fK/fπ in full QCD with domain wall valence quarks

We compute the ratio of pseudoscalar decay constants f(K)/f(pi) using domain-wall valence quarks and rooted improved Kogut-Susskind sea quarks, at a lattice spacing of b similar to 0.125 fm. By employing continuum chiral perturbation theory, we extract the Gasser-Leutwyler low-energy constant L-5 and extrapolate f(K)/f(pi) to the physical point. We find f(K)/f(pi)=1.218 +/- 0.002(-0.024)(+0.011) where the first error is statistical and the second error is an estimate of the systematic due to chiral extrapolation and fitting procedures. This value agrees within the uncertainties with the determination by the MILC collaboration, calculated using Kogut-Susskind valence quarks, suggesting that systematic errors arising from the choice of lattice valence quark are small.