1. Dipartimento di Fisica Nucleare e Teorica, Università degli Studi di Pavia; INFN, Sezione di Pavia, Pavia, Italy 2. Institut für Kernphysik, Johannes Gutenberg-Universit?t, D-55099, Mainz, Germany
Abstract:
The relativistic amplitudes of pion photoproduction are evaluated by dispersion relations at t = const. The imaginary parts of the amplitudes are taken from the MAID model covering the absorption spectrum up to center-of-mass
energies W = 2.2GeV. For sub-threshold kinematics the amplitudes are expanded in powers of the two independent variables ν and t related to energy and momentum transfer. Subtracting the loop corrections from this power series allows one to determine
the counter terms of covariant baryon chiral perturbation theory. The proposed continuation of the amplitudes into the unphysical
region provides a unique framework to derive the low-energy constants to any given order as well as an estimate of the higher-order
terms by global properties of the absorption spectrum.