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Register a new userAzimuthal anisotropy in Au+Au collisions at sqrtsNN = 200 GeV
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STAR Collaboration
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The results from the STAR Collaboration on directed flow (v_1), elliptic flow (v_2), and the fourth harmonic (v_4) in the anisotropic azimuthal distribution of particles from Au+Au collisions at sqrtsNN = 200 GeV are summarized and compared with results from other experiments and theoretical models. Results for identified particles are presented and fit with a Blast Wave model. Different anisotropic flow analysis methods are compared and nonflow effects are extracted from the data. For v_2, scaling with the number of constituent quarks and parton coalescence is discussed. For v_4, scaling with v_2^2 and quark coalescence is discussed.
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The azimuthal anisotropic flow of identified and unidentified charged particles has been systematically studied in Cu+Au collisions at $sqrt{s_{_{NN}}}$ = 200 GeV for harmonics $n=$ 1-4 in the pseudorapidity range $|eta|<1$. The directed flow in Cu+Au collisions is compared with the rapidity-odd and, for the first time, the rapidity-even components of charged particle directed flow in Au+Au collisions at $sqrt{s_{_{NN}}}$ = 200~GeV. The slope of the directed flow pseudorapidity dependence in Cu+Au collisions is found to be similar to that in Au+Au collisions, with the intercept shifted toward positive $eta$ values, i.e., the Cu-going direction. The mean transverse momentum projected onto the spectator plane, $langle p_xrangle$, in Cu+Au collision also exhibits approximately linear dependence on $eta$ with the intercept at about $etaapprox-0.4$, closer to the rapidity of the Cu+Au system center-of-mass. The observed dependencies find natural explanation in a picture of the directed flow originating partly due the tilted source and partly due to the rapidity dependent asymmetry in the initial density distribution. Charge-dependence of the $langle p_xrangle$ was also observed in Cu+Au collisions, indicating an effect of the initial electric field created by charge difference of the spectator protons in two colliding nuclei. The rapidity-even component of directed flow in Au+Au collisions is close to that in Pb+Pb collisions at $sqrt{s_{_{NN}}}$ = 2.76 TeV, indicating a similar magnitude of dipole-like fluctuations in the initial-state density distribution. Higher harmonic flow in Cu+Au collisions exhibits similar trends to those observed in Au+Au and Pb+Pb collisions and is qualitatively reproduced by a viscous hydrodynamic model and a multi-phase transport model. For all harmonics with $nge2$ we observe an approximate scaling of $v_n$ with the number of constituent quarks.
We report measurements of the third harmonic coefficient of the azimuthal anisotropy, v_3, known as triangular flow. The analysis is for charged particles in Au+Au collisions at sqrtsNN = 200 GeV, based on data from the STAR experiment at the Relativistic Heavy Ion Collider. Two-particle correlations as a function of their pseudorapidity separation are fit with narrow and wide Gaussians. Measurements of triangular flow are extracted from the wide Gaussian, from two-particle cumulants with a pseudorapidity gap, and also from event plane analysis methods with a large pseudorapidity gap between the particles and the event plane. These results are reported as a function of transverse momentum and centrality. A large dependence on the pseudorapidity gap is found. Results are compared with other experiments and model calculations.
Azimuthal angle (Delta phi) correlations are presented for a broad range of transverse momentum (0.4 < pT < 10 GeV/c) and centrality (0-92%) selections for charged hadrons from di-jets in Au+Au collisions at sqrt(s_NN) = 200 GeV. With increasing pT, the away-side Delta phi distribution evolves from a broad and relatively flat shape to a concave shape, then to a convex shape. Comparisons to p+p data suggest that the away-side distribution can be divided into a partially suppressed head region centered at Delta phi ~ pi, and an enhanced shoulder region centered at Delta phi ~ pi pm 1:1. The pT spectrum for the associated hadrons in the head region softens toward central collisions. The spectral slope for the shoulder region is independent of centrality and trigger pT . The properties of the near-side distributions are also modified relative to those in p + p collisions, reflected by the broadening of the jet shape in Delta phi and Delta eta, and an enhancement of the per-trigger yield. However, these modifications seem to be limited to pT < 4 GeV/c, above which both the dihadron pair shape and per-trigger yield become similar to p + p collisions. These observations suggest that both the away- and near-side distributions contain a jet fragmentation component which dominates for pT ge 5GeV and a medium-induced component which is important for pT le 4 GeV/c. We also quantify the role of jets at intermediate and low pT through the yield of jet-induced pairs in comparison to binary scaled p + p pair yield. The yield of jet-induced pairs is suppressed at high pair proxy energy (sum of the pT magnitudes of the two hadrons) and is enhanced at low pair proxy energy. The former is consistent with jet quenching; the latter is consistent with the enhancement of soft hadron pairs due to transport of lost energy to lower pT.
We report the first measurement of the elliptic anisotropy ($v_2$) of the charm meson $D^0$ at mid-rapidity ($|y|$,$<$,1) in Au+Au collisions at sNN = 200,GeV. The measurement was conducted by the STAR experiment at RHIC utilizing a new high-resolution silicon tracker. The measured $D^0$ $v_2$ in 0--80% centrality Au+Au collisions can be described by a viscous hydrodynamic calculation for transverse momentum ($p_{rm T}$) less than 4,GeV/$c$. The $D^0$ $v_2$ as a function of transverse kinetic energy ($m_{rm T} - m_0$, where $m_{rm T} = sqrt{p_{rm T}^2 + m_0^2}$) is consistent with that of light mesons in 10--40% centrality Au+Au collisions. These results suggest that charm quarks have achieved local thermal equilibrium with the medium created in such collisions. Several theoretical models, with the temperature--dependent, dimensionless charm spatial diffusion coefficient ($2{pi}TD_s$) in the range of $sim$2--12, are able to simultaneously reproduce our $D^0$ $v_2$ result and our previously published results for the $D^0$ nuclear modification factor.
The azimuthal anisotropy coefficients v_2 and v_4 of pi^0 and eta mesons are measured in Au+Au collisions at sqrt(s_NN)=200 GeV, as a function of transverse momentum p_T (1-14 GeV/c) and centrality. The extracted v_2 coefficients are found to be consistent between the two meson species over the measured p_T range. The ratio of v_4/v_2^2 for pi^0 mesons is found to be independent of p_T for 1-9 GeV/c, implying a lack of sensitivity of the ratio to the change of underlying physics with p_T. Furthermore, the ratio of v_4/v_2^2 is systematically larger in central collisions, which may reflect the combined effects of fluctuations in the initial collision geometry and finite viscosity in the evolving medium.
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