Electromagnetic form factor of the pion: Vector mesons or quarks?

Abstract

An updated analysis of the pion form-factor data in the spacelike region up to ${q}^{2}=\ensuremath{-}10$ Ge${\mathrm{V}}^{2}$ is performed using the extended vector-meson-dominance (EVMD) model, with parameters fixed by the dual model, as well as asymptotic quantum-chromodynamics (QCD) expressions. EVMD provides a good fit to the data and at the same time it predicts a deviation from universality in very good agreement with experiment. Perturbative QCD results to first order in ${\ensuremath{\alpha}}_{s}$ do very poorly while the nonperturbative QCD prediction of Pagels and Stokar gives an excellent fit to the data in the entire region $1\ensuremath{\le}\ensuremath{-}{q}^{2}\ensuremath{\le}10$ Ge${\mathrm{V}}^{2}$.