Physical results from unphysical simulations

We calculate various properties of pseudoscalar mesons in partially quenched QCD using chiral perturbation theory through next-to-leading order. Our results can be used to extrapolate to QCD from partially quenched simulations, as long as the latter use three light dynamical quarks. In other words, one can use unphysical simulations to extract physical quantities—in this case the quark masses, meson decay constants, and the Gasser-Leutwyler parameters $L_4-L_8.$ Our proposal for determining $L_7$ makes explicit use of an unphysical (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 masses, except for our result for $L_7,$ which is derived for degenerate sea quarks.