First results from the PHOBOS experiment at RHIC

Gunther Roland, B. B. Back, M. D. Baker, D. S. Barton, R. R. Betts, R. Bindel, A. Budzanowski, W. Busza, A. Carroll, M. P. Decowski, E. Garcia, N. George, K. Gulbrandsen, S. Gushue, C. Halliwell, J. Hamblen, G. A. Henitzelman, C. Henderson, R. Hołyński, D. J. HofmanB. Holzman, E. Johnson, J. L. Kane, J. Katzy, N. Khan, W. Kucewicz, P. Kulinich, W. T. Lin, S. Manly, D. McLeod, J. Michałowski, A. C. Mignerey, J. Mülmenstädt, R. Nouicer, A. Olszewski, R. Pak, I. C. Park, H. Pernegger, C. Reed, L. P. Remsberg, M. Reuter, C. Roland, G. Roland, L. Rosenberg, P. Sarin, P. Sawicki, W. Skulski, S. G. Steadman, G. S.F. Stephans, P. Steinberg, M. Stodulski, A. Sukhanov, J. L. Tang, R. Teng, A. Trzupek, C. Vale, G. J. van Nieuwenhuizen, R. Verdier, B. Wadsworth, F. L.H. Wolfs, B. Wosiek, K. Woźniak, A. H. Wuosmaa, B. Wysłouch

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Abstract

We have presented data taken by the PHOBOS experiment during the first RHIC run. Our studies have shown that Au+Au collisions at RHIC produce the highest particle densities yet observed in heavy-ion collisions, exceeding the densities seen at the CERN SPS by 70%. Normalizing per participant pair, particle production at mid-rapidity is enhanced by 40% compared to pp̄ collisions at the same energy. Model comparisons have shown that our data place very specific constraints on particle production mechanism. In particular the observed centrality dependence of dNch/dη\\η\<1/ (0.5 × Npart), with a fast initial rise up to (Npart) = 80, is different from the predictions of many popular models. Our study of the full pseudo-rapidity distribution up to η = 5.4 shows that much of the additional particle production is concentrated near mid-rapidity, consistent with the expectations of hard-scattering models. However, we also observe a somewhat wider plateau of the η distribution than predicted by e.g. HIJING and clear evidence for processes at large pseudo-rapidities which are not correctly modeled in this code. One of the crucial questions in the understanding of interpretation of data from heavy-ion collisions is the validity of descriptions employing (local) equilibration. The data on elliptic flow show, that the interactions in the produced collision system very effectively transform the initial-state spatial anisotropy into a final state momentum space anisotropy. Comparisons with hadronic cascade models suggest that this interaction must happen in the early, dense state of the system to achieve the observed values of elliptic flow. Finally, we have taken a first step towards characterizing the hadro-chemical composition of the particle source by measuring antiparticle/particle ratios at mid-rapidity. These ratios can be related to the baryo-chemical potential μB in statistical models of particle production. Our ratios indicate a value of μB = 45 MeV, about a factor of 5-6 smaller than observed at SPS energies. The particle source at RHIC therefore has a significantly smaller net baryon-density than at lower energies, but still is not in a regime that could be described as completely baryon-free. Based on these initial results characterizing the global conditions achieved in heavy-ion collisions at RHIC, we hope to see detailed theoretical predictions for the 2001 running period at RHIC, where we expect to collect 100 times larger statistics than in the initial running. This will allow a detailed investigation of two-particle correlations and particle distributions at low and high transverse momenta and the search for non-statistical event-by-event distributions. We expect that a scan of the observables shown here as a function of collision energy and nuclear species will also be an important part of the near-term RHIC program.

Original languageEnglish
Pages (from-to)54-63
Number of pages10
JournalNuclear Physics A
Volume698
Issue number1-4
DOIs
StatePublished - 11 Feb 2002

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