Azimuthal anisotropy in Au+Au collisions at sqrtsNN = 200 GeV

TL;DR

The paper presents comprehensive measurements of azimuthal anisotropy in Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV, focusing on directed, elliptic, and higher harmonic flows. Using multiple analysis methods (event-plane, cumulants, and mixed-harmonics), it addresses nonflow and fluctuations while comparing to hydrodynamic, transport, and Blast Wave models. Key findings include in-plane $v_2$ with mass ordering at low $p_T$, constituent-quark-number scaling for identified hadrons at intermediate $p_T$, and $v_4$ that scales with $v_2^2$ but exceeds simple coalescence expectations, challenging some model assumptions. Blast Wave fits provide a coherent framework to systematize the data across centralities, suggesting significant but nuanced collective behavior and partial early thermalization in the produced medium, with ongoing tension between hydrodynamic descriptions and high-$p_T$ phenomena.

Abstract

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.

Figures (39)

• Figure 1: (color online). The event plane resolutions as a function of centrality for $v_{k2}\{\mathrm{EP}_2\}$.
• Figure 2: (color online). Directed flow of charged hadrons as a function of pseudorapidity. The measurements of $v_1\!\left\{\mathrm{EP}_1,\mathrm{EP}_2\right\}$ (circles; centrality 20--60%) agree with the published results of $v_1\{3\}$ (stars; centrality 10--70%).
• Figure 3: (color online). The product of $v_1^2$ and $v_2$. The shaded band is the mean value of this quantity with its error, averaged over centralities 20--60%. Since this quantity is positive, elliptic flow is measured to be in-plane.
• Figure 4: (color online). Charged hadron $v_2$ vs. $p_t$ for the centrality bins (bottom to top) 5 to 10% and in steps of 10% starting at 10, 20, 30, 40, 50, 60, and 70 up to 80%. The solid lines are Blast Wave fits.
• Figure 5: (color online). $v_2$ vs. $p_t$ for charged hadrons from 0--50% centrality collisions in comparison to data from PHOBOS PHOBOSQM04. The line is a polynomial fit to the STAR data. The gray error boxes represent the PHOBOS systematic errors. The bottom panel shows the ratio of the PHOBOS data to the polynomial fit.