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 present an update of the MILC studies of the physics of light pseudoscalars using improved staggered fermions. New runs at lighter quark mass, as well as increased statistics for older sets, are enabling us to improve the results for decay constants in full QCD. In addition, we have analyzed quenched runs at two different lattice spacings. This makes possible a test of the applicability of staggered chiral perturbation theory in a different context.

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We determine the charmed meson decay constants in 2 + 1 flavor lattice QCD. We find f D + = 201 ± 3 ± 17 MeV and f D s = 249 ± 3 ± 16 MeV where the errors are statistical and the combined systematic uncertainty. We simulate quarks as light as m q /m s = 0.1 and use NLO predictions of Chiral Perturbation Theory to control the extrapolation to the physical results. We study three lattice spacings in order to estimate the continuum limit.

In the past year, we calculated with lattice QCD three quantities that were unknown or poorly known. They are the q2 dependence of the form factor in semileptonic D → Klν decay, the decay constant of the D meson, and the mass of the Bc meson. In this talk, we summarize these calculations, with emphasis on their (subsequent) confirmation by experiments.