On the antiparticle - particle ratios in symmetric and antisymmetric nuclear collisions at relativistic energies

At high enough temperatures or densities it is believed that normal hadronic matter will undergo a phase change and become a plasma of quarks and gluons called the Quark Gluon Plasma (QGP). Experiments with relativistic heavy ion collisions can achieve the energy densities required to create this new phase of matter. Such experiments have been conducted at the CERN, SPS and will be conducted at the future LHC. Data are currently being taken at four experiments at RHIC facility in Brookhaven, USA. The BRAHMS experiment is carried out at this facility and is capable of analysing charged particle production over a wide range of rapidity and uses charged particle yields and ratios to provide a measure of these Ultra-Relativistic reactions. Measurement of various charged particles as a function of pseudo-rapidity densities at BRAHMS allow the dynamics of the expanding fireball to be investigated. This shows a large variation with rapidity that is consistent with a Gaussian fit over the full range of rapidity and is in agreement with the Landau hydrodynamical model. Measurement of the kaon to pion ratio allows the strangeness content of the interaction to be measured and shows that, at mid-rapidity, strangeness production is greatly enhanced over what would be expected from normal hadronic interactions. This data can be compared with strangeness enhancement data from the SPS experiment at lower beam energies, NA49 and the STAR and PHENIX experiment at RHIC. These data seems to show an increasing strangeness production and saturation with increasing beam energy. The anti-proton to proton ratio allows the baryochemical potential of the interaction fireball to be measured as a function of baryon number in the mid rapidity region. Two scenarios are possible, full stopping, with a large baryochemical potential at mid rapidity, or transparency, where the baryon number would be carried off down the beam line and found at higher rapidities. Both these scenarios are covered by the rapidity range of the BRAHMS detector and can be measured over various beam energies. This seems to show that stopping is increasing at the lower beam energy but the interaction region is still not fully transparent at the highest RHIC energies. (authors)