Guided by the large-N C limit of QCD, we construct the most general chiral resonance Lagrangian that can generate chiral low-energy constants up to O(p 6 ). By integrating out the resonance fields, the low-energy constants are parametrized in terms of resonance masses and couplings. Information on those couplings and on the low-energy constants can be extracted by analysing QCD Green functions of currents both for large and small momenta. The chiral resonance theory generates Green functions that interpolate between QCD and chiral perturbation theory. As specific examples we consider the V A P and S P P Green functions.

Using the 1/N C expansion scheme and truncating the hadronic spectrum to the lowestlying resonances, we match a meromorphic approximation to the SP P Green function onto QCD by imposing the correct large-momentum falloff, both off-shell and on the relevant hadron mass shells. In this way we determine a number of chiral low-energy constants of O(p 6 ), in particular the ones governing SU (3) breaking in the K ℓ3 vector form factor at zero momentum transfer. The main result of our matching procedure is that the known loop contributions largely dominate the corrections of O(p 6 ) to f + (0). We discuss the implications of our final value f K 0 π − + (0) = 0.984 ± 0.012 for the extraction of V us from K ℓ3 decays.

The gauge invariant two-point correlation function of the gauge field strength tensor is calculated in perturbation theory at the next-to-leading order. Besides a direct calculation in perturbative QCD we also present a derivation of the correlation function in the heavy quark effective theory. Our results are briefly compared with recent determinations of the field strength correlator on the lattice.Comment: 11 pages, LaTeX, 3 eps figure

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