High transverse momentum hadron spectra at $\sqrt{s_{\mathit{NN}}}=17.3$ GeV in $\mathrm{Pb}+\mathrm{Pb}$ and $p+p$ collisions

Transverse momentum spectra up to $4.5\mathrm{GeV}/c$ around midrapidity of ${\pi }^{\pm },p,\overline{p}$, and $K^{\pm }$ in $\mathrm{Pb}+\mathrm{Pb}$ reactions were measured at $\sqrt{s_{{}_{\mathit{NN}}}}=17.3\mathrm{GeV}$ by the CERN-NA49 experiment. The nuclear modification factors $R_{\mathit{AA}}$ for ${\pi }^{\pm }$ and $R_{\mathrm{CP}}$ for ${\pi }^{\pm },p,\overline{p},K^{\pm }$ were extracted and compared with results obtained at the BNL Relativistic Heavy Ion Collider (RHIC) at $\sqrt{s_{{}_{\mathit{NN}}}}=200\mathrm{GeV}$. The modification factor $R_{\mathit{AA}}$ shows a rapid increase with transverse momentum in the covered region. This indicates that the Cronin effect is the dominating effect in our energy range. The modification factor $R_{\mathrm{CP}}$, in which the contribution of the Cronin effect is reduced, shows a saturation well below unity in the ${\pi }^{\pm }$ channel. The extracted $R_{\mathrm{CP}}$ values follow the $200\mathrm{GeV}$ RHIC results closely in the available transverse momentum range, except for ${\pi }^{\pm }$ above $2.5\mathrm{GeV}/c$ transverse momentum. There the measured suppression is smaller than that observed at RHIC.

Figure 1

Setup of the CERN-NA49 experiment showing different beam definitions and target arrangements.

Figure 2

Distribution of the longitudinal coordinate of the reconstructed collision point in minimum-bias Pb+Pb events, together with the cuts. The other peaks correspond to material in the beamline, while the small constant background under the target peak corresponds to beam+gas reactions.

Figure 3

Distribution of the energy deposited in the VCAL by the projectile spectators in Pb+Pb minimum-bias reactions. The centrality intervals selected by fraction of total inelastic cross section are shown by the vertical lines.

Figure 4

Upper panel: example plot for the momentum space selection in the rapidity slice $0⩽y<0.1$, with ${\pi }^{\pm }$ mass hypothesis. ϕ means charge-reflected azimuth. The size of the boxes indicate the fraction of those track candidates in the given momentum space bin which have a ratio of number of measured to number of potential points below $60\%$ (i.e., the fraction of those track candidates that most likely do not correspond to real particle trajectories). The color map indicates the number of potential points as a function of momentum space. As can be seen, the bad track candidates mainly populate the borders of the acceptance. The dotted line shows the momentum space cut, guided by potential point isosurfaces. Lower panel: example plot for the ϕ distribution at $0⩽y<0.1$ and $2.5⩽p_{{}_T}<2.6\mathrm{GeV}/c$.

Figure 5

Example plot for the identification of negative particles by specific energy loss fits in central $\mathrm{Pb}+\mathrm{Pb}$ collisions (0–5%). The parametrization of the response function of each particle species with fixed momentum is known, but the yields (the amplitudes in the summed $\frac{\mathit{dE}}{\mathit{dx}}$ spectrum model) are fit parameters.

Figure 6

Ratio of the estimated feed-down contribution to the raw measured particle yields. Plots correspond to $\mathrm{Pb}+\mathrm{Pb}$ collisions with three different centrality ranges of the inelastic cross section (see Table 2).

Figure 7

Detection inefficiencies for ${\pi }^{\pm },K^{\pm },p$, and $\overline{p}$. Plots correspond to $\mathrm{Pb}+\mathrm{Pb}$ collisions with three different centrality ranges of the inelastic cross section (see Table 2). The decay loss for ${\pi }^{\pm }$ and $K^{\pm }$ is included in the inefficiencies.

Figure 8

Invariant yields per inelastic collision of ${\pi }^{\pm },K^{\pm }$, and $p,\overline{p}$ vs transverse momentum $p_{{}_T}$ at midrapidity and $\sqrt{s_{{}_{\mathit{NN}}}}=17.3\mathrm{GeV}$ energy. Results are shown for $\mathrm{Pb}+\mathrm{Pb}$ collisions with three centrality ranges of the inelastic cross section (see Table 2). Previously published results for central collisions in the lower $p_{{}_T}$ range [24, 28] are indicated by the bands in the top row. The bottom row shows published NA49 results [7] for ${\pi }^{+}$ and ${\pi }^{-}$ production in inelastic $p+p$ collisions.

Figure 9

Ratios between antiparticle and particle yields vs transverse momentum $p_{{}_T}$ at midrapidity and $\sqrt{s_{{}_{\mathit{NN}}}}=17.3\mathrm{GeV}$ energy. Results are shown for ${\pi }^{-}/{\pi }^{+},K^{-}/K^{+},$ and $\overline{p}/p$ (scaled up by a factor 6). Plots correspond to $\mathrm{Pb}+\mathrm{Pb}$ collisions with three centrality ranges of the inelastic cross section (see Table 2). Ratios from published NA49 results on ${\pi }^{-}$ and ${\pi }^{+}$ yields in $p+p$ collisions [7] are plotted in the bottom right panel.

Figure 10

Ratios of charged pion yields at midrapidity in central nucleus+nucleus and inelastic nucleon+nucleon collisions scaled by the number of binary collisions ($R^{\mathrm{BC}}$) and number of wounded nucleons ($R^W$). The left column shows results from NA49 at the SPS ($\sqrt{s_{{}_{\mathit{NN}}}}=17.3\mathrm{GeV}$) for the ratio of central (0–5%) Pb+Pb collisions to inelastic $p+p$ reactions compared with the ratio of $p$+W to $p+p$ collisions at $\sqrt{s_{{}_{\mathit{NN}}}}=19.4\mathrm{GeV}$ [9] (the dotted line is shown to guide the eye). The right column shows measurements from RHIC ($\sqrt{s_{{}_{\mathit{NN}}}}=200\mathrm{GeV}$) [2, 4] for the ratio of central (0–5%) Au+Au collisions and $d$+Au reactions. (The error bars attached to the constant 1 line indicate normalization uncertainty.)

Figure 11

Ratio of yields in central to peripheral $\mathrm{Pb}+\mathrm{Pb}$ and Au+Au collisions at $\sqrt{s_{{}_{\mathit{NN}}}}=17.3\mathrm{GeV}$ and $200\mathrm{GeV}$ [2]. Left column shows ratio $R_{\mathrm{CP}}^{\mathrm{BC}}$ using binary collision scaling, right column shows ratio $R_{\mathrm{CP}}^W$ using wounded nucleon scaling. Centrality intervals are specified in the legend of the top left panel. (The error bars attached to the constant 1 line indicate normalization uncertainty.)

Figure 12

Comparison of nuclear modification factors for ${\pi }^{\pm }$ and ${\pi }^0$ using binary collision scaling ($R_{\mathrm{CP}}^{\mathrm{BC}}$) and wounded nucleon scaling ($R_{\mathrm{CP}}^W$). Results at the SPS ($\sqrt{s_{{}_{\mathit{NN}}}}=17.3\mathrm{GeV}$) in $\mathrm{Pb}+\mathrm{Pb}$ collisions from NA49 on ${\pi }^{\pm }$ are shown by squares, from WA98 on ${\pi }^0$ [6] by circles. Measurements from RHIC ($\sqrt{s_{{}_{\mathit{NN}}}}=200\mathrm{GeV}$) in Au+Au collisions for ${\pi }^{\pm }$ [2] are plotted as triangles, for ${\pi }^0$ [30] as diamonds. (The error bars attached to the constant 1 line indicate normalization uncertainty.)