$\Delta \varphi \Delta \eta$ correlations in central Au+Au collisions at $\sqrt{s_{\mathit{NN}}}=200$ GeV

We report charged particle pair correlation analyses in the space of $\Delta \varphi$ (azimuth) and $\Delta \eta$ (pseudorapidity), for central Au+Au collisions at $\sqrt{s_{\mathit{NN}}}=200$ GeV in the STAR detector. The analysis involves unlike-sign charged pairs and like-sign charged pairs, which are transformed into charge-dependent (CD) signals and charge-independent (CI) signals. We present detailed parametrizations of the data. A model featuring dense gluonic hot spots as first proposed by Van Hove predicts that the observables under investigation would have sensitivity to such a substructure should it occur, and the model also motivates selection of transverse momenta in the range $0.8<p_t<2.0$ GeV/$c$. Both CD and CI correlations of high statistical significance are observed, and possible interpretations are discussed.

Figure 1

$\Delta \varphi \Delta \eta$ correlation data for (a) unlike-sign and (b) like-sign charged particle pairs from the STAR central trigger charged pair signal shown in 2-D perspective plots. Particle tracks have $0.8<p_t<2.0$ GeV/$c$ and $\left|\eta \right|<1.0$. The structure that looks like tiles on a roof is due to the readout boundary effects of the 12 sector TPC.

Figure 2

Correlation data for ratio of histograms of same-event pairs to mixed-event pairs for (a) unlike-sign and (b) like-sign charged pairs, shown in 2-D perspective plots $\Delta \varphi \text{−}\Delta \eta$. The plot was normalized to a mean of 1.

Figure 3

Folded correlation data for (a) unlike-sign and (b) like-sign charged pairs on $\Delta \varphi$ and $\Delta \eta$ based on demonstrated symmetries in the data. This increases the statistics in a typical bin by a factor of 4. Henceforth, we will be dealing with folded data only.

Figure 4

(a) 2-D approximately Gaussian signal shape of Eq. (4) from the fit to the normalized and folded unlike-sign charged pair signal data. (b) Unlike-sign charged pair signal data corresponding to the adjoining signal function in (a).

Figure 5

(a) Perspective plot of fit to 2-D like-sign charged pair signal shape of Eq. (5). (b) Like-sign charged pair signal data corresponding to the adjacent signal fit in (a).

Figure 6

(a) CD signal shape is a 2-D approximate Gaussian which is symmetrical. Average Gaussian width is $27.5^{°}\pm 3.2^{°}$ (0.48 ± 0.056 rad) both in $\Delta \varphi$ and $\Delta \eta$. Thus we are observing a greater probability for unlike-sign charged pairs of particles than for like-sign charged pairs emitted randomly on 2-D $\eta \varphi$ directions. (b) CD signal data corresponding to the adjacent signal fit in (a).

Figure 7

(a) Fit to CI signal shape which is the sum of the fits of like-sign plus unlike-sign charged pair signals. The 2-D Gaussian equivalent rms signal has a $\Delta \varphi$ width of about ${32}^{°}$ (0.52 rad), and $\Delta \eta$ width of about ${66}^{°}(1.15\mathrm{rad})$ which is about double the $\Delta \varphi$ width. (b) CI signal data corresponding to the adjacent fit in (a).

Figure 8

(a) CI signal data with the lower range of elliptic flow amplitude and the best ${\chi }^2$ [same as Fig. 7] plotted as a 2-D perspective plot on $\Delta \varphi$ vs $\Delta \eta$. (b) CI signal data with the maximum elliptic flow amplitude used in our analysis (${\chi }^2$ was worse by $1\sigma$).