Pion and Light Fragment Production in Nucleus - Collisions and the Importance of Coulomb Final - Interactions

Nucleus-nucleus collisions offer the possibility of studying nuclear matter in new density and temperature domains; the properties of the excited nuclear matter, however, need to be inferred from complex multi-particle final states. At beam energies around 1 GeV/nucleon, the final state may contain a significant number of pions in addition to neutrons, protons and other nuclear fragments. Utilizing an existing microcanonical model of nuclear disassembly, we study the production of light fragments in nucleus-nucleus collisions. We pay particular attention to pions because they are expected to be a sensitive probe of the hot and dense nuclear matter and because of the availability of data. Introducing the (DELTA)-resonance in the available phase space and taking into account the collective motion of nuclear matter, we obtained good agreement with the pion data for beam energies from 0.4 to 2.0 GeV/nucleon. This result shows that the (DELTA)-isobar is a source for pions and that nuclear matter exhibits a degree of collective motion. Study of Coulomb effects may tell us about the nuclear charge distribution and its evolution in time. Such effects are seen in isobaric comparisons, e.g., neutrons to protons. Guided by recent neutron data, we first develop a simple model that incorporates a description of Coulomb effects to study low-energy nucleon angular distributions and neutron-to-proton cross section ratios. We explain the observed relative isotropy in the proton angular distributions and the rising tendency in the n/p cross section ratios of the final system with decreasing energy. With the insight afforded by this simple model, we present an approximate classical dynamic treatment of electromagnetic final-state interactions. Such a description augments the statistical predictions of the microcanonical model. We utilize the Coulomb-augmented model to address the observed structures in the (pi)('-)/(pi)('+), ('3)H/('3)He and n/p cross section ratios from various reactions at various beam energies. This work demonstrates the success of the enhanced microcanonical model in the analysis of data and thereby its utility in the design of new detection systems and in the search for new phenomena in energetic nucleus-nucleus collisions.