(Formaula) meson decay constants in lattice QCD

We have calculated the decay constants of (Formula presented) and (Formula presented) mesons with lattice QCD. We use an (Formula presented)-improved action that takes light quark actions as a starting point, tuned so that it can be directly applied at the physical masses of the (Formula presented) and (Formula presented) quarks. Our results are (Formula presented) (Formula presented) (Formula presented) and (Formula presented) in the quenched approximation. The first error in each case is statistical, and the second is from perturbation theory. We show that discretization errors are under control in our approach and smaller than our statistical errors. The effects of the quenched approximation may raise our quenched result by up to 10%.