C.E. Smith

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.

Phobos results on charged particle multiplicity and pseudorapidity distributions in Au+Au, Cu+Cu, d+Au, and p+p collisions at ultra-relativistic energies

Pseudorapidity distributions of charged particles emitted in Au+Au, Cu+Cu, d+Au, and p+p collisions over a wide energy range have been measured using the PHOBOS detector at the BNL Relativistic Heavy-Ion Collider (RHIC). The centrality dependence of both the charged particle distributions and the multiplicity at midrapidity were measured. Pseudorapidity distributions of charged particles emitted with |{eta}|<5.4, which account for between 95% and 99% of the total charged-particle emission associated with collision participants, are presented for different collision centralities. Both the midrapidity density dN{sub ch}/d{eta} and the total charged-particle multiplicity N{sub ch} are found to factorize into a product of independent functions of collision energy, {radical}(s{sub N{sub N}}), and centrality given in terms of the number of nucleons participating in the collision, N{sub part}. The total charged particle multiplicity, observed in these experiments and those at lower energies, assumes a linear dependence of (lns{sub N{sub N}}){sup 2} over the full range of collision energy of {radical}(s{sub N{sub N}})=2.7-200 GeV.

Centrality dependence of charged-hadron transverse-momentum spectra in d+Au collisions at sqrt[s(NN)]=200 GeV.

We have measured transverse momentum distributions of charged hadrons produced in d+Au collisions at sqrt[s(NN)]=200 GeV. The spectra were obtained for transverse momenta 0.25<p(T)<6.0 GeV/c, in a pseudorapidity range of 0.2<eta<1.4 in the deuteron direction. The evolution of the spectra with collision centrality is presented in comparison to p+pmacr; collisions at the same collision energy. With increasing centrality, the yield at high transverse momenta increases more rapidly than the overall particle density, leading to a strong modification of the spectral shape. This change in spectral shape is qualitatively different from observations in Au+Au collisions at the same energy. The results provide important information for discriminating between different models for the suppression of high-p(T) hadrons observed in Au+Au collisions.

Pseudorapidity distribution of charged particles in d+Au collisions at sqrt[sNN]=200 GeV.

The measured pseudorapidity distribution of primary charged particles in minimum-bias d+Au collisions at sqrt[s(NN)]=200 GeV is presented for the first time. This distribution falls off less rapidly in the gold direction as compared to the deuteron direction. The average value of the charged particle pseudorapidity density at midrapidity is |eta|< or =0.6)=9.4+/-0.7(syst) and the integrated primary charged particle multiplicity in the measured region is 82+/-6(syst). Estimates of the total charged particle production, based on extrapolations outside the measured pseudorapidity region, are also presented. The pseudorapidity distribution, normalized to the number of participants in d+Au collisions, is compared to those of Au+Au and p+(-)p systems at the same energy. The d+Au distribution is also compared to the predictions of the parton saturation model, as well as microscopic models.

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

Scaling of charged particle production in d + Au collisions at s(NN)**(1/2) = 200-GeV

The measured pseudorapidity distributions of primary charged particles over a wide pseudorapidity range of |{eta}|{ N{sub ch}{sup pp}, and the energy dependence of the density of charged particles produced in the fragmentation region exhibits extended longitudinal scaling.

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 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.