Energy and centrality dependence of (p)over-bar and p production and the (Lambda)over-bar/(p)over-bar ratio in Pb plus Pb collisions between 20A GeV and 158A GeV

The transverse mass m(t) 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 < m(t)> derived from the m(t) 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 m(t) distributions of (p) over bar and p are very similar. The ratios of the particle yields, (p) over bar /p and (p) over bar/(Lambda) over bar, are also analyzed. The (p) over bar /p ratio exhibits an increase with decreasing centrality and a steep rise with increasing beam energy. The (Lambda) over bar/(Lambda) over bar ratio increases beyond unity with decreasing beam energy.