Physical results from unphysical simulations - art. no. 094503

We calculate various properties of pseudoscalar mesons in partially quenche d QCD using chiral perturbation theory through next-to-leading order. Our r esults can be used to extrapolate to QCD from partially quenched simulation s, as long as the latter use three light dynamical quarks. In other words, one can use unphysical simulations to extract physical quantities-in this c ase the quark masses, meson decay constants, and the Gasser-Leutwyler param eters L-4-L-8. Our proposal for determining L-7 makes explicit use of an un physical (yet measurable) effect of partially quenched theories, namely the double-pole that appears in certain two-point correlation functions. Most of our calculations are done for sea quarks having up to three different ma sses, except for our result for L-7, which is derived for degenerate sea qu arks.