58 research outputs found
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)
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
Form Factors and the Fragmentation Fraction Ratio
We present a lattice quantum chromodynamics determination of the scalar and
vector form factors for the decay over the full
physical range of momentum transfer. In conjunction with future experimental
data, our results will provide a new method to extract , 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 form factors, we
determine the ratios and . These results give the fragmentation
fraction ratios and ,
respectively. The fragmentation fraction ratio is an important ingredient in
experimental determinations of meson branching fractions at hadron
colliders, in particular for the rare decay . In addition to the form factor results, we make the first prediction
of the branching fraction ratio , where is an electron or muon. Current
experimental measurements of the corresponding ratio for the semileptonic
decays of mesons disagree with Standard Model expectations at the level of
nearly four standard deviations. Future experimental measurements of
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)
Form factor ratios for and semileptonic decays and
We present a lattice quantum chromodynamics determination of the ratio of the
scalar and vector form factors for two semileptonic decays of the meson:
and . In conjunction
with future experimental data, our results for these correlated form factors
will provide a new method to extract , which may elucidate the
current tension between exclusive and inclusive determinations of these
Cabibbo-Kobayashi-Maskawa mixing matrix parameters. In addition to the form
factor results, we determine the ratio of the differential decay rates, and
forward-backward and polarization asymmetries, for the two decays.Comment: 16 pages, 9 figures. Version published in PRD: minor typos fixed and
two figures updated. arXiv admin note: text overlap with arXiv:1703.0972
An accurate calculation of the nucleon axial charge with lattice QCD
We report on a lattice QCD calculation of the nucleon axial charge, ,
using M\"{o}bius Domain-Wall fermions solved on the dynamical HISQ
ensembles after they are smeared using the gradient-flow algorithm. The
calculation is performed with three pion masses,
MeV. Three lattice spacings ( fm) are used with the
heaviest pion mass, while the coarsest two spacings are used on the middle pion
mass and only the coarsest spacing is used with the near physical pion mass. On
the MeV, fm point, a dedicated volume study is
performed with . Using a new strategy
motivated by the Feynman-Hellmann Theorem, we achieve a precise determination
of with relatively low statistics, and demonstrable control over the
excited state, continuum, infinite volume and chiral extrapolation systematic
uncertainties, the latter of which remains the dominant uncertainty. Our final
determination at 2.6\% total uncertainty is , with the
first uncertainty including statistical and systematic uncertainties from
fitting and the second including model selection systematics related to the
chiral and continuum extrapolation. The largest reduction of the second
uncertainty will come from a greater number of pion mass points as well as more
precise lattice QCD results near the physical pion mass.Comment: 17 pages + 11 pages of references and appendices. 15 figures.
Interested readers can download the Python analysis scripts and an hdf5 data
file at https://github.com/callat-qcd/project_gA_v
B-s -\u3e D(s)l nu form factors and the fragmentation fraction ratio f(s)/f(d)
We present a lattice quantum chromodynamics determination of the scalar and vector form factors for the B-s -\u3e D(s)l. decay over the full physical range of momentum transfer. In conjunction with future experimental data, our results will provide a new method to extract vertical bar V-cb vertical bar, which may elucidate the current tension between exclusive and inclusive determinations of this parameter. Combining the form factor results at nonzero recoil with recent HPQCD results for the B -\u3e Dl(v) form factors, we determine the ratios f(0)(Bs -\u3e Ds) (M-pi(2))/f(0)(B -\u3e D) (M-K(2))=1.000(62) and f(0)(Bs -\u3e Ds) (M-pi(2))/f(0)(B -\u3e D)(M-pi(2))=1.006(62). These results give the fragmentation fraction ratios f(s)/f(d) = 0.310(30)(stat)(21)(syst)(6)(theor)(38)(latt) and f(s)/f(d) = 0.307(16)(stat)(21)(syst)(23)(theor)(44)(latt), respectively. The fragmentation fraction ratio is an important ingredient in experimental determinations of Bs meson branching fractions at hadron colliders, in particular for the rare decay B(B-s -\u3emu(+)mu(-). In addition to the form factor results, we make the first prediction of the branching fraction ratio R(D-s) = B(B-s -\u3e D-s tau nu)/B(B-s -\u3e D-s tau nu)= 0.301(6), where l 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
Mobius domain-wall fermions on gradient-flowed dynamical HISQ ensembles
We report on salient features of a mixed lattice QCD action using valence Mobius domain-wall fermions solved on the dynamical N-f = 2 + 1 + 1 highly improved staggered quark sea- quark ensembles generated by the MILC Collaboration. The approximate chiral symmetry properties of the valence fermions are shown to be significantly improved by utilizing the gradient- flow scheme to first smear the highly improved staggered quark configurations. The greater numerical cost of the Mobius domain- wall inversions is mitigated by the highly efficient QUDA library optimized for NVIDIA GPU accelerated compute nodes. We have created an interface to this optimized QUDA solver in CHROMA. We provide tuned parameters of the action and performance of QUDA using ensembles with the lattice spacings a similar or equal to {0.15; 0.12; 0.09} fm and pion masses m(pi) similar or equal to {310; 220; 130} MeV. We have additionally generated two new ensembles with a similar to 0.12 fm and m(pi) similar to {400; 350} MeV. With a fixed flow time of t(gf) = 1 in lattice units, the residual chiral symmetry breaking of the valence fermions is kept below 10% of the light quark mass on all ensembles, m(res) less than or similar to 0.1 x m(l), with moderate values of the fifth dimension L-5 and a domain- wall height M-5 \u3c= 1.3. As a benchmark calculation, we perform a continuum, infinite volume, physical pion and kaon mass extrapolation of F-K +/-/F-pi +/- and demonstrate our results are independent of flow time and consistent with the FLAG determination of this quantity at the level of less than one standard deviation
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