M. Hauer

The PHOBOS Perspective on Discoveries at RHIC

This paper describes the conclusions that can be drawn from the data taken thus far with the PHOBOS detector at RHIC. In the most central Au+Au collisions at the highest beam energy, evidence is found for the formation of a very high energy density system whose description in terms of simple hadronic degrees of freedom is inappropriate. Furthermore, the constituents of this novel system are found to undergo a significant level of interaction. The properties of particle production at RHIC energies are shown to follow a number of simple scaling behaviors, some of which continue trends found at lower energies or in simpler systems. As a function of centrality, the total number of charged particles scales with the number of participating nucleons. When comparing Au+Au at different centralities, the dependence of the yield on the number of participants at higher p T (∼4 GeV/c) is very similar to that at low transverse momentum. The measured values of charged particle pseudorapidity density and elliptic flow were found to be independent of energy over a broad range of pseudorapidities when effectively viewed in the rest frame of one of the colliding nuclei, a property we describe as “extended longitudinal scaling”. Finally, the centrality and energy dependences of several observables were found to factorize to a surprising degree.

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.

Centrality dependence of charged hadron transverse momentum spectra in Au + Au collisions from s(NN)**(1/2) = 62.4-GeV to 200-GeV

We have measured transverse momentum distributions of charged hadrons produced in Au+Au collisions at sqrt(s_NN) = 62.4 GeV. The spectra are presented for transverse momenta 0.25<p_T<4.5 GeV/c, 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 the same collision energy as a function of collision centrality. For p_T>2 GeV/c, R_AA is found to be significantly larger than in Au+Au collisions at sqrt(s_NN) =130 and 200 GeV. In contrast, we find that the evolution of the invariant yields per participant pair from peripheral to central collisions is approximately energy independent over this range of collision energies. This observation challenges models of high p_T hadron suppression in terms of parton energy loss.

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 have measured transverse momentum distributions of charged hadrons produced in Au+Au collisions at sqrt(s_NN) = 62.4 GeV. The spectra are presented for transverse momenta 0.25<p_T<4.5 GeV/c, 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 the same collision energy as a function of collision centrality. For p_T>2 GeV/c, R_AA is found to be significantly larger than in Au+Au collisions at sqrt(s_NN) =130 and 200 GeV. In contrast, we find that the evolution of the invariant yields per participant pair from peripheral to central collisions is approximately energy independent over this range of collision energies. This observation challenges models of high p_T hadron suppression in terms of parton energy loss.

GeV

Two-particle correlations of identical charged pion pairs from Au+Au collisions at {radical}(s{sub NN})=62.4 and 200 GeV were measured by the PHOBOS experiment at BNL Relativistic Heavy Ion Collider (RHIC). Data for the 15% most central events were analyzed with Bertsch-Pratt and Yano-Koonin-Podgoretskii parametrizations using pairs with rapidities of 0.4<y{sub {pi}}{sub {pi}}<1.3 and transverse momenta 0.1<k{sub T}<1.4 GeV/c. The Bertsch-Pratt radii R{sub o} and R{sub l} decrease as a function of pair transverse momentum. R{sub o} and R{sub s} are independent of collision energy, while R{sub l} shows a slight increase. The source rapidity y{sub YKP} scales roughly with the pair rapidity y{sub {pi}}{sub {pi}}, indicating strong dynamical correlations.

Centrality Dependence of Charged Hadron Transverse Momentum Spectra inAu+AuCollisions fromsNN=62.4to 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.

Transverse momentum and rapidity dependence of Hanbury-Brown-Twiss correlations in Au+Au collisions at √sNN = 62.4 and 200 gev

We have performed the first measurement of elliptic flow (v2) fluctuations in nucleus-nucleus collisions. In this paper, we describe the analysis method we have developed for this measurement. In this method, v2 is determined event-by-event by a maximum likelihood fit. The non-statistical fluctuations are determined by unfolding the contribution of statistical fluctuations and detector effects using Monte Carlo simulations. Application of this method to measure dynamical fluctuations in events from a different Monte Carlo event generator is presented.We introduce an analysis method to measure elliptic flow (v_2) fluctuations using the PHOBOS detector for Au+Au collisions at sqrt(s) = 200 GeV. In this method, v_2 is determined event-by-event by a maximum likelihood fit. The non-statistical fluctuations are determined by unfolding the contribution of statistical fluctuations and detector effects using Monte Carlo simulations(MC). Application of this method to measure dynamical fluctuations embedded in special MC are presented. It is shown that the input fluctuations are reconstructed successfully for>= 0.03.

MULTIPLICITY FLUCTUATIONS IN Au+Au COLLISIONS AT RHIC

Collision centrality is a valuable parameter used in relativistic nuclear physics which relates to geometrical quantities such as the number of participating nucleons. PHOBOS utilizes a multiplicity measurement as a means to estimate fractional cross-section of a collision event-by-event. From this, the centrality of this collision can be deduced. The details of the centrality determination depend both on the collision system and collision energy. Presented here are the techniques developed over the course of the RHIC program that are used by PHOBOS to extract the centrality. Possible biases that have to be overcome before a final measurement can be interpreted are discussed.

A METHOD FOR MEASURING ELLIPTIC FLOW FLUCTUATIONS WITH THE PHOBOS DETECTOR

for the PHOBOS Collaboration: B.Alver4, B.B.Back1, M.D.Baker2, M.Ballintijn4, D.S.Barton2, R.R.Betts 6, A.A.Bickley7, R.Bindel 7, W.Busza 4, A.Carroll2, Z.Chai2, V.Chetluru6, M.P.Decowski 4, E.García6, T.Gburek3, N.George2, K.Gulbrandsen 4, C.Halliwell6, J.Hamblen8, I.Harnarine6, M.Hauer2, C.Henderson 4, D.J.Hofman6, R.S.Hollis6, R.Hołyński 3, B.Holzman2, A.Iordanova6, E.Johnson 8, J.L.Kane4, N.Khan8, P.Kulinich4, C.M.Kuo5, W.Li4, W.T.Lin5, C.Loizides4, S.Manly8, A.C.Mignerey7, R.Nouicer 2, A.Olszewski 3, R.Pak2, C.Reed4, E.Richardson 7, C.Roland4, G.Roland4, J.Sagerer 6, H.Seals2, I.Sedykh2, C.E.Smith6, M.A.Stankiewicz 2, P.Steinberg 2, G.S.F.Stephans 4, A.Sukhanov 2, A.Szostak 2, M.B.Tonjes7, A.Trzupek 3, C.Vale4, G.J.van Nieuwenhuizen 4, S.S.Vaurynovich 4, R.Verdier 4, G.I.Veres4, P.Walters8, E.Wenger 4, D.Willhelm7, F.L.H.Wolfs8, B.Wosiek 3, K.Woźniak3, S.Wyngaardt 2, B.Wysłouch 4