47 v 1 3 0 O ct 1 99 6 Preprint Numbers : ANL - PHY - 8585 - TH - 96 TRI - PP - 96 - 63 K l 3 and π e 3 transition form factors

Kl3 and πe3 transition form factors are calculated as an application of DysonSchwinger equations. The role of nonanalytic contributions to the quark–W-boson vertex is elucidated. A one-parameter model for this vertex provides a uniformly good description of these transitions, including the value of the scalar form factor of the kaon at the Callan-Treiman point. The Kl3 form factors, f K ± , are approximately linear on t ∈ [me,mμ] and have approximately the same slope. fK − (0) is a measure of the Euclidean constituent-quark mass ratio: ME s /M E u . In the isospin symmetric limit: −fπ +(0) = Fπ(t), the electromagnetic pion form factor, and fπ −(t) ≡ 0.