Low-energy expansion of meson form factors

Abstract

We calculate the corrections to various low-energy theorems concerning the behaviour of the pseudoscalar meson form factors near t =0. In particular we discuss (i) the Ademollo-Gatto theorem, (ii) Sirlin's relation between the K l 3 form factor ƒ + K π (t) and the electromagnetic form factors, (iii) the Callan-Treiman relation, and (iv) the Dashen-Weinstein relation, which connects the slope λ 0 of ƒ 0 K π (t) with the ratio F K / F π . Furthermore, we point out a remarkable isospin breaking effect which is clearly visible in the experimental rates of the decays K + → π 0 e + ν , K 0 → π − e + ν .