Contribution of real and virtual Pions to nuclear Photoabsorption at Intermediate Energies

The universal curve $\sigma /A$ of nuclear photoabsorption is investigated within a Fermi gas model of nuclear matter. An energy range from pion threshold up to 400 MeV is considered. The interactions between nucleon, pion, $\Delta $-isobar and photon are considered in the non-relativistic approximation with corrections of the order $1/M$ taken into account with respect to proton mass. Analytical expressions are obtained, in which the influence of nuclear correlations, two-nucleon contributions and relativistic corrections is studied explicitely. An extension of the model calculation to nucleon knock-out reactions is discussed