When is it possible to use perturbation technique in field theory ?

The vector pion form factor is used as an example to analyze this question. Given the experimental radius of the pion, the crucial question is whether perturbative methods could be used for the effective chiral lagrangian to calculate the pion form factor. Our analysis shows that the pion rms radius is far too large (or the related rho resonance mass is too low) for the perturbation theory to be valid.Comment: 11 pages, Latex, 4 eps figs. Contribution to the proceedings of the workshop "QCD2000" Villefranche-sur-Mer, France, January 200

Dispersion Relation Analyses of Pion Form Factor, Chiral Perturbation Theory and Unitarized Calculations

The Vector Pion form factor below 1 GeV is analyzed using experimental data on its modulus, the P-wave pion pion phase shifts and dispersion relation. It is found that causality is satisfied. Using dispersion relation, terms proportional to s squared and s cubed are calculated using the experimental data, where s is the momentum transfer. They are much larger than the one-loop and two-loop Chiral Perturbation Theory calculations. Unitarized model calculations agree very well with dispersion relation results.Comment: 10 pages, 4 PostScript figures some minor changes and added reference