On absorption of slow pions by nuclei

Methods of quantum field theory have been applied to investigation of the imaginary part of the pion polarization operator (the optical potential) ImPI(ω). The present approach is based upon the general properties of the scattering matrix (unitarity, analyticity) and does not use either the perturbation theory, or concrete nuclear models. Relations have been derived between ImPI and the imaginary parts of the mass operators (optical potentials) of nucleon and the Δ isobar inside nucleus, enabling one to calculate ImPI in the region of ω approximately equal to msub(π) with the use of phenomenological nucleon optical potential. Contributions to the quantity in view from multi-nucleon absorption mechanisms are estimated