Rapidity and energy dependence of the electric charge correlations in $A+A$ collisions from $20A$ to $158A$ GeV

Results from electric charge correlations studied with the Balance Function (BF) method in $A+A$ collisions from $20A$ to $158A$ GeV are presented in two different rapidity intervals: In the midrapidity region we observe a decrease of the width of the BF distribution with increasing centrality of the collision, whereas this effect vanishes in the forward rapidity region. Results from the energy dependence study in central $\mathrm{Pb}+\mathrm{Pb}$ collisions show that the narrowing of the BF expressed by the normalized width parameter $W$ increases with energy toward the highest CERN Super Proton Synchrotron and BNL Relativistic Heavy Ion Collider energies. Finally we compare our experimental data points with the predictions of several models. The hadronic string models Ultra-relativistic Quantum Molecular Dynamics and Heavy Ion Jet INteraction Generator (HIJING) do not reproduce the observed narrowing of the BF. However, A MultiPhase Transport medel (AMPT), which contains a quark-parton transport phase before hadronization, can reproduce the narrowing of the BF's width with centrality. This confirms the proposed sensitivity of the BF analysis to the time of hadronization.

Figure 1

The BF as a function of the pseudorapidity difference $\Delta \eta$ for central (centrality class 1 of Tables 1 and 2; left panels of this figure) and peripheral (centrality class 6 of Tables 1 and 2; right panels of this figure) $\mathrm{Pb}+\mathrm{Pb}$ collisions at $\sqrt{s_{\mathit{NN}}}=17.3$ GeV (top panels) and $\sqrt{s_{\mathit{NN}}}=8.8$ GeV (bottom panels) for the two rapidity regions. The curves drawn are Gaussian fits used to guide the eye.

Figure 2

The system size and centrality dependence of the measured width $\langle \Delta \eta \rangle$ of the BF for charged particles at $\sqrt{s_{\mathit{NN}}}=17.3$ GeV (top panels) and $\sqrt{s_{\mathit{NN}}}=8.8$ GeV (bottom panels) plotted as a function of the mean number of wounded nucleons $\langle N_W\rangle$ for the midrapidity (left panels) and the forward rapidity (right panels) regions (see Table 4).

Figure 3

The BF as a function of the pseudorapidity difference $\Delta \eta$ for central $\mathrm{Pb}+\mathrm{Pb}$ collisions at $\sqrt{s_{\mathit{NN}}}=12.3$ GeV (left panel), $\sqrt{s_{\mathit{NN}}}=7.6$ GeV (middle panel), and $\sqrt{s_{\mathit{NN}}}=6.3$ GeV (right panel) for two rapidity regions (see Table 4). The curves drawn are Gaussian fits used to guide the eye.

Figure 4

The dependence of the normalized parameter $W$ on $\sqrt{s_{\mathit{NN}}}$ for central $\mathrm{Pb}+\mathrm{Pb}$ collisions in the SPS energy range after applying the NA49 acceptance filter in comparison with the UrQMD results (left panel) and together with data for central $\mathrm{Au}+\mathrm{Au}$ collisions at RHIC (right panel).

Figure 5

The dependence of the normalized parameter $W$ on $\sqrt{s_{\mathit{NN}}}$ for central $\mathrm{Pb}+\mathrm{Pb}$ collisions in the SPS energy range after applying the NA49 acceptance filter for the midrapidity and for the forward rapidity region (the intervals analyzed are given in Table 4).

Figure 6

The centrality dependence of the measured width $\langle \Delta \eta \rangle$ of the BF at midrapidity ($2.5⩽\eta ⩽3.9$) for charged particles produced in $\mathrm{Pb}+\mathrm{Pb}$ collisions at $\sqrt{s_{\mathit{NN}}}=17.3$ GeV as a function of the mean number of wounded nucleons $\langle N_W\rangle$. The plot shows the experimental data along with the predictions from different microscopic models.