% ****** Start of file template.aps ****** % This file is part of the APS files in the REVTeX 3.0 distribution. % Version 3.0 of REVTeX,! November 10, 1992. % % Copyright (c) 1992 The American Physical Society. % % See the REVTeX 3.0 README file for restrictions and more information. % % % This is a template for producing files for use with REVTEX 3.0. % Copy this file to another name and then work on that file. % That way, you always have this original template file to use. % % ********************************************************************************** % THIS FILE: dndeta200-draft26.tex % % October 13-2001. 23.00 GMT+1 % Based on first text by SS and JJG % REVISED: Oct 14-2001 JJG. Update text % Oct 16-2001 JJG. Update text % Oct 17-2001 JJG. Include hard/soft fit results + update text % Oct 17-2001 JJG. Fill out Table + update text, author list, figure 4. % Oct 18-2001 JJG. update references and include eta = 1.5 for fits. % Oct 19-2001 JJG. smaller updates and corrections after input from CEJ % and TST % ********************************************************************************** \documentstyle[prl,aps,epsfig,preprint]{revtex} %\documentstyle[prl,aps,epsfig,multicol]{revtex} \newcommand{\Version} {Version: dndeta200-draft26.tex, Oct. 19, 2001} \newcommand{\bnl} {$\rm^{1}$} \newcommand{\ires} {$\rm^{2}$} \newcommand{\kraknuc} {$\rm^{3}$} \newcommand{\krakow} {$\rm^{4}$} \newcommand{\baltimore} {$\rm^{5}$} \newcommand{\newyork} {$\rm^{6}$} \newcommand{\nbi} {$\rm^{7}$} \newcommand{\texas} {$\rm^{8}$} \newcommand{\bergen} {$\rm^{9}$} \newcommand{\bucharest} {$\rm^{10}$} \newcommand{\kansas} {$\rm^{11}$} \newcommand{\oslo} {$\rm^{12}$} \begin{document} % \draft command makes pacs numbers print \draft % repeat the \author\address pair as needed \title{Charged particle densities from Au+Au collisions at $\sqrt{s_{\small{NN}}}$=200 GeV at RHIC} \author{ I.~G.~Bearden\nbi, % \and D.~Beavis\bnl, % \and C.~Besliu\bucharest, % \and Y.~Blyakhman\newyork, % \and J.~Brzychczyk\krakow, % \and B.~Budick\newyork, % \and H.~B{\o}ggild\nbi, % \and C.~Chasman\bnl, % \and C.~H.~Christensen\nbi, % \and P.~Christiansen\nbi, % \and J.~Cibor\kraknuc, % \and R.~Debbe\bnl, % \and E. Enger\oslo, %\and J.~J.~Gaardh{\o}je\nbi, % \and K.~Grotowski\krakow, % \and K.~Hagel\texas, % \and O.~Hansen\nbi, % \and A.~Holm\nbi, % \and A.~K.~Holme\oslo, % \and H.~Ito\kansas, % \and E.~Jakobsen\nbi, % \and A.~Jipa\bucharest, % \and J.~I.~J{\o}rdre\bergen, % \and F.~Jundt\ires, % \and C.~E.~J{\o}rgensen\nbi, % \and T.~Keutgen\texas, % \and E.~J.~Kim\baltimore, % \and T.~Kozik\krakow, % \and T.~M.~Larsen\oslo, % \and J.~H.~Lee\bnl, % \and Y.~K.~Lee\baltimore, % \and G.~L{\o}vh{\o}iden\oslo, % \and Z.~Majka\krakow, % \and A.~Makeev\texas, % \and B.~McBreen\bnl, % \and M.~Mikelsen\oslo % \and M.~Murray\texas, % \and J.~Natowitz\texas, % \and B.~S.~Nielsen\nbi, % \and K.~Olchanski\bnl, % \and J.~Olness\bnl, % \and D.~Ouerdane\nbi, % \and R.~P\l aneta\krakow, % \and F.~Rami\ires, % \and D.~R{\"o}hrich\bergen, % \and B.~H.~Samset\oslo, % \and D.~Sandberg\nbi, % \and S.~J.~Sanders\kansas, % \and R.~A.~Sheetz\bnl, % \and Z.~Sosin\krakow, % \and P.~Staszel\nbi, % \and T.~F.~Thorsteinsen\bergen$^+$,% \and T.~S.~Tveter\oslo, % \and F.~Videb{\ae}k\bnl, % \and R.~Wada\texas, % \and A.~Wieloch\krakow, and I.~S.~Zgura\bucharest\\% \and (BRAHMS Collaboration )\\[1ex] \bnl~Brookhaven National Laboratory, Upton,New York 11973, \ires~Institut de Recherches Subatomiques and Universit{\'e} Louis Pasteur, Strasbourg, France, \kraknuc~Institute of Nuclear Physics, Krakow, Poland, \krakow~Jagiellonian University, Krakow, Poland, \baltimore~Johns Hopkins University, Baltimore, Maryland 21218, \newyork~New York University, New York, New York 10003, \nbi~Niels Bohr Institute, University of Copenhagen, Denmark, \texas~Texas A$\&$M University, College Station,Texas 77843, \bergen~University of Bergen, Department of Physics, Bergen,Norway, \bucharest~University of Bucharest,Romania, \kansas~University of Kansas, Lawrence, Kansas 66049, \oslo~University of Oslo, Department of Physics, Oslo,Norway, $^+ Deceased$} \date{\Version} \maketitle \begin{abstract} % insert abstract here { We present charged particle densities as a function of pseudorapidity and collision centrality for the $^{197}$Au+$^{197}$Au reaction at $\sqrt{s_{NN}}$=200 GeV, the top energy at RHIC. The charged particle multiplicity for the 5\% most central events is found to be 630 $\pm$30, i.e a 14\% increase relative to $\sqrt{s_{NN}}$=130 collisions. The total multiplicity of charged particles in the pseudorapidity range $-4.7\le \eta \le 4.7$ is 4520 (CHECK) $\pm$220, an increase by 17\% (CHECK) over the lower energy. The data support the 'limiting' fragmentation picture close to beam rapidity, but evidence an increase from 2.9 to 3.7 in the production of charged particles per pair of participant nucleons from peripheral (40-50\%) to central collisions (0-5\%) around midrapidity. The data constrain current models based on high density QCD gluon saturation and on the superposition of particle production from soft hadronic and hard partonic collisions. } \end{abstract} % insert suggested PACS numbers in braces on next line {PACS number(s): 25.75.Dw} % body of paper here %% The following two lines are commented out for preprint style %\begin{multicols}{2} %\narrowtext A central question in the study of collisions between heavy nuclei at the top energy afforded by the RHIC collider, $\sqrt{s_{NN}}$=200 GeV, is the role of hard scatterings between partons and the interactions of these partons in a high density environment. Indeed, it has been conjectured that new phenomena related to non-perturbative QCD may come into play at this energy. Among these a saturation of the number of parton (mainly gluon) collisions, which would limit the number of gluon-gluon collisions in central nucleus-nucleus collisions, has been predicted~\cite{partonsat83,Escola00, Kharzeev_and_Levin}, which would limit the production of charged particles. Recently, indications for a reduction in the number of hadrons at high transverse momentum for $\sqrt{s_{NN}}$=130 GeV collisions have been presented that may hint at suppression of hadronic jets at high matter densities ~\cite{Phenix-jets,Star-jets}. The present Letter addresses these issues via the first comprehensive investigation of multiplicity distributions of emitted charged particles in relativistic heavy ion collisions between $^{197}$Au nuclei at the maximum RHIC energy of 100AGeV+100AGeV. In particular, we present pseudorapidity distributions of charged particles in the range $-4.7 \le \eta \le 4.7$ for such collisions at $\sqrt{s_{\small{NN}}}$=200 GeV and as a function of collision centrality. The production of charged particles in these highly energetic nuclear collisions depends on the relative roles of hadronic and partonic collision processes and thus on the presence of gluon saturation effects and, in general, on the relative importance of soft and hard scattering processes. We find in this work that the production of charged particles at midrapidity increases by about 14\% for the most central collisions relative to $\sqrt{s_{NN}}$=130 GeV collisions. This result is in agreement with first results of the PHOBOS experiment at midrapidity~\cite{Phobos-mult200-1}. In addition, we present the dependence of the pseudorapidity distributions on the degree of overlap between the colliding nuclei and compare these to current models. The BRAHMS experiment consists primarily of two magnetic spectrometers for measuring exclusive charged particle spectra over wide ranges of pseudorapidity and transverse momentum. In addition, a number of global detectors are used for determining the location of the collision vertex and the time of the collision, and for characterizing general reaction properties such as the collision centrality and the inclusive charged particle densities. The data for this paper were obtained using three of the global detector systems at BRAHMS: the Multiplicity Array (MA), the Beam-Beam Counter arrays (BBC), and the Zero-degree Calorimeters (ZDC). A detailed discussion of the BRAHMS experimental arrangement can be found in ref.~\cite{bearden01b}. In addition, an analysis of charged particle densities for Au+Au reactions at $\sqrt{s_{\small{NN}}}$=130 GeV that is very similar in method to that presented here is described fully in ref.~\cite{bearden01a}. The Multiplicity Array determines charged particle densities around mid-rapidity. An inner, hexagonal-sided barrel of modestly segmented Si strip detectors (SiMA) is surrounded by an outer, barrel of plastic scintillator tiles (TMA). For the current data, four sides of the inner barrel were fully populated with six Si detectors, each, one side had one detector, and one side was left unpopulated. Each Si wafer is dimensioned 4~cm x 6~cm x 300~$\mu$m and is subdivided into seven active strips. The detectors are located 5.3~cm from the beam axis. The plastic scintillator tiles, dimensioned 12~cm x 12~cm x 0.5~cm and located 13.9 cm from the beam axis, were arranged with four sides of the hexagonal barrel populated with eight detectors, each, and the remaining two sides populated with two detectors and one detector, respectively. Both the SiMA and TMA cover a pseudorapidity range of $-2.2\le\eta\le2.2$ for collisions occurring at the nominal interaction vertex. Using an extended range of collision vertices, the effective coverage of the array is $-3.0\le\eta\le3.0$ . Particle densities are deduced from the observed energy loss in the SiMA and TMA elements using GEANT simulations ~\cite{Geant} to relate energy loss to the number of particles hitting a given detector element. This procedure is described in ref.~\cite{bearden01a}. The BBC Arrays consist of two sets of Cherenkov UV transmitting plastic radiators coupled to photomultiplier tubes. They are positioned around the beam pipe on either side of the nominal interaction point at a distance of 2.20~m (CHECK). The BBC elements have a time resolution of 50~ps allowing for the determination of the position of the interaction point with an accuracy of $\approx$ 1.6~cm. Charged particle multiplicities in the pseudorapidity range $2.1\le |\eta| \le 4.7$ are deduced from the number of particles hitting each tube, as found by dividing the measured ADC signal by that corresponding to a single particle hitting the detector. The ZDC detectors~\cite{adler00}, consisting of alternating layers of tungsten and fiber-ribbon plastic scintillator and located about $\pm$18m from the nominal interaction vertex, measure neutrons that are emitted at small angles with respect to the beam direction. For the present analysis, these detectors serve in two roles: the relative time of arrival of the ``first'' particles to hit each detector can be used to locate the interaction vertex with an accuracy of $\approx$~3.6~cm; the ZDC detectors also provide the minimum bias trigger for the experiment (estimated to include 97\% of the nuclear reaction cross section). Total particle multiplicities in the MA and BBC are developed separately as the two arrays can utilize different ranges of collision vertices, with the MA analysis restricted to collision vertex positions within 30~cm of the nominal vertex and with the BBC array sensitive to the wider range extending out to $\pm$120~cm from the nominal vertex. The SiMA and TMA total multiplicities are averaged after accounting for the different geometric acceptances of the two arrays and used together with the BBC total multiplicity information to determine, the reaction centrality by assuming that a cut on total multiplicity translates to a cut on collision centrality. In analyzing particle densities in dN/d$\eta$, the centrality dependence of the MA and BBC distributions are based on the centrality measurements of the corresponding array. In general, for interaction vertices close to the array center, the MA gives a better multiplicity determination because of the larger number of particles hitting elements of this array. In the pseudorapidity range of 3.0$\le\eta\le$4.2, where it was possible to analyze the BBC data using both centrality selections, the two centrality analyses give identical results to within 1-2\%. In general, statistical error on the measurements are less than 1\%, while we estimate that the systematic errors are 5\%. In figure~\ref{dndeta} (upper panel) we show the measured pseudorapidity distributions for charged particles for centrality cuts 0-5\%, 5-10\%, 10-20\%, 20-30\%, 30-40\% and 40-50\% of the minimum bias distribution. The $dN/d\eta$ values at $\eta$=0,1,5,3 are also listed, together with the estimated number of participating baryons from the reacting nuclei, in table 1. For the most central collisions (0-5\%) the multiplicities reach $dN/d\eta$=630 at midrapidity. This corresponds, by division with the number of participating baryon pairs, to $3.7 \pm 0.2$ charged particles per pair. This value, indicates an increase in the multiplicities of about 14\% relative to $^{197}$Au+$^{197}$Au reactions at $\sqrt{s_{NN}}$=130 GeV ~\cite{bearden01a,back00,back01,adcox01,Star-mult130}. By integrating the most central distribution we deduce that $N=4520 (CHECK) \pm 220$ charged particles are emitted in the considered rapidity range, the largest number of particles observed so far in energetic nuclear collisions. This number exceeds the corresponding number for $\sqrt{s_{NN}}$=130 GeV reactions by about 17\%. More detailed comparison shows that the distributions at the two energies are quite similar in shape. Indeed, the FWHM of the most central distributions is $\Delta \eta = 7.4$ for $\sqrt{s_{NN}}$=200 GeV, as compared to $\Delta \eta = 7.2 \pm 0.8$ for $\sqrt{s_{NN}}$=130 GeV collisions. For the most peripheral collisions analyzed here (40-50\%) the multiplicities at $\eta=0$ reach $dN/d\eta=110\pm 6$ while the corresponding value scaled to the number of participating pairs is 2.9$\pm 0.15$. For comparison, the similar number for proton-proton collisions at this energy is 2.5. Figure~\ref{dndeta_fragment} shows, on the other hand, that the charged particle multiplicities in an interval of approximately 2 units below the beam rapidity are independent of the collision centrality and energy, from CERN-SPS energy ($\sqrt{s_{NN}}$=17 GeV) to the present RHIC energy. This is consistent with a limiting fragmentation picture in which the excitations of the fragment baryons saturate already at moderate collision energies independently of the system size ~\cite{bearden01a}. In contrast, the increased projectile kinetic energy is utilized for particle production in the region around midrapidity, as evidenced by the observed increase of the multiplicities per participant pair around the center of mass rapidity, which is approximately \50% above the value for p+p collisions clearly demonstrating significant medium effects. Figure~\ref{dndeta_models} presents the $dN/d\eta$ distributions obtained by mirroring and averaging the negative and positive halves of the measured distributions to further decrease errors. We also compare the distributions with model calculations. The full drawn lines are calculations using the model of Kharzeev and Levin~\cite{Kharzeev_and_Levin} which is based on a classical QCD calculation using parameters fixed to the $\sqrt{s_{NN}}$=130 GeV data. It is seen that this approach is able to reproduce the magnitude and shape of the observed multiplicity distributions quite well. Also shown in Figure~\ref{dndeta_models} (dotted lines) are the results of calculations with the AMPT model~\cite{zhang01,lin01a,lin01b}, which is cascade model based on the HIJING model ~\cite{wang91} but including final state rescattering of produced particles. This model is also able to account for the general trend of the measured distributions, particularly for the most central collisions but tends to overpredict the data for the more peripheral collisions. Finally in Figure~\ref{ratios} (top panel) we plot the dependence of the multiplicity of charged particles per pair of participant baryons as a function of the number of participants in 3 narrow pseudrapidity regions ($\Delta \eta \approx 0.2$ around $\eta$ =0, 3.0 and 4.5. While the figure shows that particle production per participant pair is remarkably constant at the forward rapidities characteristic of the fragmentation region, this is not the case for the central rapidities. Indeed, we find a significant increase of particle production per pair for the more central collisions for $\eta=0$ and 1.5 which may be attributed to the onset of hard scatterings dependent, not on the number of participants, but on the number of binary parton collisions, $N_{coll}$. Using for $N_{coll}$ the values estimated in ~\cite{Kharzeev_and_Nardi} we fit the observed dependencies to a functional $dN/\eta/(N_part/2)=\alpha\cdot N_{part}+\beta \cdot N_{coll}$. For rapidities $\eta=0,1.5,3.0,4.5$ we obtain: $\alpha=1.14 \pm 0.23 ,1.27 \pm 0.23 , 1.10 \pm 0.20, 0.58 \pm 0.10$ and $\beta=0.20 \pm 0.10, 1.17 \pm 0.10, 0.07 \pm 0.08, -0.02 \pm 0.04$ respectively. For comparison we find $\alpha=1.09 \pm 0.16, 1.20 \pm 0.17 , 1.00 \pm 0.99, 0.49 \pm 0.08$ and $\beta=0.15 \pm 0.07, 1.12 \pm 0.07 , 0.02 \pm 0.08, -0.06 \pm 0.04$ at $\sqrt{s_{NN}}$=130 GeV.At $\eta=0.0$ we find that the hard scattering component to the charged particle production increases from 29\% $\pm$14\% at the lower energy to about 35\% $\pm$ 16\% at $\sqrt{s_{NN}}$=200 GeV for the most central collisions. In Figure~\ref{dndeta_ratios}(bottom panel) we show the ratio of the $dN/d\eta$ distributions measured at $\sqrt{s_{NN}}$=200 GeV and $\sqrt{s_{NN}}$=130 GeV for three different centralities and compare them to the similar model predictions. It is seen that this ratio is practically constant as a function of number of participant nucleons in each rapidity interval, again suggesting that the dominant particle production mechanism is similar at the two energies. The reason the ratio increases towards forward rapidities is that the rapiditry range at the top RHIC energy is broadened due to the higher beam energy. The overlaid curves show the ratio of the Kharzeev and Levin calculations at the two energies, which are able to accurately reproduce the observed dependence. In conclusion, we find that the charged particle production scales smoothly from $\sqrt{s_{NN}}$=130 GeV to $\sqrt{s_{NN}}$=200 GeV. The data are well reproduced by calculations based on high density QCD and quite well by the AMPT/HIJING microcopic parton model. A phenomenological two component analysis in terms of a supersposition of particle production due to soft/hard scatterings also accounts well for the data and indicates that hard scatterings and associated mini-jets are increasingly important for more central collisions and for the higher energy and amount to about 35\% of the charged particle production. We find good consistency with the gluon saturation model of Kharzeev and Levin, but stress that within errors of models and data alike, the data can be equally well reproduced by other models not requiring saturation effects in the description of parton collisions. While the current work establishes the baseline for particle production at the maximum energy available for nucleus-nucleus collisions for several years to come, the full understanding of these energetic collisions must await more detailed analyses of hadronic and leptonic observables over a wide region of phase space and rapidity. \vskip 1.2cm %\section{Acknowledgments} We thank the RHIC collider team for their support to the experiment. This work was supported by the Division of Nuclear Physics of the Office of Science of the U.S. Department of Energy under contracts DE-AC02-98-CH10886, DE-FG03-93-ER40773, DE-FG03-96-ER40981, and DE-FG02-99-ER41121, the Danish Natural Science Research Council, the Research Council of Norway, the Jagiellonian University Grants, the Korea Research Foundation, and the Romanian Ministry of Research (5003/1999,6077/2000). We are grateful to Drs. D Kharzeev, BNL, and E. Levin, Tel Aviv, and Zi-Wei Lin, Texas, for stimulating discussions and for supplying us with the model calculations shown and siscussed in this article. \vskip 0.8cm \begin{references} %jet quenching \bibitem{star-jets} Reference to STAR jet quenching (FIND.Is there any???) % \bibitem{phenix-jets} K. Adcox et al., subm. to Phys. Rev. Lett. (2001), nucl-exp-0109003. % %Parton saturation. \bibitem{partonsat83} L.V. Gribov,E.M. Levin and M.G. Ryskin,Phys.Rep.100 (1983) 1. % % ``Energy and Centrality Dependence of Rapidity Densities at RHIC'' %\bibitem{wang00} X. N. Wang and M. Gyulassy, Phys. Rev. Lett. {\bf 86}, 3496 (2001). % % ``Centrality dependence of multiplicities in ultrarelativistic nuclear collisions'' \bibitem{eskola00} K.J. Eskola, K. Kajantie, and K. Tuominen, Phys. Lett. B {\bf 497}, 39(2001); hep-ph/0009246. % %Wounded nucleon %\bibitem{bialas76} A.~Bialas, M.~Bleszynski, and W.~Czyz, Nucl. Phys. %{\bf B111}, 461(1976). % %SPS Pb+Pb \bibitem{deines00} P.~Deines-Jones {\it et al.} Phys. Rev. C {\bf 62}, 014903(2000); hep-ex/9912008. % %SPS Pb+Pb %\bibitem{aggarwal01} M.M.~Aggaral {\it et al.} Eur. Phys. J. {\bf C18}, %651 (2001); nucl-ex/0008004. % %BRAHMS mult 130 reference PLB \bibitem{bearden01a} I.G. Bearden {\it et al.}, to be published in Phys. Lett. B.; nucl-ex/0108016. % %BRAHMS NIM reference \bibitem{bearden01b} I.G. Bearden {\it et al.}, NIM preprint. % %ZDC reference \bibitem{adler00} C. Adler, A. Denisov, E. Garcia, M. Murray, H. Stroebele, S. White, Nucl. Instrum. Methods Phys. Res., {\bf }A461 337(2001); nucl-ex/0008005. % %STAR reference on scintillator tiles %\bibitem{aota95} S. Aota {\it et al.}, NIM {\bf A352}, 557(1995). % %GEANT reference \bibitem{geant} GEANT 3.2.1, CERN program library. % %Hijing Paper \bibitem{wang91} X.N. Wang and M. Gyulasy, Phys. Rev. D {\bf 44}, 3501 (1991). % %Fritiof reference: Check if to correct version... %\bibitem{pi92} H. Pi, Comput. Phys. Commun. {\bf 71}, 173(1992). % %Glauber paper %\bibitem{glauber} R.J. Glauber and G. Matthiae, Nucl. Phys. {\bf B21}, 135(1970). % %Phobos dN/deta paper 2000 (56 and 130) \bibitem{back00} B.B. Back {\it et al.}, Phys. Rev. Lett. {\bf 85}, 3100 (2000). % %STAR paper on mult at 130GeV \bibitem{star-mult130} C. Adler et al., Phys. Rev Lett. 87, 112303 (2001) % %Phobos 200 Gev mult at midrapidity 2001. subm to PRL \bibitem{Phobos-mult200-1} B.B. Back {\it et al.}, subm. to Phys. Rev. Lett. (2001), nucl.exp-0108009. % %PHENIX dN/deta paper 130GeV \bibitem{adcox01} K. Adcox {\it et al.}, Phys. Rev. Lett. {\bf 86}, 3500 (2001). % %limiting fragmentation \bibitem{benecke69} J.~Benecke, T.T.~Chou, C.N.~Yang, and E.~Yen, Phys. Rev. {\bf 188}, 2159(1969). % %\bibitem{urqmd} S.A. Bass {\it et al.}, Prog. Part. Nucl. Phys. %{\bf 41}, 225(1998); nucl-th/9803035. % %AMPT model \bibitem{zhang01} Bin Zhang, C.M.~Ko, Bao-An~Li, and Ziwei~Lin, Phys. Rev. C {\bf 61} 067901(2001). \bibitem{lin01a} Zi-wei Lin, Subrata Pal, C.M. Ko, Bao-An Li, and Bin Zhang, Phys. Rev. C {\bf 64} 011902R(2001). \bibitem{lin01b} Zi-wei Lin, Subrata Pal, C.M. Ko, Bao-An Li, and Bin Zhang, preprint nucl-th/0105044; and Zi-wei Lin, private communication. % %Kharzeev Papers \bibitem{Kharzeev_and_Levin}D. Kharzeev and E. Levin. nucl- th/0108006. Private communication. % \bibitem{Kharzeev_and_Nardi}D. Kharzeev and M. Nardi. Phys. Lett. B507, 121 (2001), nucl-th/0012025. % % %UA5 p-pbar paper %\bibitem{alner86} G. Alner {\it et al.}, Z. Phys. C, {\bf 33}, 1 (1986). \end{references} \begin{table}[h!] \caption{\label{TABLE}\textit{\sl Charged particle densities in $dN_{ch}/d\eta$ as a function of centrality and pseudorapidity. Total uncertainties, dominated by the systematics, are indicated. The average number of participants $$ is given for each centrality class based on HIJING model calculations. The last column gives the integral charged particle multiplicity within the pseudorapidity range $-4.7 \le \eta \le 4.7$.}} \begin{tabular}{|c|c|c|c|c|c|c|c|} \hline Centrality & $$& $$& $\eta = 0$ & $\eta =1.5$ & $\eta = 3.0$ & $\eta = 4.5$ & $N_{ch}$ \\ 0-5\% & 1074& 352 & 632$\pm$55 & 628$\pm$57 & 453$\pm$41 & 181 $\pm$21 & 4514$\pm$300 \\ 5-10\% & 842 & 299 & 498$\pm$44 & 509$\pm$46 & 379$\pm$37 & 156$\pm$17 & 0$\pm$250 \\ 10-20\% & 590 & 235 & 373$\pm$33 & 385$\pm$35 & 296$\pm$29 & 124$\pm$13 & 0$\pm$190 \\ 20-30\% & 356 & 165 & 257$\pm$23 & 265$\pm$24 & 207$\pm$21 & 89 $\pm$10 & 0$\pm$130 \\ 30-40\% & 204 & 114 & 170$\pm$15 & 178$\pm$16 & 140$\pm$15 & 62 $\pm$7 & 0$\pm$90 \\ 40-50\% & 108 & 75 & 110$\pm$10 & 115$\pm$10 & 90$\pm$9 & 42 $\pm$5 & 0$\pm$60 \\ \hline \end{tabular} \end{table} \begin{figure} \epsfig{file=fig1.eps,width=8.5cm} \caption{ Top panel: Distributions of $dN_{ch}/d\eta$ for centrality ranges of, top to bottom, 0-5\%, 5-10\%, 10-20\%, 20-30\%, 30-40\%, and 40-50\%. The SiMA and BBC results are indicated by the circles and triangles, respectively. Bottom panel: Distributions of $dN_{ch}/d\eta$ as shown above divided by the average number of participating nucleon pairs. } \label{dndeta} \end{figure} \begin{figure} \epsfig{file=fig2.eps,width=8.5cm} \caption{ Charged particle densities normalized to the number of participant pairs for the present 0-5\% central (open circles) and 30-40\% central (open squared) Au+Au results at $\sqrt{s_{NN}}$=200 GeV, the BRAHMS Au+Au results at $\sqrt{s_{NN}}$=130 GeV (closed circles) and the 9.4\% central Pb+Pb data at $\sqrt{s_{NN}}$=17 GeV(closed triangles) of ref.~\cite{deines00}. Here, the different data are plotted as a function of the pseudorapidity shifted by the relevant beam rapidity, as discussed in the text. } \label{dndeta_fragment} \end{figure} \begin{figure} \epsfig{file=fig3.eps,width=8.5cm} \caption{ Distribution of the measured $dN_{ch}/d\eta$ for the 6 indicated centrality ranges. Total uncertainties (statistical and systematic) are indicated. Data are compared to theoretical predictions by Kharzeev and Levin (full drawn line) and to the predictions of the AMPT model (dashed line). } \label{dndeta_models} \end{figure} \begin{figure} \epsfig{file=fig4.eps,width=8.5cm} \caption{ Top panel: distributions of $dN_{ch}/d\eta$ per participant pair as a function of the number of participants (see table) for $\eta$= 0,1.5, 3.0 and 4.5. Bottom panel: Ratio of the measured $dN_{ch}/d\eta$ distributions at $\sqrt{s_{NN}}$=200 GeV and $\sqrt{s_{NN}}$=130 GeV as a function of $N_{part} for $\eta$= 0, 1.5, 3.0 and 4.5. The curves show predictions by the Kharzeev and Levin model (full drawn line) and the AMPT model (dashed line). } \label{dndeta_ratios} \end{figure} % The following line is commented out for preprint style %\end{multicols} \end{document}