We calculate the form factor f + (q 2 ) for B-meson semileptonic decay in unquenched lattice QCD with 2+1 flavors of light sea quarks. We use Asqtad-improved staggered light quarks and a Fermilab bottom quark on gauge configurations generated by the MILC Collaboration. We simulate with several light quark masses and at two lattice spacings, and extrapolate to the physical quark mass and continuum limit using heavy-light meson staggered chiral perturbation theory. We then fit the lattice result for f + (q 2 ) simultaneously with that measured by the BABAR experiment using a parameterization of the form factor shape in q 2 which relies only on analyticity and unitarity in order to determine the CKM matrix element |V ub |. This approach reduces the total uncertainty in |V ub | by combining the lattice and experimental information in an optimal, model-independent manner. We find a value of |V ub | × 10 3 = 3.38 ± 0.36.

We present the first three-flavor lattice QCD calculations for D-->pilnu and D-->Klnu semileptonic decays. Simulations are carried out using ensembles of unquenched gauge fields generated by the MILC Collaboration. With an improved staggered action for light quarks, we are able to simulate at light quark masses down to 1/8 of the strange mass. Consequently, the systematic error from the chiral extrapolation is much smaller than in previous calculations with Wilson-type light quarks. Our results for the form factors at q(2)=0 are f(D-->pi)(+)(0)=0.64(3)(6) and f(D-->K)(+)(0)=0.73(3)(7), where the first error is statistical and the second is systematic, added in quadrature. Combining our results with experimental branching ratios, we obtain the Cabibbo-Kobayashi-Maskawa matrix elements |V(cd)|=0.239(10)(24)(20) and |V(cs)|=0.969(39)(94)(24), where the last errors are from experimental uncertainties.

We present the first lattice QCD calculation with realistic sea quark content of the D+-meson decay constant f(D+). We use the MILC Collaboration's publicly available ensembles of lattice gauge fields, which have a quark sea with two flavors (up and down) much lighter than a third (strange). We obtain f(D+)=201+/-3+/-17 MeV, where the errors are statistical and a combination of systematic errors. We also obtain f(Ds)=249+/-3+/-16 MeV for the Ds meson.

We perform lattice simulations of two-flavor QCD using Neuberger's overlap fermion, with which the exact chiral symmetry is realized at finite lattice spacings. The regime is reached by decreasing the light quark mass down to 3 MeV on a 16 3 32 lattice with a lattice spacing 0:11 fm. We find a good agreement of the low-lying Dirac eigenvalue spectrum with the analytical predictions of the chiral random matrix theory, which reduces to the chiral perturbation theory in the regime. The chiral condensate is extracted as MS 2 GeV 251 7 11 MeV 3 , where the errors are statistical and an estimate of the higher order effects in the expansion.In quantum chromodynamics (QCD), it is widely believed that chiral symmetry is spontaneously broken, making pions nearly massless while giving masses of order QCD , the QCD scale, to the other hadrons. In fact, chiral perturbation theory (ChPT), an effective theory based on the spontaneously broken chiral symmetry, describes low energy interactions of pions very accurately. Nevertheless, theorists have not been successful in analytically solving QCD and deriving the chiral symmetry breaking, due to its highly nonperturbative dynamics.The most promising approach to establishing the link between QCD and ChPT is to utilize the numerical simulation of lattice QCD, with which the every prediction of ChPT can be tested in principle. For instance, the presence of the so-called chiral logarithms, the effect of a pion cloud, should be reproduced. Such a numerical test is, however, not an easy task, because of rapidly increasing computational cost in the small quark mass region where ChPT is reliably applied. Another serious problem is the explicit violation of the chiral symmetry at finite lattice spacings in the conventional fermion formulations, with which the conclusive test of ChPT requires wellcontrolled and thus computationally demanding continuum extrapolation.In this work we improve this situation in two ways. First, we employ Neuberger's overlap fermion [1,2] for dynamical quarks. It preserves exact chiral symmetry at finite lattice spacings, and hence ChPT can be applied before taking the continuum limit. Although the numerical cost of the overlap fermion is almost 100 times higher than that of the other fermions, new computational facilities at KEK enable us to carry out such a work.Second, we study the correspondence between QCD and ChPT in the so-called regime [3][4][5], which is characterized by the small pion mass m satisfying m L & 1 with L the box size. In this regime ChPT is safely applied as an expansion in terms of 2 m = QCD , provided that the condition 1= QCD L 2 1, the usual condition that the box size is larger than the inverse QCD scale, is satisfied. We set the sea quark mass to 3 MeV, for which m L ' 1:0. With the space-time volume L 3 T ' 1:8 fm 3 3:5 fm, the numerical cost is still not prohibitive even with such a small sea quark mass, since the finite volume provides a natural lower bound on the lowest eigenvalue of the Dirac operator.In the regime, zero-momentum...

A full determination of the CKM matrix using recent results from lattice QCD is presented. To extract the CKM matrix in a uniform fashion, I exclusively use results from unquenched lattice QCD as the theory input for nonperturvative QCD effects. All 9 CKM matrix elements and all 4 Wolfenstein parameters are obtained from results for gold-plated quantities, which include semileptonic decay form factors and leptonic decay constants of B, D and K mesons, and B 0 −B 0 and K 0 −K 0 mixing amplitudes. XXIIIrd International Symposium on Lattice Field Theory