GeV2)]. The first error is from the experimental branching fractions, and the second is a conservative bound on the residual form-factor shape uncertainty, both of which will improve with additional data. Together with current and future lattice determinations of the form-factor normalization this result gives an accurate, model independent determination of |Vub|. We further extract semileptonic shape observables such as |VubF+(0)|=0.92±0.11±0.03 and show how these observables can be used to test factorization and to determine low-energy parameters in hadronic B decays.