| Azimuthal anisotropy, or collective flow, has been studied in heavy-ion collisions for two decades. It is one of the most important signals that gives insight into the early stages of the evolution of the matter created in such collisions. The E895 experiment at the Alternating Gradient Synchrotron (AGS) and PHENIX experiment at Relativistic Heavy Ion Collider (RHIC) explore different regions of the phase diagram of nuclear matter. Collective flow measurements from these two experiments are important in understanding the dynamics of the matter produced and constraining its equation of state.; Directed and elliptic flow of charged hadrons at beam energies of 2, 4, 6 and 8 GeV/nucleon have been measured using the cumulant method of flow analysis. The directed flow of pions is observed to change from positive to negative as a function of centrality. At RHIC, there is much evidence that a deconfined system of quarks and gluons, called the Quark-Gluon Plasma (QGP), has been produced. The PHENIX Collaboration has collected data from Au+AU collisions at center-of-mass energies of 62.4, 130 and 200 GeV and Cu+Cu collisions at center-of-mass energies of 62.4 and 200 GeV. Elliptic flow has been studied in these systems as a function of transverse momentum, centrality, rapidity, beam energy and particle type. These measurements show that the magnitude of the elliptic flow is strikingly similar in the energy range 62.4--200 GeV, hinting at a softening of the equation of state of the matter at RHIC. An estimation of the speed of sound in the medium in the medium suggests a soft equation of state. The properties of the matter have been probed through the scaling characteristics of elliptic flow. Eccentricity scaling shows that the produced matter is highly thermalized. The elliptic flow of identified particles is found to scale with transverse kinetic energy up to ≈1 GeV, revealing the hydrodynamic nature of the expanding fluid. Constituent quark number scaling, predicted by recombination models, is observed to hold over the whole transverse kinetic energy range of the data, hence revealing the pertinence of partonic degrees of freedom in the development of flow. |
