Experimental Study of Collective Flow Phenomena in High-Energy nucleus-Nucleus Collisions

摘要

The results of the experimental study of collective flow phenomena, such as the sideward and elliptic flow of nuclear matter, discovered during the last 10-15 years in high-energy nucleus-nucleus collisions are presented in this review. Sideward (often termed directed) and elliptic flows have been observed for protons, antiprotons, light nuclei, pions, kaons, and lambdas emitted in nucleus-nucleus collisions at 0.1-1.8 GeVucleon of LBL Bevalac and GSI/SIS by Plasti-Ball, Streamer Chamber, EOS-NPC, FOPI, LAND, TAPS, and KAOS collaborations; at 2-4 GeVucleon of Dubna JINR by SKM-200-GIBS, Propane Buble Chamber, and Emulsion Chamber collaborations; at 2-14 GeVucleon of BNL AGS, by the E877, E895, and E917 collaborations; and at 60 and 200 GeVucleon of CERN SPS, by the WA98 and NA49 collaborations and more recently by the STAR at RHIC BNL. In the review, the results of the SKM-200-GIBSS collaboration of JINR are presented and compared with the results of different experiments by Bevalac, GSI/SIS, BNL, and CERN SPS. Different methods proposed for the study of the flow effects in relativistic nuclear collisions, such as the directed transverse momentum analysis technique developed by Danielewicz and Odyniec and the method of the Fourier expansion of azimuthal distributions, are described. The obtained signatures show the persistence of collective flow phenomena from the Bevalac and GSI/SIS up to Dubna, BNL, RHIC, and CERN SPS energies. The F flow (measure of the amount of collective transverse momentum transfer in the reaction plane) and the a_2 = 2v_2 parameter (measure of the anisotropic emission strength) have been extracted in experiments from the transverse momentum and azimuthal distributions of particles with respect to the reaction plane defined by the participant protons. The scaled flow F_s = F/(A_P~(1/3) + A_T~(1/3)) has been used for comparison of the flow results for various energies and projectile-targed configurations. The F_s demonstrates a common scaling behavior of proton flow values for different energies (Bevalac, GSI/SIS, Dubna, AGS, CERN SPS) and systems. The Quark-Gluon String Model (OGSM) and the relativistic transport models [the Boltzmann-Uehling-Uhlenbeck equation transport model (BEM) ART] yield a signature of directed and elliptic flow effects in nucleus-nucleus collisions and have been used for comparison with the experimental results of Bevalac, GSI/SIS, Dubna, GS, RHIC, and CERN SPS. Data on the proton elliptic flow (EOS, E895, M-200-GIBS, E877) show a transition from a negative (out-of-plane) to a positive (in-plane) elliptic flow at the beam energy E_(tr) ≈ 4 GeVucleon. Studies based on transport models have indicated that the value for the transition energy E_(tr) depends on the nuclear equation of state (EOS) at high densities.