Collective and individual processes of sub-threshold particles production. Pions neutral and η mesons production in the Ar+Ca 180 A MeV reaction

The subject of this work is the study of the collective processes and individual nucleon-nucleon collisions Involved in sub-threshold particles production. For this purpose, aad for the first time, neutral pions and etas have been simultaneously measured well below the free nucleon-nucleon threshold, in the reaction Ar+Ca at 180 A MeV. This experiment has been performed at GSI with the multidetector TAPS. The measured eta probability seems to be 20 times lower than expected from the systematics. Surprisingly, the mt scaling law appears to be still valid at so low energy. Experimental results point out that most of the observed pions are mediated by Δ resonances. Finally, the high mt pions multiplicity increases much faster than low mt pion multiplicity with the number of participants. This fact is a strong evidence of the role of collective and cooperative processes for the production of energetic particles beyond the picture of particles produced in first chance nucleon-nucleon collisions. Experimental results have been confronted to the predictions of two dynamical and one statistical models, but these models fail to provide a consistent picture of the processes involved in sub-threshold particle production.