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A new approach is developed for evaluating contributions of various isobar channels to the double charged-pion production in the reactions γ r,v p → π + π − p. This approach makes it possible to determine both respective cross sections and amplitudes for quasi-two-particle channels from a fit to data on cross sections for a three-particle final state. The diffractive anzatz, which is widely used to describe rho-meson production, is modified to meet the purpose of the description of data in the near-threshold and subthreshold regions. The cross sections for rho-meson electroproduction in the energy region of nucleon-resonance excitation are determined for the first time on the basis of the latest data of the CLAS Collaboration. [ABSTRACT FROM AUTHOR]

A model has been developed for describing the cross section for the reaction γ[sub r, v]p → π[sup -]Δ[sup ++] in the energy region of nucleon-resonance excitation, and relevant calculations have been performed. The model takes into account the contribution of all reliably established resonances with masses less than 2 GeV and the contributions of nonresonance mechanisms in the approximation of Born diagrams. On the basis of data on the amplitudes of pion-nucleon interactions, a method has been evolved for phenomenologically describing the coupling of initial and final states with open inelastic channels. Calculations performed in this study represent a first step in developing a method for determining the electromagnetic form factors for nucleon resonances in exclusive reactions of pion-pair production on a nucleon by real and virtual photons. [ABSTRACT FROM AUTHOR]