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Strange Brew: Probing the Flow of (Strange) Quarks With Identified Particles

by Jamie Dunlop

At the Relativistic Heavy Ion Collider (RHIC), gold nuclei are collided at high energy to produce a state of hot, dense matter as it existed in the first few microseconds of our universe. This matter appears to flow strongly, behaving almost as a perfect fluid.

This unexpected property of the matter created at RHIC has led to great excitement both inside our community and in the greater scientific community.

There are, however, a number of outstanding questions about this surprising behavior. We believe that the fluid is rather perfect, but need to quantify just how perfect it is, and measure its properties in detail. In order to do this, we need to analyze the flow patterns in greater detail, turning as many experimental knobs as we can to see how the flow pattern varies. From accurate measurements of the flow pattern and its dependence on the full range of accessible experimental variables we can move towards this goal.

The STAR experiment has excellent capability to identify exactly which particles are emerging from the final state of the interaction zone, and to analyze the flow patterns particle species by particle species. We recently submitted a highly detailed paper to Physical Review C doing just this \cite{B. Abelev et al, nucl-ex/0701010}.

The dominant flow phenomenon is the second Fourier harmonic of particle production relative to the reaction plane, the "elliptic flow", shown schematically in figure 1. In this paper, elliptic flow was measured for a bevy of particles (pions, kaons, protons, Lambdas, Cascades, and Omegas) for two different energies of the incoming gold nuclei and as a function of the particles' transverse momentum. For each incident energy, this large number of measurements can be collapsed into one simple curve when scaled by the number of constituent quarks, shown in figure 2, which leads intuitively to the conclusion that the flow of the different species of hadrons reflects the flow of their constituent quarks at an earlier stage of the collision. In this paper, higher harmonics were also measured for these particles, and precise comparisons made between these measurements and similar measurements from collisions at lower incident energy.

One particle in particular, the phi meson, probes this partonic collectivity incisively. Once formed, the phi meson is expected to interact weakly with surrounding hadronic matter, so the flow of the phi meson is expected to reflect quite cleanly the flow of the strange quarks from which it is formed. STAR has recently submitted a paper to Physical Review Letters \cite{B. Abelev et al, nucl-ex/0703033} in which the elliptic flow of the phi meson is measured with unprecedented accuracy. As shown in figure 3, the flow of the phi follows the scaling pattern expected for particles with the same number of quarks, which implies rather strongly that hot and dense matter with collectivity at the level of constituent partons has been formed at RHIC. This observation, besides being compelling in its own right, greatly simplifies the extraction of the properties of the fluid produced at RHIC from the flow of the hadrons observed in the final state.

Figure 1: Left: Schematic of the collision zone between two incoming nuclei. Right: Initial-state anisotropy in the collision zone converting into final-state elliptic flow, measured as anisotropy in particle momentum.

Figure 2: Elliptic flow parameter v2 as a function of transverse mass, scaled by the number of constituent quarks (2 for mesons, 3 for baryons) from Au+Au collisions at sqrt(s<sub>NN</sub>)=62.4 GeV. The combined data in the top panel have been fit to a polynomial; the bottom panel shows the data divided by the resulting fit. From B. Abelev et al, nucl-ex/0701010.

Figure 3: Elliptic flow parameter v2 as a function of transverse momentum from Au+Au collisions at sqrt(s<sub>NN</sub>) = 200 GeV. Figure from B. Abelev et al, nucl-ex/0703033.