Physical Review D
We present a lattice-QCD calculation of the B -> pi l nu semileptonic form factors and a new determination of the CKM matrix element vertical bar V-ub vertical bar. We use the MILC asqtad (2 + 1)-flavor lattice configurations at four lattice spacings and light-quark masses down to 1/20 of the physical strange-quark mass. We extrapolate the lattice form factors to the continuum using staggered chiral perturbation theory in the hard-pion and SU (2) limits. We employ a model-independent z parametrization to extrapolate our lattice form factors from large-recoil momentum to the full kinematic range. We introduce a new functional method to propagate information from the chiral-continuum extrapolation to the z expansion. We present our results together with a complete systematic error budget, including a covariance matrix to enable the combination of our form factors with other lattice-QCD and experimental results. To obtain vertical bar V-ub vertical bar, we simultaneously fit the experimental data for the B -> pi l nu differential decay rate obtained by the BABAR and Belle collaborations together with our lattice form-factor results. We find vertical bar V-ub vertical bar = (3.72 +/- 0.16) x 10(-3), where the error is from the combined fit to lattice plus experiments and includes all sources of uncertainty. Our form-factor results bring the QCD error on vertical bar V-ub vertical bar to the same level as the experimental error. We also provide results for the B -> pi l nu vector and scalar form factors obtained from the combined lattice and experiment fit, which are more precisely determined than from our lattice-QCD calculation alone. These results can be used in other phenomenological applications and to test other approaches to QCD.
Bailey, J. A., Bernard, C., DeTar, C., Di Pierro, M., El-Khadra, A. X., Evans, R. T., ... & Hetrick, J. E. (2008). The B-> pi l nu semileptonic form factor from three-flavor lattice QCD: A model-independent determination of| V (ub)|. arXiv preprint arXiv:0811.3640.