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 $m_{ρ''''}=2455\pm53 MeV, Γ_{ρ''''}=728\pm2 MeV$ and $(f^{(1)}_{ρ''''NN}/f_{ρ''''})=0.0549\pm0.0005$, $(f^{(2)}_{ρ''''NN}/f_{ρ''''})=-0.0103\pm0.0001$. 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 $t^s_0=9m^2_π$ are complex functions, the isoscalar spectral function behaviours are predicted as well.