B→πlν semileptonic form factor from three-flavor lattice QCD: a model-independent determination of [Vub]

We calculate the form factor f+(q2) for B-meson semileptonic decay in unquenched lattice QCD with 2 + 1 flavors of light sea quarks. We use Asqtad-improved staggered light quarks and a Fermilab bottom quark on gauge configurations generated by the MILC Collaboration. We simulate with several light-quark masses and at two lattice spacings, and extrapolate to the physical quark mass and continuum limit using heavy-light meson staggered chiral perturbation theory. We then fit the lattice result for f+(q2) simultaneously with that measured by the BABAR experiment using a parameterization of the form-factor shape in q2, which relies only on analyticity and unitarity in order to determine the Cabibbo-Kobayashi-Maskawa matrix element [Vub]. This approach reduces the total uncertainty in [Vub] by combining the lattice and experimental information in an optimal, model-independent manner.We find a value of [Vub] X 103 = 3:38 + 0:36.