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

J. M. Flynn, T. Izubuchi, T. Kawanai, C. Lehner, A. Soni, R. S. Van De Water, O. Witzel

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Abstract / Description of output

We calculate the form factors for B→π ν and Bs→K ν 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≈0.11, 0.086 fm with unitary pion masses as light as Mπ≈290MeV. 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+(q2) and f0(q2) for both B→π ν and Bs→K ν at three momenta that span the q2 range accessible in our numerical simulations. Next we extrapolate these results to q2=0 using a model-independent z-parametrization based on analyticity and unitarity. We present our final results for f+(q2) and f0(q2) 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→π ν and Bs→K ν decays enable determinations of the Cabibbo-Kobayashi-Maskawa matrix element |Vub|. To illustrate this, we perform a combined z-fit of our numerical B→π ν 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 |Vub|=3.61(32)×10-3, where the error includes statistical and all systematic uncertainties. The same approach can be applied to the decay Bs→K ν to provide an alternative determination of |Vub| once the process has been measured experimentally. Finally, in anticipation of future experimental measurements, we make predictions for B→π ν and Bs→K ν differential branching fractions and forward-backward asymmetries in the Standard Model.

Original languageEnglish
Article number074510
JournalPhysical Review D
Issue number7
Publication statusPublished - 14 Apr 2015


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