B. Alver

System Size, Energy, Pseudorapidity, and Centrality Dependence of Elliptic Flow

This Letter presents measurements of the elliptic flow of charged particles as a function of pseudorapidity and centrality from Cu-Cu collisions at 62.4 and 200 GeV using the PHOBOS detector at the Relativistic Heavy Ion Collider. The elliptic flow in Cu-Cu collisions is found to be significant even for the most central events. For comparison with the Au-Au results, it is found that the detailed way in which the collision geometry (eccentricity) is estimated is of critical importance when scaling out system-size effects. A new form of eccentricity, called the participant eccentricity, is introduced which yields a scaled elliptic flow in the Cu-Cu system that has the same relative magnitude and qualitative features as that in the Au-Au system.

Triangular flow in hydrodynamics and transport theory

In ultrarelativistic heavy-ion collisions, the Fourier decomposition of the relative azimuthal angle, {Delta}{phi}, distribution of particle pairs yields a large cos(3{Delta}{phi}) component, extending to large rapidity separations {Delta}{eta}>1. This component captures a significant portion of the ridge and shoulder structures in the {Delta}{phi} distribution, which have been observed after contributions from elliptic flow are subtracted. An average finite triangularity owing to event-by-event fluctuations in the initial matter distribution, followed by collective flow, naturally produces a cos(3{Delta}{phi}) correlation. Using ideal and viscous hydrodynamics and transport theory, we study the physics of triangular (v{sub 3}) flow in comparison to elliptic (v{sub 2}), quadrangular (v{sub 4}), and pentagonal (v{sub 5}) flow. We make quantitative predictions for v{sub 3} at RHIC and LHC as a function of centrality and transverse momentum. Our results for the centrality dependence of v{sub 3} show a quantitative agreement with data extracted from previous correlation measurements by the STAR collaboration. This study supports previous results on the importance of triangular flow in the understanding of ridge and shoulder structures. Triangular flow is found to be a sensitive probe of initial geometry fluctuations and viscosity.

System size dependence of cluster properties from two-particle angular correlations in Cu+Cu and Au+Au collisions at sq root(s{sub NN})=200 GeV

B.Alver, B.B.Back, M.D.Baker, M.Ballintijn, D.S.Barton, R.R.Betts, R.Bindel, W.Busza, Z.Chai, V.Chetluru, E.Garćıa, T.Gburek, K.Gulbrandsen, J.Hamblen, I.Harnarine, C.Henderson, D.J.Hofman, R.S.Hollis, R.Ho lyński, B.Holzman, A.Iordanova, J.L.Kane, P.Kulinich, C.M.Kuo, W.Li, W.T.Lin, C.Loizides, S.Manly, A.C.Mignerey, R.Nouicer, A.Olszewski, R.Pak, C.Reed, E.Richardson, C.Roland, G.Roland, J.Sagerer, I.Sedykh, C.E.Smith, M.A.Stankiewicz, P.Steinberg, G.S.F.Stephans, A.Sukhanov, A.Szostak, M.B.Tonjes, A.Trzupek, G.J.van Nieuwenhuizen, S.S.Vaurynovich, R.Verdier, G.I.Veres, P.Walters, E.Wenger, D.Willhelm, F.L.H.Wolfs, B.Wosiek, K.Woźniak, S.Wyngaardt, B.Wys louch

Event-by-Event Fluctuations of Azimuthal Particle Anisotropy in Au + Au Collisions at

This paper presents the first measurement of event-by-event fluctuations of the elliptic flow parameter v_2 in Au+Au collisions at sqrt(s_NN) = 200GeV as a function of collision centrality. The relative non-statistical fluctuations of the v_2 parameter are found to be approximately 40%. The results, including contributions from event-by-event elliptic flow fluctuations and from azimuthal correlations that are unrelated to the reaction plane (non-flow correlations), establish an upper limit on the magnitude of underlying elliptic flow fluctuations. This limit is consistent with predictions based on spatial fluctuations of the participating nucleons in the initial nuclear overlap region. These results provide important constraints on models of the initial state and hydrodynamic evolution of relativistic heavy ion collisions.

\sqrt{s_{NN}}= 200

We present transverse momentum distributions of charged hadrons produced in Cu+Cu collisions at sqrt(s) = 62.4 and 200 GeV. The spectra are measured for transverse momenta of 0.25<p_T<5.0 GeV/c at sqrt(s) = 62.4 GeV and 0.25<p_T<7.0 GeV/c at sqrt(s) = 200 GeV, in a pseudo-rapidity range of 0.2<eta<1.4. The nuclear modification factor R_AA is calculated relative to p+p data at both collision energies as a function of collision centrality. At a given collision energy and fractional cross-section, R_AA is observed to be systematically larger in Cu+Cu collisions compared to Au+Au. However, for the same number of participating nucleons, R_AA is essentially the same in both systems over the measured range of p_T, in spite of the significantly different geometries of the Cu+Cu and Au+Au systems.

GeV

B.Alver, B.B.Back, M.D.Baker, M.Ballintijn, D.S.Barton, R.R.Betts, R.Bindel, W.Busza, Z.Chai, V.Chetluru, E.Garćıa, T.Gburek, K.Gulbrandsen, J.Hamblen, I.Harnarine, C.Henderson, D.J.Hofman, R.S.Hollis, R.Ho lyński, B.Holzman, A.Iordanova, J.L.Kane, P.Kulinich, C.M.Kuo, W.Li, W.T.Lin, C.Loizides, S.Manly, A.C.Mignerey, R.Nouicer, A.Olszewski, R.Pak, C.Reed, E.Richardson, C.Roland, G.Roland, J.Sagerer, I.Sedykh, C.E.Smith, M.A.Stankiewicz, P.Steinberg, G.S.F.Stephans, A.Sukhanov, A.Szostak, M.B.Tonjes, A.Trzupek, G.J.van Nieuwenhuizen, S.S.Vaurynovich, R.Verdier, G.I.Veres, P.Walters, E.Wenger, D.Willhelm, F.L.H.Wolfs, B.Wosiek, K.Woźniak, S.Wyngaardt, B.Wys louch

System size and centrality dependence of charged hadron transverse momentum spectra in Au + Au and Cu + Cu collisions at s(NN)**(1/2) = 62.4-GeV and 200-GeV

This article presents results on event-by-event elliptic flow fluctuations in Au+Au collisions at sq root(s{sub NN})= 200 GeV, where the contribution from non-flow correlations has been subtracted. An analysis method is introduced to measure non-flow correlations, relying on the assumption that non-flow correlations are most prominent at short ranges (|{Delta}{eta}| 2), relative elliptic flow fluctuations of approximately 30-40% are observed. These results are consistent with predictions based on spatial fluctuations of the participating nucleons in the initial nuclear overlap region. It is found that the long-range non-flow correlations in Au+Au collisions would have to be more than an order of magnitude strongercompared to the p+p data to lead to the observed azimuthal anisotropy fluctuations with no intrinsic elliptic flow fluctuations.

Cluster properties from two-particle angular correlations in p + p collisions at s**(1/2) = 200-GeV and 410-GeV

We present transverse momentum distributions of charged hadrons produced in Cu+Cu collisions at sqrt(s) = 62.4 and 200 GeV. The spectra are measured for transverse momenta of 0.25<p_T<5.0 GeV/c at sqrt(s) = 62.4 GeV and 0.25<p_T<7.0 GeV/c at sqrt(s) = 200 GeV, in a pseudo-rapidity range of 0.2<eta<1.4. The nuclear modification factor R_AA is calculated relative to p+p data at both collision energies as a function of collision centrality. At a given collision energy and fractional cross-section, R_AA is observed to be systematically larger in Cu+Cu collisions compared to Au+Au. However, for the same number of participating nucleons, R_AA is essentially the same in both systems over the measured range of p_T, in spite of the significantly different geometries of the Cu+Cu and Au+Au systems.

Non-flow correlations and elliptic flow fluctuations in Au+Au collisions at sq root(s{sub NN})=200 GeV

In the PHOBOS experiment, charged particles are measured in almost the full solid angle. This enables the study of fluctuations and correlations in the particle production over a very wide kinematic range. In this paper, we show results of a direct search for fluctuations identified by an unusual shape of the pseudorapidity distribution. In addition, we use analysis of correlations of the multiplicity in similar pseudorapidity bins, placed symmetrically in the forward and backward hemispheres, to test the hypothesis of production of particles in clusters.

System Size and Centrality Dependence of Charged Hadron Transverse Momentum Spectra inAu+AuandCu+CuCollisions atsNN=62.4and 200 GeV

Antiparticle to particle ratios for identified protons, kaons, and pions at {radical}(s{sub NN})=62.4 and 200 GeV in Cu+Cu collisions are presented as a function of centrality for the midrapidity region of 0.2 / ratio at {approx_equal}0.51 GeV/c, we observe an average value of 0.50{+-}0.003{sub (stat)}{+-}0.04{sub (syst)} and 0.77{+-}0.008{sub (stat)}{+-}0.05{sub (syst)} for the 10% most central collisions of 62.4 and 200 GeV Cu+Cu, respectively. The values for all three particle species measured at {radical}(s{sub NN})=200 GeV are in agreement within systematic uncertainties with that seen in both heavier and lighter systems measured at the same RHIC energy. This indicates that system size does not appear to play a strong role in determining the midrapidity chemical freeze-out properties affecting the antiparticle to particle ratios of the three most abundant particle species produced in these collisions.