T. J. Hallman

Azimuthal correlations for large transverse momentum charged hadrons have been measured over a wide pseudorapidity range and full azimuth in $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ and $p+p$ collisions at $\sqrt{{s}_{NN}}=200\text{ }\text{ }\mathrm{G}\mathrm{e}\mathrm{V}$. The small-angle correlations observed in $p+p$ collisions and at all centralities of $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions are characteristic of hard-scattering processes previously observed in high-energy collisions. A strong back-to-back correlation exists for $p+p$ and peripheral $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$. In contrast, the back-to-back correlations are reduced considerably in the most central $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions, indicating substantial interaction as the hard-scattered partons or their fragmentation products traverse the medium.

Inclusive transverse momentum distributions of charged hadrons within $0.2<{p}_{T}<6.0\text{ }\mathrm{G}\mathrm{e}\mathrm{V}/c$ have been measured over a broad range of centrality for $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions at $\sqrt{{s}_{NN}}=130\text{ }\text{ }\mathrm{G}\mathrm{e}\mathrm{V}$. Hadron yields are suppressed at high ${p}_{T}$ in central collisions relative to peripheral collisions and to a nucleon-nucleon reference scaled for collision geometry. Peripheral collisions are not suppressed relative to the nucleon-nucleon reference. The suppression varies continuously at intermediate centralities. The results indicate significant nuclear medium effects on high-${p}_{T}$ hadron production in heavy-ion collisions at high energy.

Charged hadrons in $0.15<{p}_{\ensuremath{\perp}}<4\text{ }\text{ }\mathrm{GeV}/c$ associated with particles of ${p}_{\ensuremath{\perp}}^{\mathrm{trig}}>4\text{ }\text{ }\mathrm{GeV}/c$ are reconstructed in $pp$ and $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{{s}_{NN}}=200\text{ }\text{ }\mathrm{GeV}$. The associated multiplicity and ${p}_{\ensuremath{\perp}}$ magnitude sum are found to increase from $pp$ to central $\mathrm{Au}+\mathrm{Au}$ collisions. The associated ${p}_{\ensuremath{\perp}}$ distributions, while similar in shape on the nearside, are significantly softened on the awayside in central $\mathrm{Au}+\mathrm{Au}$ relative to $pp$ and not much harder than that of inclusive hadrons. The results, consistent with jet quenching, suggest that the awayside fragments approach equilibration with the medium traversed.

Elliptic flow holds much promise for studying the early-time thermalization attained in ultrarelativistic nuclear collisions. Flow measurements also provide a means of distinguishing between hydrodynamic models and calculations which approach the low density (dilute gas) limit. Among the effects that can complicate the interpretation of elliptic flow measurements are azimuthal correlations that are unrelated to the reaction plane (nonflow correlations). Using data for Au + Au collisions at $\sqrt{{s}_{\mathrm{NN}}}=130\mathrm{GeV}\mathrm{}$ from the STAR time projection chamber, it is found that four-particle correlation analyses can reliably separate flow and nonflow correlation signals. The latter account for on average about 15% of the observed second-harmonic azimuthal correlation, with the largest relative contribution for the most peripheral and the most central collisions. The results are also corrected for the effect of flow variations within centrality bins. This effect is negligible for all but the most central bin, where the correction to the elliptic flow is about a factor of 2. A simple new method for two-particle flow analysis based on scalar products is described. An analysis based on the distribution of the magnitude of the flow vector is also described.

We report first results on elliptic flow of identified particles at midrapidity in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{{s}_{\mathrm{NN}}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}130\mathrm{GeV}$ using the STAR TPC at RHIC. The elliptic flow as a function of transverse momentum and centrality differs significantly for particles of different masses. This dependence can be accounted for in hydrodynamic models, indicating that the system created shows a behavior consistent with collective hydrodynamical flow. The fit to the data with a simple model gives information on the temperature and flow velocities at freeze-out.