Physics – High Energy Physics – High Energy Physics - Phenomenology
Scientific paper
2007-05-10
Eur.Phys.J.A34:223-236,2007
Physics
High Energy Physics
High Energy Physics - Phenomenology
14 pages in 2-column format, 4 figures, references added, typos corrected, minor changes in the text, accepted in Eur. Phys. J
Scientific paper
10.1140/epja/i2007-10493-3
We analyzed the presently available experimental data on nucleon electromagnetic form factors within a multipole model based on dispersion relations. A good fit of the data is achieved by considering the coefficients of the multipole expansions as logarithmic functions of the momentum transfer squared. The superconvergence relations, applied to this coefficients, makes the model agree with unitary constraints and pQCD asymptotics for the Dirac and Pauli form factors. The soft photon emission is proposed as a mechanism responsible for the difference between the Rosenbluth, polarization and beam--target--asymmetry data. It is shown, that the experimentally measured cross sections depend not only on the Dirac and Pauli form factors, but also on the average number of the photons emitted. For proton this number is shown to be different for different types of experimental measurements and then estimated phenomenologically. For neutron the same mechanism predicts, that the data form different types of experiments must coincide with high accuracy. A joint fit of all the experimental data reproduce the $Q^2-$dependence with the accuracy $\chi^2/dof=0.86$. Predictions of the model, that 1) the ratios of the proton form factors $G_E/G_M$ are different for Rosenbluth, polarization and beam--target--asymmetry experiments and 2) similar ratios are nearly the same for neutron, can be used for experimental verification of the model.
Lalakulich Olga
Vereshkov Gregory
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