Unitarity, Chiral Perturbation Theory, and Meson Form Factors

The inverse-amplitude method is applied to the one-loop chiral expansion of the pion, kaon, and $K_{l3}$ form factors. Since these form factors are determined by the same chiral low-energy constants, it is possible to obtain finite predictions for the inverse-amplitude method. It is shown that this method clearly improves one-loop chiral perturbation theory, and a very good agreement between the inverse-amplitude method and the experimental information is obtained. This suggests that the inverse-amplitude method is a rather systematic way of improving chiral perturbation theory.Comment: 15 pages, 5 figs, uses REVTeX and epsfig.st

Measurement of K^0_e3 form factors

The semileptonic decay of the neutral K meson, KL -> pi e nu (Ke3), was used to study the strangeness-changing weak interaction of hadrons. A sample of 5.6 million reconstructed events recorded by the NA48 experiment was used to measure the Dalitz plot density. Admitting all possible Lorentz-covariant couplings, the form factors for vector (f_+(q^2)), scalar (f_S) and tensor (f_T) interactions were measured. The linear slope of the vector form factor lambda_+ = 0.0284+-0.0007+-0.0013 and values for the ratios |f_S/f_+(0)| = 0.015^{+0.007}_{-0.010}+-0.012 and |f_T/f_+(0)| = 0.05^{+0.03}_{-0.04}+-0.03 were obtained. The values for f_S and f_T are consistent with zero. Assuming only Vector-Axial vector couplings, lambda_+ = 0.0288+-0.0004+-0.0011 and a good fit consistent with pure V-A couplings were obtained. Alternatively, a fit to a dipole form factor yields a pole mass of M = 859+-18 MeV, consistent with the K^*(892) mass.Comment: 16 pages, 7 figures. submitted to Phys. Lett.