B → πlν and Bs → Klν form factors and |Vub| from 2+1-flavor lattice QCD with domain-wall light quarks and relativistic heavy quarks

We calculate the form factors for B --> pi l nu and B-s --> Kl nu decay in dynamical lattice quantum chromodynamics (QCD) using domain-wall light quarks and relativistic b-quarks. We use the (2 + 1)-flavor gauge-field ensembles generated by the RBC and UKQCD collaborations with the domain-wall fermion action and Iwasaki gauge action. For the b-quarks we use the anisotropic clover action with a relativistic heavy-quark interpretation. We analyze data at two lattice spacings of a approximate to 0.11, 0.086 fm with unitary pion masses as light as M-pi approximate to 290 MeV. We simultaneously extrapolate our numerical results to the physical light-quark masses and to the continuum and interpolate in the pion/kaon energy using SU(2) "hard-pion" chiral perturbation theory for heavy-light meson form factors. We provide complete systematic error budgets for the vector and scalar form factors f(+)(q(2)) and f(0)(q(2)) for both B --> pi l nu and B-s --> Kl nu at three momenta that span the q(2) range accessible in our numerical simulations. Next we extrapolate these results to q(2) = 0 using a model-independent z-parametrization based on analyticity and unitarity. We present our final results for f(+)(q(2)) and f(0)(q(2)) as the coefficients of the series in z and the matrix of correlations between them; this provides a parametrization of the form factors valid over the entire allowed kinematic range. Our results agree with other three-flavor lattice-QCD determinations using staggered light quarks, and have comparable precision, thereby providing important independent cross-checks. Both B --> pi l nu and B-s --> Kl nu decays enable determinations of the Cabibbo-Kobayashi-Maskawa matrix element \V-ub\. To illustrate this, we perform a combined z-fit of our numerical B --> pi l nu form-factor data with the experimental measurements of the branching fraction from BABAR and Belle leaving the relative normalization as a free parameter; we obtain \V-ub\ = 3.61(32) x 10(-3), where the error includes statistical and all systematic uncertainties. The same approach can be applied to the decay B-s --> Kl nu to provide an alternative determination of \V-ub\ once the process has been measured experimentally. Finally, in anticipation of future experimental measurements, we make predictions for B --> pi l nu and B-s --> Kl nu differential branching fractions and forward-backward asymmetries in the Standard Model.