Charged-to-neutral correlation at forward rapidity in Au + Au collisions at $\sqrt{s_{\mathit{NN}}}=200$ GeV

Event-by-event fluctuations of the multiplicities of inclusive charged particles and photons at forward rapidity in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{\mathit{NN}}}=200$ GeV have been studied. The dominant contribution to such fluctuations is expected to come from correlated production of charged and neutral pions. We search for evidence of dynamical fluctuations of different physical origins. Observables constructed out of moments of multiplicities are used as measures of fluctuations. Mixed events and model calculations are used as base lines. Results are compared to the dynamical net-charge fluctuations measured in the same acceptance. A nonzero statistically significant signal of dynamical fluctuations is observed in excess to the model prediction when charged particles and photons are measured in the same acceptance. We find that, unlike dynamical net-charge fluctuation, charge-neutral fluctuation is not dominated by correlation owing to particle decay. Results are compared to the expectations based on the generic production mechanism of pions owing to isospin symmetry, for which no significant ($<1\%$) deviation is observed.

Figure 1

Multiplicity distributions of raw charged particles (a) and photons (b). The solid lines are Gaussian fits to the real event data points to guide the eyes.

Figure 2

The ${\nu }_{\mathrm{dyn}}$ and the three terms of ${\nu }_{\mathrm{dyn}}$ vs $\sqrt{\langle N_{\mathrm{ch}}\rangle \langle N_{\gamma }\rangle }$ for real (circles) and mixed (squares) events. $\langle N_{\mathrm{ch}}\rangle$ and $\langle N_{\gamma }\rangle$ are the mean charged-particle and photon multiplicities in each centrality bin. Statistical uncertainties are represented by vertical lines, while systematic uncertainties are shown by boxes. The statistical uncertainties for the models are shown by bands. The central limit theorem (CLT) fit to ${\nu }_{\mathrm{dyn}}$ for real events with the functional form $0.005+0.37/\sqrt{\langle N_{\mathrm{ch}}\rangle \langle N_{\gamma }\rangle }$ is plotted as a solid curve.

Figure 3

The values of the observable ${\nu }_{\mathrm{dyn}}^{\mathrm{ch}-\gamma }$ for the same side (a) and the away side (b). The same side corresponds to a measurement of photons and charged particles in the same acceptance $-3.7<\eta <-2.8$. The away side corresponds to measurement of photons in the range $-3.7<\eta <-2.8$ and charged particle in the range $3.7>\eta >2.8$. Model and mixed-event calculations are performed using the same kinematic cuts. For data, the statistical uncertainties are shown by vertical lines and the systematic uncertainties are shown by boxes. For model curves statistical uncertainties are shown by bands.

Figure 4

The correlation between positive and negative charged particles measured by the FTPC and photons measured by the PMD using ${\nu }_{\mathrm{dyn}}$ for real events. For the positive-negative charged-particle correlation, the statistical uncertainties are represented by lines and the systematic uncertainties are shown by boxes. For the different combinations of $\mathrm{ch}-\gamma$ correlation the systematic uncertainties in $\sqrt{\langle N_{\mathrm{ch}}\rangle \langle N_{\gamma }\rangle }$ are shown by boxes, whereas the systematic uncertainties in ${\nu }_{\mathrm{dyn}}$ are shown by the yellow band.

Figure 5

$r_{m,1}$ vs multiplicity for first three orders of $m$. Data and mixed-event results are compared to hijing and $hijing+geant$ values, which are shown by the curves. Statistical uncertainties are shown by vertical lines and the systematic uncertainties are shown by boxes. For model curves statistical uncertainties are shown by bands.

Figure 6

The moments $r_{m,1}(m=1–3)$ for real and mixed events as a function of order $m$ in a fixed multiplicity bin of $47<\sqrt{\langle N_{\mathrm{ch}}\rangle \langle N_{\gamma }\rangle }<54$. hijing, $hijing+geant$ results are represented by curves. For data, the statistical uncertainties are shown by vertical lines and the systematic uncertainties are shown by boxes. For model curves, statistical uncertainties are shown by bands.