Abstract
We investigate the strong decrease of the isovector nucleon form factors in the spacelike region—which empirically can be adequately described by the dipole fit—through a dispersion-theoretical description of the ππ →N\(\bar {\mathcal{N}}\) scattering amplitude in the ρ-resonance region. To that extent we consider on the right-hand cut besides the elastic contribution the influence of the uncorrelated 2π exchange and on the left-hand cut besides the nucleon pole and the isobar exchange the contributions of box diagrams in the sense of a strip approximation in the Mandelstam plane. We find that the electric Sachs form factor can be described completely through the nucleon pole and the uncorrelated 2π exchange, whereas for the magnetic form factor one needs in the region below the ρ resonance a contribution which is about 2 1/2 times the magnitude of the nucleon pole. The decrease in the spacelike region is very sensitive to variations of the ππ scattering phase shift above the ρ-resonance.
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