A dispersive treatment of Kl4 decays

K-l4 decays have several features of interest: they allow an accurate measurement of pi pi-scattering lengths; they provide the best source for the determination of some low-energy constants of chi PT; one form factor is directly related to the chiral anomaly, which can be measured here. We present a dispersive treatment of K-l4 decays that provides a resummation of pi pi- and K pi-rescattering effects. The free parameters of the dispersion relation are fitted to the data of the high-statistics experimentsE865 and NA48/2. The matching to chi PT at NLO and NNLO enables us to determine the LECs L-1(r), L-2(r) and L-3(r). With recently published data from NA48/2, the LEC L-9(r) can be determined as well. In contrast to a pure chiral treatment, the dispersion relation describes the observed curvature of one of the form factors, which we understand as a rescattering effect beyond NNLO.