Azimuthal Anisotropy at the Relativistic Heavy Ion Collider: The First and Fourth Harmonics

We report the first observations of the first harmonic (directed flow, v1) and the fourth harmonic (v4), in the azimuthal distribution of particles with respect to the reaction plane in Au+Au collisions at the BNL Relativistic Heavy Ion Collider (RHIC). Both measurements were done taking advantage of the large elliptic flow (v2) generated at RHIC. From the correlation of v2 with v1 it is determined that v2 is positive, or in-plane. The integrated v4 is about a factor of 10 smaller than v2. For the sixth (v6) and eighth (v8) harmonics upper limits on the magnitudes are reported

Open Charm Yields in d+Au Collisions at √s\u3csub\u3eNN\u3c/sub\u3e = 200 GeV

Midrapidity open charm spectra from direct reconstruction of D0(D0)→K∓π± in d+Au collisions and indirect electron-positron measurements via charm semileptonic decays in p+p and d+Au collisions at √sNN=200  GeV are reported. The D0(D0) spectrum covers a transverse momentum (pT) range of 0.1T\u3c3  GeV/c, whereas the electron spectra cover a range of 1T\u3c4  GeV/c. The electron spectra show approximate binary collision scaling between p+p and d+Au collisions. From these two independent analyses, the differential cross section per nucleon-nucleon binary interaction at midrapidity for open charm production from d+Au collisions at BNL RHIC is dσNNc¯c/dy=0.30±0.04(stat)±0.09(syst)   mb. The results are compared to theoretical calculations. Implications for charmonium results in A+A collisions are discussed

Measurements of transverse energy distributions in Au+Au collisions at √s\u3csub\u3eNN\u3c/sub\u3e=200 GeV

Transverse energy (ET) distributions have been measured for Au+Au collisions at √sNN=200GeV by the STAR Collaboration at RHIC. ET is constructed from its hadronic and electromagnetic components, which have been measured separately. ET production for the most central collisions is well described by several theoretical models whose common feature is large energy density achieved early in the fireball evolution. The magnitude and centrality dependence of ET per charged particle agrees well with measurements at lower collision energy, indicating that the growth in ET for larger collision energy results from the growth in particle production. The electromagnetic fraction of the total ET is consistent with a final state dominated by mesons and independent of centrality

Centrality and pseudorapidity dependence of charged hadron production at intermediate p\u3csub\u3eT\u3c/sub\u3e in Au+Au collisions at √s\u3csub\u3eNN\u3c/sub\u3e=130 GeV

We present STAR measurements of charged hadron production as a function of centrality in Au+Au collisions at √sNN=130 GeV. The measurements cover a phase space region of 0.2T\u3c6.0GeV∕c in transverse momentum and −1\u3cη\u3c1 in pseudorapidity. Inclusive transverse momentum distributions of charged hadrons in the pseudorapidity region 0.5\u3c∣η∣\u3c1 are reported and compared to our previously published results for ∣η∣\u3c0.5. No significant difference is seen for inclusive pT distributions of charged hadrons in these two pseudorapidity bins. We measured dN∕dη distributions and truncated mean pT in a region of pT\u3epTcut, and studied the results in the framework of participant and binary scaling. No clear evidence is observed for participant scaling of charged hadron yield in the measured pT region. The relative importance of hard scattering processes is investigated through binary scaling fraction of particle production

K(892)* resonance production in Au+Au and p+p collisions at √s\u3csub\u3eNN\u3c/sub\u3e = 200 GeV

The short-lived K(892)∗ resonance provides an efficient tool to probe properties of the hot and dense medium produced in relativistic heavy-ion collisions. We report measurements of K∗ in √sNN=200GeV Au+Au and p+p collisions reconstructed via its hadronic decay channels K(892)*0→Kπ and K(892)*±→K0Sπ± using the STAR detector at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory. The K*0 mass has been studied as a function of pT in minimum bias p+p and central Au+Au collisions. The K∗pT spectra for minimum bias p+pinteractions and for Au+Au collisions in different centralities are presented. The K∗/K yield ratios for all centralities in Au+Au collisions are found to be significantly lower than the ratio in minimum bias p+p collisions, indicating the importance of hadronic interactions between chemical and kinetic freeze-outs. A significant nonzero K*0 elliptic flow (v2) is observed in Au+Au collisions and is compared to the K0S and Λ v2. The nuclear modification factor of K∗ at intermediate pT is similar to that of K0S but different from Λ. This establishes a baryon-meson effect over a mass effect in the particle production at intermediate pT (

Pion-Kaon Correlations in Central Au+Au Collisions at √s\u3csub\u3eNN\u3c/sub\u3e=130 GeV

Pion-kaon correlation functions are constructed from central Au+Au STAR data taken at √sNN=130  GeV by the STAR detector at the Relativistic Heavy Ion Collider (RHIC). The results suggest that pions and kaons are not emitted at the same average space-time point. Space-momentum correlations, i.e., transverse flow, lead to a space-time emission asymmetry of pions and kaons that is consistent with the data. This result provides new independent evidence that the system created at RHIC undergoes a collective transverse expansion

Pion interferometry in Au+Au collisions at √s\u3csub\u3eNN\u3c/sub\u3e = 200 GeV

We present a systematic analysis of two-pion interferometry in Au+Au collisions at √sNN=200GeV using the STAR detector at Relativistic Heavy Ion Collider. We extract the Hanbury-Brown and Twiss radii and study their multiplicity, transverse momentum, and azimuthal angle dependence. The Gaussianness of the correlation function is studied. Estimates of the geometrical and dynamical structure of the freeze-out source are extracted by fits with blast-wave parametrizations. The expansion of the source and its relation with the initial energy density distribution is studied

Production of e\u3csup\u3e+\u3c/sup\u3ee\u3csup\u3e−\u3c/sup\u3e pairs accompanied by nuclear dissociation in ultraperipheral heavy-ion collisions

We present data on e+e− pair production accompanied by nuclear breakup in ultraperipheral gold-gold collisions at a center of mass energy of 200GeV per nucleon pair. The nuclear breakup requirement selects events at small impact parameters, where higher-order diagrams for pair production should be enhanced. We compare the data with two calculations: one based on the equivalent photon approximation, and the other using lowest-order quantum electrodynamics (QED). The data distributions agree with both calculations, except that the pair transverse momentum spectrum disagrees with the equivalent photon approach. We set limits on higher-order contributions to the cross section

ϕ meson production in Au+Au and p+p collisions at sNN=200GeV

We report the STAR measurement of ϕ meson production in Au+Au and p+p collisions at sNN=200GeV. Using the event mixing technique, the ϕ spectra and yields are obtained at mid-rapidity for five centrality bins in Au+Au collisions and for non-singly-diffractive p+pcollisions. It is found that the ϕ transverse momentum distributions from Au+Au collisions are better fitted with a single-exponential while the p+p spectrum is better described by a double-exponential distribution. The measured nuclear modification factors indicate that ϕproduction in central Au+Au collisions is suppressed relative to peripheral collisions when scaled by the number of binary collisions (〈Nbin〉). The systematics of 〈pt〉 versus centrality and the constant ϕ/K− ratio versus beam species, centrality, and collision energy rule out kaon coalescence as the dominant mechanism for ϕ production

Azimuthal Anisotropy and Correlations at Large Transverse Momenta in p+p and Au+Au Collisions at √s\u3csub\u3eNN\u3c/sub\u3e=200 GeV

Results on high transverse momentum charged particle emission with respect to the reaction plane are presented for Au+Au collisions at √sNN=200  GeV. Two- and four-particle correlations results are presented as well as a comparison of azimuthal correlations in Au+Au collisions to those in p+p at the same energy. The elliptic anisotropy v2 is found to reach its maximum at pt∼3  GeV/c, then decrease slowly and remain significant up to pt≈7–10  GeV/c. Stronger suppression is found in the back-to-back high-pt particle correlations for particles emitted out of plane compared to those emitted in plane. The centrality dependence of v2 at intermediate pt is compared to simple models based on jet quenching