366 research outputs found

    Centrality and transverse momentum dependence of elliptic flow of multi-strange hadrons and Ōē\phi meson in Au+Au collisions at sNN\sqrt{s_{NN}} = 200 GeV

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    We present high precision measurements of elliptic flow near midrapidity (‚ą£y‚ą£<1.0|y|<1.0) for multi-strange hadrons and Ōē\phi meson as a function of centrality and transverse momentum in Au+Au collisions at center of mass energy sNN=\sqrt{s_{NN}}= 200 GeV. We observe that the transverse momentum dependence of Ōē\phi and ő©\Omega v2v_{2} is similar to that of ŌÄ\pi and pp, respectively, which may indicate that the heavier strange quark flows as strongly as the lighter up and down quarks. This observation constitutes a clear piece of evidence for the development of partonic collectivity in heavy-ion collisions at the top RHIC energy. Number of constituent quark scaling is found to hold within statistical uncertainty for both 0-30%\% and 30-80%\% collision centrality. There is an indication of the breakdown of previously observed mass ordering between Ōē\phi and proton v2v_{2} at low transverse momentum in the 0-30%\% centrality range, possibly indicating late hadronic interactions affecting the proton v2v_{2}.Comment: 7 pages and 4 figures, Accepted for publication in Physical Review Letter

    Observation of charge asymmetry dependence of pion elliptic flow and the possible chiral magnetic wave in heavy-ion collisions

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    We present measurements of ŌÄ‚ąí\pi^- and ŌÄ+\pi^+ elliptic flow, v2v_2, at midrapidity in Au+Au collisions at sNN=\sqrt{s_{_{\rm NN}}} = 200, 62.4, 39, 27, 19.6, 11.5 and 7.7 GeV, as a function of event-by-event charge asymmetry, AchA_{ch}, based on data from the STAR experiment at RHIC. We find that ŌÄ‚ąí\pi^- (ŌÄ+\pi^+) elliptic flow linearly increases (decreases) with charge asymmetry for most centrality bins at sNN=27¬†GeV\sqrt{s_{_{\rm NN}}} = \text{27 GeV} and higher. At sNN=200¬†GeV\sqrt{s_{_{\rm NN}}} = \text{200 GeV}, the slope of the difference of v2v_2 between ŌÄ‚ąí\pi^- and ŌÄ+\pi^+ as a function of AchA_{ch} exhibits a centrality dependence, which is qualitatively similar to calculations that incorporate a chiral magnetic wave effect. Similar centrality dependence is also observed at lower energies.Comment: 6 pages, 4 figure

    Isolation of Flow and Nonflow Correlations by Two- and Four-Particle Cumulant Measurements of Azimuthal Harmonics in sNN=\sqrt{s_{_{\rm NN}}} = 200 GeV Au+Au Collisions

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    A data-driven method was applied to measurements of Au+Au collisions at sNN=\sqrt{s_{_{\rm NN}}} = 200 GeV made with the STAR detector at RHIC to isolate pseudorapidity distance őĒő∑\Delta\eta-dependent and őĒő∑\Delta\eta-independent correlations by using two- and four-particle azimuthal cumulant measurements. We identified a component of the correlation that is őĒő∑\Delta\eta-independent, which is likely dominated by anisotropic flow and flow fluctuations. It was also found to be independent of ő∑\eta within the measured range of pseudorapidity ‚ą£ő∑‚ą£<1|\eta|<1. The relative flow fluctuation was found to be 34%¬Ī2%(stat.)¬Ī3%(sys.)34\% \pm 2\% (stat.) \pm 3\% (sys.) for particles of transverse momentum pTp_{T} less than 22 GeV/cc. The őĒő∑\Delta\eta-dependent part may be attributed to nonflow correlations, and is found to be 5%¬Ī2%(sys.)5\% \pm 2\% (sys.) relative to the flow of the measured second harmonic cumulant at ‚ą£őĒő∑‚ą£>0.7|\Delta\eta| > 0.7

    Azimuthal anisotropy in U+U and Au+Au collisions at RHIC

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    Collisions between prolate uranium nuclei are used to study how particle production and azimuthal anisotropies depend on initial geometry in heavy-ion collisions. We report the two- and four-particle cumulants, v2{2}v_2\{2\} and v2{4}v_2\{4\}, for charged hadrons from U+U collisions at sNN\sqrt{s_{\rm NN}} = 193 GeV and Au+Au collisions at sNN\sqrt{s_{\rm NN}} = 200 GeV. Nearly fully overlapping collisions are selected based on the amount of energy deposited by spectators in the STAR Zero Degree Calorimeters (ZDCs). Within this sample, the observed dependence of v2{2}v_2\{2\} on multiplicity demonstrates that ZDC information combined with multiplicity can preferentially select different overlap configurations in U+U collisions. An initial-state model with gluon saturation describes the slope of v2{2}v_2\{2\} as a function of multiplicity in central collisions better than one based on Glauber with a two-component multiplicity model.Comment: Final paper version accepted for publication in Phys. Rev. Lett. New version includes comparisons to a constituent quark glauber mode

    Observation of Transverse Spin-Dependent Azimuthal Correlations of Charged Pion Pairs in p‚ÜĎ+pp^\uparrow+p at s=200\sqrt{s}=200 GeV

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    We report the observation of transverse polarization-dependent azimuthal correlations in charged pion pair production with the STAR experiment in p‚ÜĎ+pp^\uparrow+p collisions at RHIC. These correlations directly probe quark transversity distributions. We measure signals in excess of five standard deviations at high transverse momenta, at high pseudorapidities eta>0.5, and for pair masses around the mass of the rho-meson. This is the first direct transversity measurement in p+p collisions. Comparing the results to data from lepton-nucleon scattering will test the universality of these spin-dependent quantities.Comment: 11 pages, 5 figures, 15 tables. Submitted to PR

    Beam energy dependent two-pion interferometry and the freeze-out eccentricity of pions in heavy ion collisions at STAR

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    We present results of analyses of two-pion interferometry in Au+Au collisions at sNN\sqrt{s_{NN}} = 7.7, 11.5, 19.6, 27, 39, 62.4 and 200 GeV measured in the STAR detector as part of the RHIC Beam Energy Scan program. The extracted correlation lengths (HBT radii) are studied as a function of beam energy, azimuthal angle relative to the reaction plane, centrality, and transverse mass (mTm_{T}) of the particles. The azimuthal analysis allows extraction of the eccentricity of the entire fireball at kinetic freeze-out. The energy dependence of this observable is expected to be sensitive to changes in the equation of state. A new global fit method is studied as an alternate method to directly measure the parameters in the azimuthal analysis. The eccentricity shows a monotonic decrease with beam energy that is qualitatively consistent with the trend from all model predictions and quantitatively consistent with a hadronic transport model.Comment: 27 pages; 27 figure

    Energy dependence of acceptance-corrected dielectron excess mass spectrum at mid-rapidity in Au+Au collisions at sNN=19.6\sqrt{s_{NN}} = 19.6 and 200 GeV

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    The acceptance-corrected dielectron excess mass spectra, where the known hadronic sources have been subtracted from the inclusive dielectron mass spectra, are reported for the first time at mid-rapidity ‚ą£yee‚ą£<1|y_{ee}|<1 in minimum-bias Au+Au collisions at sNN\sqrt{s_{NN}} = 19.6 and 200 GeV. The excess mass spectra are consistently described by a model calculation with a broadened ŌĀ\rho spectral function for Mee<1.1M_{ee}<1.1 GeV/c2c^{2}. The integrated dielectron excess yield at sNN\sqrt{s_{NN}} = 19.6 GeV for 0.4<Mee<0.750.4<M_{ee}<0.75 GeV/c2c^2, normalized to the charged particle multiplicity at mid-rapidity, has a value similar to that in In+In collisions at sNN\sqrt{s_{NN}} = 17.3 GeV. For sNN\sqrt{s_{NN}} = 200 GeV, the normalized excess yield in central collisions is higher than that at sNN\sqrt{s_{NN}} = 17.3 GeV and increases from peripheral to central collisions. These measurements indicate that the lifetime of the hot, dense medium created in central Au+Au collisions at sNN\sqrt{s_{NN}} = 200 GeV is longer than those in peripheral collisions and at lower energies.Comment: 9 pages, 6 figure

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

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    Event-by-event fluctuations of the ratio of inclusive charged to photon multiplicities at forward rapidity in Au+Au collision at sNN\sqrt{s_{NN}}=200 GeV have been studied. Dominant contribution to such fluctuations is expected to come from correlated production of charged and neutral pions. We search for evidences 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 baselines. Results are compared to the dynamical net-charge fluctuations measured in the same acceptance. A non-zero 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 due to particle decay. Results are compared to the expectations based on the generic production mechanism of pions due to isospin symmetry, for which no significant (<1%) deviation is observed.Comment: 14 pages, 6 figure

    Centrality dependence of identified particle elliptic flow in relativistic heavy ion collisions at sqrt(s)= 7.7--62.4 GeV

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    Elliptic flow (v_2) values for identified particles at midrapidity in Au + Au collisions measured by the STAR experiment in the Beam Energy Scan at the Relativistic Heavy Ion Collider at sqrt{s_{NN}}= 7.7--62.4 GeV are presented for three centrality classes. The centrality dependence and the data at sqrt{s_{NN}}= 14.5 GeV are new. Except at the lowest beam energies we observe a similar relative v_2 baryon-meson splitting for all centrality classes which is in agreement within 15% with the number-of-constituent quark scaling. The larger v_2 for most particles relative to antiparticles, already observed for minimum bias collisions, shows a clear centrality dependence, with the largest difference for the most central collisions. Also, the results are compared with A Multiphase Transport Model and fit with a Blast Wave model.Comment: 14 pages, 12 figures, Phys. Rev. C, to be published. Data tables available at https://drupal.star.bnl.gov/STAR/publications/centrality-dependence-identified-particle-elliptic-flow-relativistic-heavy-ion-collisi
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