Pion and kaon production in central $\mathrm{Pb}+\mathrm{Pb}$ collisions at $20A$ and $30A$ GeV: Evidence for the onset of deconfinement

Results on charged pion and kaon production in central $\mathrm{Pb}+\mathrm{Pb}$ collisions at $20A$ and $30A$ GeV are presented and compared to data at lower and higher energies. Around $30A$ GeV a rapid change of the energy dependence for the yields of pions and kaons as well as for the shape of the transverse mass spectra is observed. The change is compatible with the prediction that the threshold for production of a state of deconfined matter at the early stage of the collisions is located at low CERN Super Proton Synchroton energies.

Figure 1

Transverse mass spectra of ${\pi }^{-}$ and ${\pi }^{+}$ mesons at $0<y<0.2$ (left) and those of K${}^{+}$ and K${}^{-}$ mesons at $\left|y\right|<0.1$ (right) produced in central $\mathrm{Pb}+\mathrm{Pb}$ collisions at $20A$ and $30A$ GeV. The lines are fits of Eq. (1) to the spectra in the interval $0.2<m_T-m<0.7$ GeV. The statistical errors are smaller than the symbol size. The systematic errors, which are not plotted, are 5% in the region used for the fit and reach 10% at the low and high ends of the $m_T$ spectra.

Figure 2

Rapidity distributions of ${\pi }^{-}$ (left), ${\mathrm{K}}^{-}$ (middle), and ${\mathrm{K}}^{+}$ mesons (right) produced in central $\mathrm{Pb}+\mathrm{Pb}$ collisions at $20A$ (lower curves) and $30A$ GeV (upper curves). Squares and circles show the results of an analysis based on $\mathrm{d}E/\mathrm{d}x$ only, whereas triangles are obtained from a $\mathrm{TOF}+\mathrm{d}E/\mathrm{d}x$ analysis. The solid symbols indicate measured points while the open points are reflected with respect to midrapidity. The lines indicate fits of Eq. (2) to the data. The errors on the points are the statistical and systematic errors added in quadrature. The statistical errors, if larger than the symbol size, are indicated by the inner error bars.

Figure 3

(Left) Energy dependence of the mean pion multiplicity per wounded nucleon measured in central $\mathrm{Pb}+\mathrm{Pb}$ and $\mathrm{Au}+\mathrm{Au}$ [15, 16] collisions (solid symbols) compared to the corresponding results from $\mathrm{p}+\mathrm{p}(\overline{\mathrm{p}})$ reactions (open circles). The errors indicate statistical and systematic (if given by an experiment) errors added in quadrature. They are smaller than the symbol size for the NA49 points. (Right) Energy dependence of the difference between the measured mean pion multiplicity per wounded nucleon and a parametrization (see text) of the $\mathrm{p}+\mathrm{p}$ data. The meanings of the solid and open symbols and the errors are the same as those in the left-hand plot. The lines show various model predictions described in the text.

Figure 4

(Left) Energy dependence of the $\langle {\mathrm{K}}^{+}\rangle /\langle {\pi }^{+}\rangle$ ratio measured in central $\mathrm{Pb}+\mathrm{Pb}$ and $\mathrm{Au}+\mathrm{Au}$ [15, 16] collisions (solid symbols) compared to the corresponding results from $\mathrm{p}+\mathrm{p}(\overline{\mathrm{p}})$ reactions (open circles). The errors indicate statistical and systematic (if given by an experiment) errors added in quadrature. The statistical errors on the NA49 points, if larger than the symbol size, are indicated by the inner error bars. (Right) Energy dependence of the relative strangeness production as measured by the $E_S$ ratio (see text) in central $\mathrm{Pb}+\mathrm{Pb}$ and $\mathrm{Au}+\mathrm{Au}$ collisions (solid symbols) compared to results from $\mathrm{p}+\mathrm{p}(\overline{\mathrm{p}})$ reactions (open circles). The meanings of the errors are the same as those in the left-hand plot. The curves in the figures show predictions of various models described in the text.

Figure 5

(Left) Energy dependence of the ratio $\langle {\mathrm{K}}^{-}\rangle /\langle {\pi }^{-}\rangle$ measured in central $\mathrm{Pb}+\mathrm{Pb}$ and $\mathrm{Au}+\mathrm{Au}$ [15, 16] collisions (solid symbols). The errors indicate statistical and systematic (if given by an experiment) errors added in quadrature. (Right) Energy dependence of the ratio of midrapidity yields of ${\mathrm{K}}^{-}$ and ${\mathrm{K}}^{+}$ mesons. The curves in the figures show model predictions described in the text. The meanings of the errors are the same as those in the left-hand plot.

Figure 6

Energy dependence of the ratios ${\mathrm{K}}^{+}/{\pi }^{+}$ (left) and ${\mathrm{K}}^{-}/{\pi }^{-}$ (right) at midrapidity measured in central $\mathrm{Pb}+\mathrm{Pb}$ and $\mathrm{Au}+\mathrm{Au}$ [15, 16] collisions. The NA44 data are taken from Ref. [20]. The errors indicate statistical and systematic (if given by an experiment) errors added in quadrature. The statistical errors on the NA49 points, if larger than the symbol size, are indicated by the inner error bars.

Figure 7

Energy dependence of the inverse slope parameter $T$ of the transverse mass spectra of ${\mathrm{K}}^{+}$ (left) and ${\mathrm{K}}^{-}$ mesons (right) measured at mid-rapidity in central $\mathrm{Pb}+\mathrm{Pb}$ and $\mathrm{Au}+\mathrm{Au}$ [15, 16] collisions. The ${\mathrm{K}}^{+}$ slope parameters are compared to those from $\mathrm{p}+\mathrm{p}(\overline{\mathrm{p}})$ reactions [21] in the left-hand plot (open circles). The errors indicate statistical errors only. The curves in the right-hand plot represent predictions from various models described in the text.

Figure 8

Energy dependence of the mean transverse mass $\langle m_T\rangle$ measured at midrapidity in central $\mathrm{Pb}+\mathrm{Pb}$ and $\mathrm{Au}+\mathrm{Au}$ collisions for ${\pi }^{\pm }$ (left) and ${\mathrm{K}}^{\pm }$ (middle). For completeness, the previously published results on p and $\overline{\mathrm{p}}$ [22] are shown in the right panel. In the plots, positively (negatively) charged hadrons are indicated by the solid (open) symbols. The results from AGS and RHIC are taken from Refs. [15] and [16]. The errors indicate statistical and systematic errors added in quadrature.