Abstract
Sum rules obtained from considering dispersion relations for the invariant amplitudes at fixed t and fixed u are presented for pion-photoproduction off nucleons. These are saturated with low-energy resonances in all channels, viz., N and N∗ in s- and u-channels and π, ϱ, ω and A1 in t-channel. The problem of saturation of the sum rules is investigated within the approximation scheme and a criterion for reliable saturation of sum rules in terms of the magnitude of the nucleon contribution is obtained. The sum rules relate interesting electromagnetic properties of the particles in the direct channel to the coupling constants associated with t-channel exchanges. The results are in general agreement with other determinations.
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