The dynamical origin of abnormal spectra encountered in inelastic collisions on light nuclei

The γd→(π-p)p reaction was studied on a function of the (π-p) system invariant mass, in the region of the Δ(1236) production. In this reaction, when the recoil momentum of the 'spectator' proton is about 200MeV/c, a strong desagreement appears between experimental results and the impulse approximation model previsions, and an anomalous effect is observed at low values of the (π-p) system mass. These characteristics are explained by taking into account multiple scattering processes. A few approximations were developed, which deal with Fermi motion in nuclei, and take into account the fact that particles involved in some intermediate reactions can be on their mass-shell. Then the ABC effect was respectively studied in the np→d(ππ)0 and pd→3He(ππ)0 reactions. In the former, it is shown that a double nucleon exchange model (similar to the Deck model used in high energy diffractive dissociations) can reproduce missing mass spectra. For the pd→3He (ππ)0 reaction, it is shown that the observed ABC effect is reproduced, using a multiple scattering model, by resonance production and decay in the following intermediate reactions: pN→dsub(i)π and dsub(i)N→3Heπ. The angular dependence of the ABC effect is deduced from those of the intermediate reactions