The 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 |×103 = 3.38 ± 0.3