Evidence of Δ(1232)-Resonance Excitation in Subthreshold Pion Production at Intermediate Energies

After almost twenty years of intense research activity, it is nowadays rather well accepted that the main responsible of subthreshold pion production in heavy-ion collisions at intermediate energies (around 100 MeV/nucleon) has to be localized in the “in-medium” incoherent nucleon-nucleon collisions which take place in the overlap volume of the two reacting nuclei at the early stage of quite central collisions. In this framework two different channels are deemed possible: the “direct” NN →NNπ one and the “indirect” NN → NΔ → NNπ one. In the latter, the excitation and decay of the baryonic resonance acts as a doorway state which might be responsible of the large energy pooling needed in the subthreshold particle production. Up to now, all previous statements rely only on the results of indirect tests such as the agreement of the predictions of microscopic theoretical models, based on that picture, with both inclusive and semi-exclusive experimental data[1,2,3,4].