B → πlv semileptonic form factors from unquenched lattice QCD and determination of |V_ub|

We compute the B <→> πlv semileptonic form factors and update the determination of the CKM matrix element |V_ub|. We use the MILC asqtad ensembles with N_f = 2 + 1 sea quarks at four different lattice spacings in the range a ≈ 0.045 fm to 0.12 fm. The lattice form factors are extrapolated to the continuum limit using SU(2) staggered chiral perturbation theory in the hard pion limit, followed by an extrapolation in q² to the full kinematic range using a functional zparameterization. The extrapolation is combined with the experimental measurements of the partial branching fraction to extract |V_ub|. Our preliminary result is |V_ub| = (3.72±0.14) x 10^-3, where the error reflects both the lattice and experimental uncertainties, which are now on par with each other.