B→πlν semileptonic form factor from three-flavor lattice QCD: A model-independent determination of |Vub|

We calculate the form factor f(+)(q(2)) 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(+)(q(2)) simultaneously with that measured by the BABAR experiment using a parameterization of the form-factor shape in q(2), which relies only on analyticity and unitarity in order to determine the Cabibbo-Kobayashi-Maskawa matrix element |V-ub|. This approach reduces the total uncertainty in |V-ub| by combining the lattice and experimental information in an optimal, model-independent manner. We find a value of |V-ub|x10(3)=3.38 +/- 0.36.