B→Dlν form factors at nonzero recoil and extraction of |Vcb|

We present a lattice QCD calculation of the B→Dlν semileptonic decay form factors f+(q2) and f0(q2) for the entire physical q2 range. Nonrelativistic QCD bottom quarks and highly improved staggered quark charm and light quarks are employed together with Nf=2+1 MILC gauge configurations. A joint fit to our lattice and BABAR experimental data allows an extraction of the Cabibbo-Kobayashi-Maskawa matrix element |Vcb|. We also determine the phenomenologically interesting ratio R(D)=B(B→Dτντ)/B(B→Dlνl) (l=e,μ). We find |Vcb|B→Dexcl=0.0402(17)(13), where the first error consists of the lattice simulation errors and the experimental statistical error and the second error is the experimental systematic error. For the branching fraction ratio we find R(D)=0.300(8)

Lattice Monte Carlo Data versus Perturbation Theory

Differences between lattice Monte Carlo data and perturbation theory (for example the lack of asymptotic scaling) are usually associated with the `bad' behaviour of the bare lattice coupling g_0 due to the effects of large (and unknown) higher order terms in g_0. In this philosophy a new, renormalised coupling g' is defined with the aim of making the higher order coefficients of the perturbative series in g' as small as possible. In this paper an alternative scenario is discussed where lattice artifacts are proposed as the cause of the disagreement between Monte Carlo data and the g_0-perturbative series. We find that with the addition of a lattice artifact term, the usual asymptotic scaling expression in g_0 is in excellent agreement with Monte Carlo data. Lattice data studied includes the string tension, the hadronic scale r_0, the discrete beta function, M_rho, f_pi and the 1P-1S splitting in charmonium.Comment: 3 pages, LaTeX. Uses espcrc2.sty and fleqn.sty (included) Talk presented at LATTICE96(improvement

B and Bs semileptonic decay form factors with NRQCD/HISQ quarks

We discuss our ongoing effort to calculate form factors for several B and Bs semileptonic decays. We have recently completed the first unquenched calculation of the form factors for the rare decay B -> K ll. Extrapolated over the full kinematic range of q^2 via model-independent z expansion, these form factor results allow us to calculate several Standard Model observables. We compare with experiment (Belle, BABAR, CDF, and LHCb) where possible and make predictions elsewhere. We discuss preliminary results for Bs -> K l nu which, when combined with anticipated experimental results, will provide an alternative exclusive determination of |Vub|. We are exploring the possibility of using ratios of form factors for this decay with those for the unphysical decay Bs -> eta_s as a means of significantly reducing form factor errors. We are also studying B -> pi l nu, form factors for which are combined with experiment in the standard exclusive determination of |Vub|. Our simulations use NRQCD heavy and HISQ light valence quarks on the MILC 2+1 dynamical asqtad configurations.Comment: 7 pages, 5 figures, presented at the 31st International Symposium on Lattice Field Theory (Lattice 2013), 29 July - 3 August 2013, Mainz, German

Bs→DsℓνB_s \to D_s \ell \nu Form Factors and the Fragmentation Fraction Ratio fs/fdf_s/f_d

We present a lattice quantum chromodynamics determination of the scalar and vector form factors for the $B_s \rightarrow D_s \ell \nu$ decay over the full physical range of momentum transfer. In conjunction with future experimental data, our results will provide a new method to extract $|V_{cb}|$, which may elucidate the current tension between exclusive and inclusive determinations of this parameter. Combining the form factor results at non-zero recoil with recent HPQCD results for the $B \rightarrow D \ell \nu$ form factors, we determine the ratios $f^{B_s \rightarrow D_s}_0(M_\pi^2) / f^{B \rightarrow D}_0(M_K^2) = 1.000(62)$ and $f^{B_s \rightarrow D_s}_0(M_\pi^2) / f^{B \rightarrow D}_0(M_\pi^2) = 1.006(62)$. These results give the fragmentation fraction ratios $f_s/f_d = 0.310(30)_{\mathrm{stat.}}(21)_{\mathrm{syst.}}(6)_{\mathrm{theor.}}(38)_{\mathrm{latt.}}$ and $f_s/f_d = 0.307(16)_{\mathrm{stat.}}(21)_{\mathrm{syst.}}(23)_{\mathrm{theor.}}(44)_{\mathrm{latt.}}$, respectively. The fragmentation fraction ratio is an important ingredient in experimental determinations of $B_s$ meson branching fractions at hadron colliders, in particular for the rare decay ${\cal B}(B_s \rightarrow \mu^+ \mu^-)$. In addition to the form factor results, we make the first prediction of the branching fraction ratio $R(D_s) = {\cal B}(B_s\to D_s\tau\nu)/{\cal B}(B_s\to D_s\ell\nu) = 0.301(6)$, where $\ell$ is an electron or muon. Current experimental measurements of the corresponding ratio for the semileptonic decays of $B$ mesons disagree with Standard Model expectations at the level of nearly four standard deviations. Future experimental measurements of $R(D_s)$ may help understand this discrepancy.Comment: 21 pages, 15 figure

B - \u3e Dl nu form factors at nonzero recoil and extraction of vertical bar V-cb vertical bar

We present a lattice QCD calculation of the B -\u3e Dl nu semileptonic decay form factors f(+)(q(2)) and f(0)(q(2)) for the entire physical q(2) range. Nonrelativistic QCD bottom quarks and highly improved staggered quark charm and light quarks are employed together with N-f = 2 + 1 MILC gauge configurations. A joint fit to our lattice and BABAR experimental data allows an extraction of the Cabibbo-Kobayashi-Maskawa matrix element vertical bar V-cb vertical bar. We also determine the phenomenologically interesting ratio R(D) = B(B -\u3e D tau nu(tau))/B(B -\u3e Dl nu(l)) (l = e, mu). We find vertical bar V-cb vertical bar(B -\u3e D)(excl) = 0.0402(17)(13), where the first error consists of the lattice simulation errors and the experimental statistical error and the second error is the experimental systematic error. For the branching fraction ratio we find R(D) = 0.300(8)

Leptonic Decays of Heavy-Light Systems

Results from recent lattice calculations of the decay constants $f_B$ and $f_D$ are reviewed. A discussion of the methods currently used is presented, together with an outline of the various systematic effects involved.Comment: Plenary talk given at LAT95, Melbourne, July 1995. 12 pages (Two column format), Latex file with 2 postscript figures uuencoded. Requires psfig.sty and espcrc2.sty [Numbers in Table 4 from LANL Collaboration updated slightly.

Rare decay B -> K ll form factors from lattice QCD

We calculate, for the first time using unquenched lattice QCD, form factors for the rare decay B -> Kll in and beyond the Standard Model. Our lattice QCD calculation utilizes a nonrelativistic QCD formulation for the b valence quarks, the highly improved staggered quark formulation for the light valence quarks, and employs the MILC 2+1 asqtad ensembles. The form factor results, based on the z expansion, are valid over the full kinematic range of q^2. We construct the ratios f0/f+ and fT/f+, which are useful in constraining new physics and verifying effective theory form factor symmetry relations. We also discuss the calculation of Standard Model observables.Comment: 30 pages, 23 figures; v2 adds a reference and fixes typos, results unchanged; v3 corrects a typo, updates references, makes minor changes in title and text, and is version published in Phys.Rev.D, results unchange