Narrowing of the Balance Function with Centrality in $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ Collisions at $\sqrt{s_{NN}}=130\mathrm{G}\mathrm{e}\mathrm{V}$

The balance function is a new observable based on the principle that charge is locally conserved when particles are pair produced. Balance functions have been measured for charged particle pairs and identified charged pion pairs in $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions at $\sqrt{s_{NN}}=130\mathrm{G}\mathrm{e}\mathrm{V}$ at the Relativistic Heavy Ion Collider using STAR. Balance functions for peripheral collisions have widths consistent with model predictions based on a superposition of nucleon-nucleon scattering. Widths in central collisions are smaller, consistent with trends predicted by models incorporating late hadronization.

Figure 1

The balance function versus $\Delta \eta$ for charged particle pairs from (a) central and peripheral $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions at $\sqrt{s_{NN}}=130\mathrm{G}\mathrm{e}\mathrm{V}$ and mixed events from central and peripheral $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions, and (b) HIJING events filtered with GEANT [16] and shuffled pseudorapidity events from measured central and peripheral $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions. To guide the eye, Gaussian fits excluding the lowest bin in $\Delta \eta$ are shown. The error bars shown are statistical. The balance function for HIJING events is independent of centrality.

Figure 2

The width of the balance function for charged particles, $⟨\Delta \eta ⟩$, as a function of normalized impact parameter ($b/b_{\mathrm{m}\mathrm{a}\mathrm{x}}$). Error bars shown are statistical. The width of the balance function from HIJING events is shown as a band whose height reflects the statistical uncertainty. Also shown are the widths from the shuffled pseudorapidity events.

Figure 3

The balance function versus $\Delta y$ for identified pion pairs from (a) central and peripheral $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions at $\sqrt{s_{NN}}=130\mathrm{G}\mathrm{e}\mathrm{V}$, and (b) HIJING events simulated in GEANT [16]. To guide the eye, Gaussian fits excluding the lowest two bins in $\Delta y$ are shown. The error bars shown are statistical. The balance function for HIJING events is independent of centrality.

Figure 4

The width of the balance function for identified charged pions, $⟨\Delta y⟩$, as a function of normalized impact parameter ($b/b_{\mathrm{m}\mathrm{a}\mathrm{x}}$). Error bars shown are statistical. The width of the balance function from HIJING events is shown as a band whose height reflects the statistical uncertainty.