On the collective isobaric resonances in pion-nucleus scattering at intermediate energies

The excitation functions of differential cross-sections in pion-nucleus scattering at intermediate energies have been obtained by fitting in the c.m.s. with Legendre polynomials all the existing data on 1H, 2H, 3He, 4He, 12C and 16O. The spectra have been deduced by calculating the maximum-likelihood Lorentz lines through a Monte Carlo method. Among the main features, a resonant behaviour of the excitation functions is found at energies much lower compared to the free Δ-resonance position. The shift is angle dependent and A-dependent. A possible explanation of the downward energy shift is given on the ground of the recent developments of the collective isobaric resonance model. (author)