Near-threshold Isospin Violation in the Pion Form Factor from Chiral Perturbation Theory

We examine the isospin violation in the timelike pion form factor near threshold at next-to-leading order in the chiral expansion using the techniques of Chiral Perturbation Theory. This next-to-leading order contribution contains the first nonvanishing isospin violation. This isospin violation is found to be very small near threshold. In particular, the isospin violation at threshold is found to be of order 10−4, which should be compared with the few percent level seen in the vector meson resonance region. Present address. Permanent address. E-mail: [email protected]; athomas, [email protected]