Energy and centrality dependence of [anti]-p and p production and the [anti-]Λ /[anti-]p ratio in Pb+Pb collisions between 20A GeV and 158A GeV

The transverse mass mt distributions for antiprotons are measured at midrapidity for minimum bias Pb+Pb collisions at 158A GeV and for central Pb+Pb collisions at 20A, 30A, 40A, and 80A GeV beam energies in the fixed target experiment NA49 at the CERN SPS. The rapidity density dn/dy, inverse slope parameter T, and mean transverse mass 〈mt〉 derived from the mt distributions are studied as a function of the incident energy and the collision centrality and compared to the relevant data on proton production. The shapes of the mt distributions of p̅ and p are very similar. The ratios of the particle yields, p̅ /p and Λ̅ /p̅ , are also analyzed. The p̅ /p ratio exhibits an increase with decreasing centrality and a steep rise with increasing beam energy. The Λ̅ /p̅ ratio increases beyond unity with decreasing beam energy.