2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/197091We compute pseudoscalar meson masses and decay constants in partially quenched QCD with three dynamical flavors of improved staggered quarks. Fitting the lattice data to staggered chiral perturbation theory forms and extrapolating in quark mass and lattice spacing, we find: $f_π= 129.5 \pm 0.9 \pm 3.5$ MeV, $f_K = 156.6 \pm 1.0 \pm 3.6$ MeV, and $f_K/f_π= 1.210(4)(13)$. The value for $f_K/f_π$ implies $|V_{us}|=0.2219(26)$. We also obtain $m_u/m_d = 0.43(0)(1)(8)$, or, with more input on electromagnetic effects from the continuum, $m_u/m_d = 0.41(0)(1)(4)$. Using the one-loop perturbative mass renormalization, we determine $m_u$ and $m_d$ in the MSbar scheme.Talk presented at Lattice2004(spectrum), Fermilab, June 21 - 26, 2004; 3 pages, 2 figuresHigh Energy Physics - LatticeHigh Energy Physics - Phenomenologytext