Energy and centrality dependence of $\overline{p}$ and p production and the $\overline{\Lambda }/\overline{p}$ ratio in Pb+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 $\mathit{dn}/\mathit{dy}$, inverse slope parameter T, and mean transverse mass $\langle m_t\rangle$ 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 $\overline{p}$ and p are very similar. The ratios of the particle yields, $\overline{p}$/p and $\overline{\Lambda }$/$\overline{p}$, are also analyzed. The $\overline{p}$/p ratio exhibits an increase with decreasing centrality and a steep rise with increasing beam energy. The $\overline{\Lambda }$/$\overline{p}$ ratio increases beyond unity with decreasing beam energy.

Figure 1

Mass squared histograms for TOF-identified kaons and antiprotons in Pb+Pb reactions at 20A, 30A, 40A, 80A, and 158A GeV beam energies. $\mathit{dE}/\mathit{dx}$ cuts have been applied to the data to reject most of the pions. The particle momenta range from 3 to 10 GeV/c.

Figure 2

Transverse mass distributions for antiprotons and protons [21] in the rapidity interval $2.4<y<2.8$ for the six centrality classes in 158A GeV Pb+Pb collisions. The circle and square points denote the data obtained with the trigger settings selecting minimum bias and central collisions, respectively. Errors are statistical. Curves through the measured points represent the two-exponential fit of Eq. (1) to the data. For clarity, the spectra are scaled down by a factor of 2 successively from the uppermost data.

Figure 3

Transverse mass distributions for antiprotons and protons at midrapidity in central Pb+Pb collisions at 20A, 30A, 40A, 80A, and 158A GeV. Errors are statistical. Solid lines illustrate the two-exponential fit of Eq. (1) to the data. For clarity, the spectra are scaled down by a factor of 2 successively from the uppermost data.

Figure 4

Mean transverse mass $\langle m_t\rangle -m$ for $\overline{p}$ and p as a function of $N_{\mathrm{wound}}$ at midrapidity in 158A GeV Pb+Pb collisions. Errors are statistical.

Figure 5

Mean transverse mass $\langle m_t\rangle -m$ for $\overline{p}$ and p at midrapidity as a function of the c.m. energy per nucleon pair $\sqrt{s_{\mathit{NN}}}$ in central Pb+Pb collisions at SPS energies (NA49) together with data for central Au+Au collisions from the AGS [35, 36, 37] and RHIC [38]. Errors are statistical.

Figure 6

Yield $\mathit{dn}/\mathit{dy}$ of $\overline{p}$ and p per $N_{\mathrm{wound}}$ and the $\overline{p}/p$ ratio plotted as a function of $N_{\mathrm{wound}}$ at midrapidity in 158A GeV Pb+Pb collisions. Errors are statistical.

Figure 7

The $\overline{p}/p$ ratio at midrapidity as a function of the c.m. energy per nucleon pair $\sqrt{s_{\mathit{NN}}}$ in central Pb+Pb collisions at SPS energies (NA49) together with the data for lower energies at the AGS [37] and higher energies at the RHIC [44, 45, 47], respectively. The data [48] at top SPS energy are also shown.

Figure 8

$\overline{\Lambda }/\overline{p}$ ratio at midrapidity as a function of the c.m. energy per nucleon pair $\sqrt{s_{\mathit{NN}}}$ in central Pb+Pb collisions at SPS energies (NA49) together with data from AGS [7, 8] and RHIC [11]. Total error bars are drawn. Curve shows the prediction of the statistical hadron gas model [49].

Figure 9

Impact parameter dependence of the $\overline{\Lambda }/\overline{p}$ ratio at midrapidity in central Pb+Pb collisions at 158A GeV. Errors are statistical. Curve presents the result of hadronic cascade (UrQMD) calculation [18].