System size, energy and pseudorapidity dependence of directed and elliptic flow at RHIC

The PHOBOS collaboration has measured transverse momentum distributions of charged hadrons produced in Cu+Cu collisions at sqrt(s_(NN)) = 200 and 62.4 GeV. The nuclear modification factor R_(AA)^(Npart) is calculated relative to p+p data at both collision energies as a function of collision centrality. For the same number of participating nucleons, R_(AA)^(Npart) is essentially the same in both systems over the full range of p_(T) that is measured. In addition, we observe tha...

Charged particle pseudorapidity distributions are presented from the PHOBOS experiment at RHIC, measured in Au+Au and Cu+Cu collisions at sqrt{s_NN}=19.6, 22.4, 62.4, 130 and 200 GeV, as a function of collision centrality. The presentation includes the recently analyzed Cu+Cu data at 22.4 GeV. The measurements were made by the same detector setup over a broad range in pseudorapidity, |eta|<5.4, allowing for a reliable systematic study of particle production as a function of e...

The measurement of elliptic flow for charged particles in Au-Au collisions at $\sqrt{s_{_{NN}}} = 130$ GeV using the PHOBOS detector at the Relativistic Heavy Ion Collider (RHIC) is presented. Charge particle distributions over a wide pseudo-rapidity region ($|\eta|<5.3$) are measured by the PHOBOS silicon multiplicity detectors. These distributions are used to extract the final-state azimuthal anisotropy. The centrality and pseudo-rapidity dependences of elliptic flow, avera...

Collective flow reflects dynamical evolution in high-energy heavy ion collisions. In particular, the strange hadron elliptic flow reflects early collision dynamics. We present results from a systematic analysis of the centrality dependence of strange hadron elliptic flow ($v_2$) measurement of $K_{S}^{0}$, $\Lambda$ and $\Xi$ for Cu + Cu collisions at 200 GeV. Results for Cu + Cu collisions are compared with results previously reported for Au + Au collisions. We will also com...

Recent differential measurements of elliptic flow are used to probe several hydrodynamic scaling predictions. Eccentricity scaling is observed for Cu+Cu and Au+Au collisions at $\sqrt{s_{NN}}=~200$ GeV, suggesting essentially complete thermalization of the high energy density matter produced in these collisions. An estimate of the speed of sound is also obtained from the eccentricity scaled elliptic flow. The predicted mass scaling is observed for particles with transverse ki...

A consistent picture of the Au+Au and D+Au, s^1/2 = 200 A GeV measurements at RHIC obtained with the PHENIX, STAR, PHOBOS and BRAHMS detectors including both the rapidity and transverse momentum spectra was previously developed with the simulation LUCIFER. The approach was modeled on the early production of a fluid of pre-hadrons after the completion of an initial, phase of high energy interactions. The formation of pre-hadrons is discussed here, in a perturbative QCD approac...

Two particle azimuthal correlation functions are presented for charged hadrons produced in Au + Au collisions at RHIC sqrt(s_NN) = 130 GeV. The measurements permit determination of elliptic flow without event-by-event estimation of the reaction plane. The extracted elliptic flow values v_2 show significant sensitivity to both the collision centrality and the transverse momenta of emitted hadrons, suggesting rapid thermalization and relatively strong velocity fields. When scal...

We present the directed flow measurement ($v_1$) from Au+Au collisions at $\sqrtsNN = 62$ GeV. Over the pseudorapidity range we have studied, which covers $\eta$ from -1.2 to 1.2 and $2.4 < |\eta| < 4$, the magnitude of $v_1$ for charged particles is found to increase monotonously with pseudorapidity for all centralities. No ``$v_1$ wiggle'', as predicted by various theoretical models, is observed at midrapidity. Elliptic flow ($v_2$) from moderate high $p_t$ particles ($3-6 ...

Two particle azimuthal correlation functions are presented for charged hadrons produced in Au+Au collisions at RHIC ($\sqrt{s_{_{NN}}}=130$ GeV). The measurements allow for the determination of elliptic flow without event-by-event estimation of the reaction plane. The measured correlation functions indicate elliptic flow values ($v_2$) which show significant sensitivity to both the collision centrality and the transverse momenta of emitted hadrons.

The common interpretation of $v_2$ in heavy ion collisions is that it is produced by hydrodynamic flow at low transverse momentum and by parton energy loss at high transverse momentum. In this talk we discuss this interpretation in view of the dependence of $v_2$ with energy, rapidity and system size, and show that it might not be trivial to reconcile these models with the relatively simple scaling found in experiment