Absorption of slow pions by nuclei

The imaginary part of the pion polarization operator (optical potential) ImPi(ω) is investigated by means of the methods of quantum field theory. The study is based on general properties of the scattering matrix (unitarity, analyticity) without using perturbation theory and specific nuclear models. Relations connecting ImPi with the imaginary parts of mass operators (optical potentials) of the nucleon and Δ isobar in the nucleus are derived which, with the use of phenomenological nucleon optical potentials, permit the calculation of imPi in the region ωroughly-equalm/sub π/. The contributions from many-nucleon absorption mechanisms to this quantity are estimated