- th / 0 10 80 64 v 2 2 9 A ug 2 00 1 ISI and FSI in NN → NNX reactions close to threshold

The present status of the theoretical description of Initial (ISI) and Final State Interactions (FSI) in nucleon induced exclusive meson production processes close to threshold is reviewed. Shortcomings are addressed and new perspectives given. It is quite surprising, that the discussion of the theoretical description of ISI and FSI in nuclear and particle collision/production processes is gaining again increasing attention, even though people thought, this issue has been well understood due to an established formalism going back to outstanding scientists such as e. entering nearly all standard text books of Scattering Theory, e.g. [1, 2]. The existing formalism for the treatment of ISI and FSI is based on the idea, that T-matrix elements of many processes (in the Distorted Wave Born Approximation (DWBA)) are well described as overlap integrals of initial and final state wavefunctions multiplied by some interaction potential. This idea works quite successfully for (mainly nonrelativistic) textbook examples, in which excitation energies are small compared to the masses of the particles involved, i.e. for the description of electromagnetic transitions in atoms (excitation energies ∼ 1 eV) or β ±-decays/transitions of atomic nuclei (excitation energies ∼ 10 2. .. 10 3 keV). In these processes the initial/final states hardly go offshell due to the transition interaction, or at least for a very short time. In re