We report the first three-particle coincidence measurement in pseudorapidity ($\Delta\eta$) between a high transverse momentum ($p_{\perp}$) trigger particle and two lower $p_{\perp}$ associated particles within azimuth $\mid$$\Delta\phi$$\mid$$<$0.7 in $\sqrt{{\it s}_{NN}}$ = 200 GeV $d$+Au and Au+Au collisions. Charge ordering properties are exploited to separate the jet-like component and the ridge (long-range $\Delta\eta$ correlation). The results indicate that the particles from the ridge are uncorrelated in $\Delta\eta$ not only with the trigger particle but also between themselves event-by-event. In addition, the production of the ridge appears to be uncorrelated to the presence of the narrow jet-like component.
We present first measurements of the evolution of the differential transverse momentum correlation function, {\it C}, with collision centrality in Au+Au interactions at $\sqrt{s_{NN}} = 200$ GeV. {\it C} exhibits a strong dependence on collision centrality that is qualitatively similar to that of number correlations previously reported. We use the observed longitudinal broadening of the near-side peak of {\it C} with increasing centrality to estimate the ratio of the shear viscosity to entropy density, $\eta/s$, of the matter formed in central Au+Au interactions. We obtain an upper limit estimate of $\eta/s$ that suggests that the produced medium has a small viscosity per unit entropy.
Forum