Nucleon electromagnetic structure revisited

Abstract

Unitary and analytic ten-resonance model of the nucleon electromagnetic (e.m.) structure with canonical normalizations and QCD (up to the logarithmic correction) asymptotics is constructed on the four-sheeted Riemann surface, which provides a superposition of vector–meson pole and continuum contributions in a very natural way. As a result it describes simultaneously all existing experimental space-like and time-like data on the proton e.m. form factors (ff's) and on the neutron e.m. ff's as well. A crucial factor in the latter achievement is the inclusion of a contribution of the fourth excited state of the ρ(770) meson with the parameters mρ′′′′ = 2455 ± 53 MeV, Γρ′′′′ = 728 ± 2 MeV and (f(1)ρ′′′′NN/fρ′′′′) = 0.0549 ± 0.0005, (f(2)ρ′′′′NN/fρ′′′′) = −0.0103 ± 0.0001. The pronounced effect of the two-pion continuum on the isovector spectral functions demonstrating a strong enhancement of the left wing of the ρ(770) resonance close to two-pion threshold, which was revealed by Höhler and Pietarinen by means of the nucleon ff unitarity condition more than a quarter of the century ago, is predicted by the model automatically. The model gives large values of the f(1,2)ϕNN coupling constants, thus indicating the violation of the OZI rule. Since in the framework of the considered model isoscalar ff's above their lowest branch point ts0 = 9m2π are complex functions, the isoscalar spectral function behaviours are predicted as well.