Timelike and spacelike electromagnetic form factors of nucleons, a unified description

The extended Lomon-Gari-Krumpelmann model of nucleon electromagnetic form factors, which embodies rho, rho', omega, omega', and phi vector meson contributions and the perturbative QCD high momentum transfer behavior has been extended to the timelike region. Breit-Wigner formulas with momentum-dependent widths have been considered for broad resonances in order to have a parametrization for the electromagnetic form factors that fulfills, in the timelike region, constraints from causality, analyticity, and unitarity. This analytic extension of the Lomon-Gari-Krumpelmann model has been used to perform a unified fit to all the nucleon electromagnetic form factor data, in the spacelike and timelike region (where form factor values are extracted from e(+) e(-) <-> N (N) over bar cross sections data). The knowledge of the complete analytic structure of form factors enables predictions at extended momentum transfer, and also of timelike observables such as the ratio between electric and magnetic form factors and their relative phase.