Energy and centrality dependence of antiproton and proton production and the antilambda to antiproton ratio in Pb+Pb collisions between 20A GeV and 158A GeV

The transverse mass 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, inverse slope parameter and mean transverse mass derived from the transverse mass 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 transverse mass distributions of antiproton and proton are very similar. The ratios of the particle yields, antiproton/proton and antilambda/antiproton, are also analysed. The antiproton to proton ratio exhibits an increase with decreasing centrality and a steep rise with increasing beam energy. The antilambda to antiproton ratio increases beyond unity with decreasing beam energy.