star focus: System-size independence of directed flow at RHIC
Highlights from the STAR paper System-size independence of directed flow at the Relativistic Heavy-Ion Collider submitted recently to Physical Review Letters.
Posted: Jul 14, 2008

Directed flow refers to collective sidewards deflection of particles and is characterized by a first-order harmonic (v1) of the Fourier expansion of particle's azimuthal distribution w.r.t. the reaction plane in heavy-ion collisions. STAR has recently submitted to Physical Review Letters multiple differential measurements of v1 for Au+Au and Cu+Cu collisions at center of mass energies of 200 and 62.4 GeV as a function of pseudorapidity (eta), transverse momentum, and collision centrality. We find that directed flow violates the "entropy-driven" multiplicity scaling which dominates all other soft observables. STAR has reported an intriguing new universal scaling of the phenomenon with collision centrality. Neither Boltzmann/cascade nor hydrodynamic models are able to explain the measured trends.

The figure shows the system size dependence of v1, Cu+Cu data compared to Au+Au in the centrality range 30 - 60% for both 200 and 62.4 GeV. There is a clear trend for v1(eta) to decrease with increasing beam energy for both Au+Au and Cu+Cu. In the studied pseudorapidity and centrality range, v1(eta) is, within errors, independent of the system size at each beam energy, despite the three-to-one mass ratio between gold and copper. This remarkable feature holds for almost all centrality bins studied, as shown in other figures of the paper, and persists even near mid-pseudorapidity (|eta|<1), where elliptic flow (v2) of charged particles in Cu+Cu is considerably lower than in Au+Au.

Unlike the ratio of the elliptic flow parameter v2 to the system initial eccentricity, which scales with the particle density in the transverse plane, v1(eta) at a given centrality is found to be independent of the system size, and varies only with the incident energy. The different scalings for v2/eccentricity and v1 might arise from the way in which they are developed: to produce v2, many momentum exchanges among particles must occur, while to produce v1, an important feature of the collision process is that different rapidity losses need to occur for particles at different distances from the center of the participant zone. This later quantity is related to incident energy. Results from A Multi Phase Transport model lies consistently below the measured data, as evident from the figure. It is noteworthy that AMPT does not exhibit the observed pattern of system-size independence. UrQMD (not shown here) is similar to AMPT in exhibiting a significant change in v1 between Au+Au and Cu+Cu.

Further details can be found in the following STAR paper -
System-size independence of directed flow at the Relativistic Heavy-Ion Collider - arXiv:0807.1518v2.

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