Microscopic description of anisotropic flow in relativistic heavy ion collisions

Anisotropic flow of hadrons is studied in heavy ion collisions at SPS and RHIC energies within the microscopic quark-gluon string model. The model was found to reproduce correctly many of the flow features, e.g., the wiggle structure of direct flow of nucleons at midrapidity, or centrality, rapidity, and transverse momentum dependences of elliptic flow. Further predictions are made. The differences in the development of the anisotropic flow components are linked to the freeze-out conditions, which are quite different for baryons and mesons.Comment: Proceedings of the Erice School on Nuclear Physics (Erice, Italy, September 16-24, 2003

Elliptic flow at RHIC: where and when does it formed?

Evolution of the elliptic flow of hadrons in heavy-ion collisions at RHIC energies is studied within the microscopic quark-gluon string model. The elliptic flow is shown to have a multi-component structure caused by (i) rescattering and (ii) absorption processes in spatially asymmetric medium. Together with different freeze-out dynamics of mesons and baryons, these processes lead to the following trend in the flow formation: the later the mesons are frozen, the weaker their elliptic flow, whereas baryon fraction develops stronger elliptic flow during the late stages of the fireball evolution. Comparison with the PHOBOS data demonstrates the model ability to reproduce the v2(eta) signal in different centrality bins.Comment: 11 pages incl. 5 figure

Violation of energy-per-hadron scaling in a resonance matter

Yields of hadrons, their average masses and energies per hadron at the stage of chemical freeze-out in (ultra)relativistic heavy-ion collisions are analyzed within the statistical model. The violation of the scaling / = 1 GeV observed in Au+Au collisions at $\sqrt{s}$ = 130 AGeV is linked to the formation of resonance-rich matter with a considerable fraction of baryons and antibaryons. The rise of the energy-per-hadron ratio in baryon-dominated matter is discussed. A violation of the scaling condition is predicted for a very central zone of heavy-ion collisions at energies around 40 AGeV.Comment: 5 pages incl. 3 figures and 2 tables, to be published in Phys. Rev.

Anisotropic flow of strange particles at RHIC

Space-time picture of the anisotropic flow evolution in Au+Au collisions at BNL RHIC is studied for strange hadrons within the microscopic quark-gluon string model. The directed flow of both mesons and hyperons demonstrates wiggle structure with the universal antiflow slope at |y| < 2 for minimum bias events. This effect increases as the reaction becomes more peripheral. The development of both components of the anisotropic flow is closely related to particle freeze-out. Hadrons are emitted continuously, and different hadronic species are decoupled from the system at different times. These hadrons contribute differently to the formation and evolution of the elliptic flow, which can be decomposed onto three components: (i) flow created by hadrons emitted from the surface at the onset of the collision; (ii) flow produced by jets; (iii) hydrodynamic flow. Due to these features, the general trend in elliptic flow formation is that the earlier mesons are frozen, the weaker their elliptic flow. In contrast, baryons frozen at the end of the system evolution have stronger v2.Comment: proceedings of the conference SQM2004 (September 2004, Cape Town, South Africa

Third flow component as QGP signal

A review of earlier fluid dynamical calculations with QGP show a softening of the directed flow while with hadronic matter this effect is absent. The effect shows up in the reaction plane as enhanced emission which is orthogonal to the directed flow. Thus, it is not shadowed by the deflected projectile and target. As both of these flow components are in the reaction plane these form an enhanced 'elliptic flow' pattern. Recent experimental data at 11 AGeV and above show the same softening, hinting at QGP formation.Comment: 12 pages, Latex, and 3 figures (.eps), 2 style files (.sty

Directed flow in heavy-ion collisions at NICA: what is interesting to measure?

We study the formation of the directed flow of hadrons in nuclear collisions at energies between AGS and SPS in Monte Carlo cascade model. The slope of the proton flow at midrapidity tends to zero (softening) with increasing impact parameter of the collision. For very peripheral topologies this slope becomes negative (antiflow). The effect is caused by rescattering of hadrons in remnants of the colliding nuclei. Since the softening of the proton flow can be misinterpreted as indication of the presence of quark-gluon plasma, we propose several measurements at NICA facility which can help one to distinguish between the cases with and without the plasma formation.Comment: 5 pages, 3 figures, Contribution to the NICA White Paper (EPJA, topical issue

Transition to meson-dominated matter at RHIC. Consequences for kaon flow

Anisotropic flow of kaons and antikaons is studied in heavy-ion collisions at CERN SPS and BNL RHIC energies within the microscopic quark-gluon string model. In the midrapidity range the directed flow of kaons v_1 differs considerably from that of antikaons at SPS energy (E_{lab} = 160 AGeV), while at RHIC energy (\sqrt{s} = 130 AGeV) the excitation functions of both, kaon and antikaon, flows coincide within the statistical error bars. The change is attributed to formation of dense meson-dominated matter at RHIC, where the differences in interaction cross-sections of kaons and antikaons become unimportant. The time evolution of the kaon anisotropic flow is also investigated. The elliptic flow of these hadrons is found to develop at midrapidity at times 3 < t < 10 fm/c, which is much larger than the nuclear passing time t^{pass} = 0.12 fm/c. As a function of transverse momentum the elliptic flow increases almost linearly with rising p_t. It stops to rise at p_t > 1.5 GeV/c reaching the saturation value $v_2^K (p_t) \approx 10$.Comment: REVTEX, 14 pages, 4 figure

Equilibration within a semiclassical off-shell transport approach

Equilibration times for nuclear matter configurations -- modelling intermediate and high energy nucleus-nucleus collisions -- are evaluated within the semiclassical off-shell transport approach developed recently. The transport equations are solved for a finite box in coordinate space employing periodic boundary conditions. The off-shell transport model is shown to give proper off-shell equilibrium distributions in the limit $t \to \infty$ for the nucleon and $\Delta$-resonance spectral functions. We find that equilibration times within the off-shell approach are only slightly enhanced as compared to the on-shell limit for the momentum configurations considered.Comment: 19 pages, LaTeX, including 4 postscript figures, submitted to Nucl. Phys.