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Results from the BRAHMS Experiment at RHIC

Results from the BRAHMS Experiment at RHIC. F.Rami* for the BRAHMS Collaboration * Institut de Recherches Subatomiques and Universit é Louis Pasteur, Strasbourg. Introduction The BRAHMS Experiment Main Physics Results  Global features and event characterization

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Results from the BRAHMS Experiment at RHIC

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  1. Results from the BRAHMS Experiment at RHIC F.Rami* for the BRAHMS Collaboration * Institut de Recherches Subatomiques and Université Louis Pasteur, Strasbourg • Introduction • The BRAHMS Experiment • Main Physics Results  Global features and event characterization Charged particle multiplicity distributions dNch/d vs. Centrality and SNN Comparison to theoretical models • Summary and Conclusion

  2. Relativistic Heavy Ion Collider BRAHMS 2 O’clock IR PHOBOS • June 2000: Startup of RHIC PHENIX • June - September 2000 • First Physics Run STAR Au+Au @ two energies SNN = 56 and 130 GeV • July 2001- January 2002 • Second Physics Run Au+Au @ SNN = 200 GeV (maximal design energy) p+p (reference data)

  3. The BRAHMS Collaboration I.G. Bearden7, D. Beavis1, C. Besliu10, Y. Blyakhman6,J. Bondorf7, J.Brzychczyk4, B. Budick6, H. Bøggild7, C. Chasman1, C. H.Christensen7, P. Christiansen7, J.Cibor4, R.Debbe1, J. J. Gaardhøje7, K. Grotowski4, K. Hagel8, O. Hansen7, H. Heiselberg7, A. Holm7, A.K. Holme12, H. Ito11, E.Jacobsen7, • Jipa10, J. I. Jordre10, F. Jundt2, C. E. Jørgensen7, T.Keutgen9, E. J. Kim5, T. Kozik3, T.M.Larsen12, J. H. Lee1, Y. K.Lee5, G. Løvhøjden2, Z. Majka3, A. Makeev8, B. McBreen1, M. Murray8, J.Natowitz8, B.S.Nielsen7, K. Olchanski1, D. Ouerdane7, R.Planeta4, F.Rami2, D.Roehrich9, B. H. Samset12, S. J. Sanders11, I. S. Sgura10, R.A.Sheetz1, Z.Sosin3, P. Staszel7,T.S. Tveter12, F.Videbæk1 R.Wada8 and A.Wieloch3. 1Brookhaven National Laboratory, USA 2IReS and Université Louis Pasteur, Strasbourg, France 3Jagiellonian University, Cracow, Poland 4Institute of Nuclear Physics, Cracow, Poland 5Johns Hopkins University, Baltimore, USA 6New York University, USA 7Niels Bohr Institute, Blegdamsvej 17, University of Copenhagen, Denmark 8Texas A&M University, College Station. USA 9University of Bergen, Norway 10University of Bucharest, Romania 11University of Kansas, Lawrence,USA 12 University of Oslo Norway ~55 physicists from 12 institutions

  4. The BRAHMS Experiment at RHIC Perspective view of BRAHMS Forward Spectrometer 2.3o <  < 30o • Good Particle Identification over wide range of rapidities • (0<|y|<4) and transverse • momenta (0.2<pt<4GeV/c) Mid-Rapidity Spectrometer 30o <  < 95o • Basic information on momentum spectra and yields of charged hadrons as a function ofy and pt

  5. Global Detectors in BRAHMS BBC SiMA TPM1 TMA BBC SiMASilicon strips TMAScintillator tiles BBCČerenkov radiator Charged Particle Multiplicity  Primary Vertex

  6. BRAHMS Physics Program Probing Hot and Dense Nuclear Matter by studying: • Reaction Mechanisms and Dynamics Different Observables: dNch/d, pt spectra • Baryon Stopping (anti-particle/particle ratios) • Strangeness Production • Collective Flow • High pt hadron spectra (Jet Quenching effects) First Results  dNch/d and anti-particle/particle ratios I.Bearden et al, PRL87(2001)112305 I.Bearden et al, PLB523(2001)227 I.Bearden et al, nucl-ex/0112001 submitted to PRL

  7. EVENT CHARACTERIZATION COLLISION CENTRALITY Au+Au @ SNN=130GeV • Measured with Multiplicity Detectors (TMA and SiMA) Central b=0    Peripheral Central Peripheral b large  • Define Event Centrality Classes  Slices corresponding to different fractions of the cross section • For each Centrality Cut  Evaluate the corresponding number of participants Npart (Glauber Model)

  8. dNch/d measurements in BRAHMS I.Bearden et al, Phys.Lett.B523(2001)227 Au+Au @ SNN=130GeV 0 - 5 % 5 -10% TPM1  = -ln (tan(/2)) TMA • Data from  detectors  Consistency • By combining all results  Cover wide  range -4.7    4.7 BBC SiMA dNch/d 10-20% 20-30% “Complete” distribution 30-40% 40-50% Total Charged Particle Multiplicities 

  9. dNch/d distributions Au+Au SNN=130GeV SNN=200GeV I.Bearden et al (BRAHMS) PLB523(2001)227 I.Bearden et al (BRAHMS) Submitted to PRL Nch(-4.7<<4.7) 3860  300 4630  370  0-5% 30-40% • Forward ’s No Centrality Dependence • Mid-rapidity (0) Increase with centrality Centrality Dependence Relative contributions of Soft and Hard processes

  10. % of hard • SNN=130GeV 20%  7% • SNN=200GeV 25%  7% dNch/d - Centrality Dependence • =0  Steady increase • =3  Flat dependence (dNch/dscales with Npart) • Increase with Npart Onset of hard processes dNch/d = ANpart  BNcoll Superposition ofSoft + Hard D.Kharzeev and M.Nardi, PLB 507(2001)121

  11. Wang & Gyulassy, PRL86(2001)3496 1 3  2 BRAHMS   |  | |  | 1 HIJING – Jet quenching 2 HIJING – No Jet quenching 3 EKRT (Gluon Saturation) Comparison to Model Predictions • Au+Au data much larger than pp  Not a simple superposition Medium effects  important role in AA collisions • Both models HIJING and EKRT reproduce the measured multiplicities For Central Collisions Central Collisions • It would be interesting to explore the Centrality Dependence in these models •  Stronger constraints

  12. SUMMARY BRAHMS has measured dNch/d distributions in Au+Au collisions at two energies SNN=130GeV and 200GeV • Combining different sub-detectors in BRAHMS  “Complete” dNch/d distributions • At Forward ’s  No Centrality Dependence (dNch/d scales with Npart)  No Energy Dependence ( Limiting Fragmentation) • At Mid-rapidity dNch/d/(0.5<Npart>) increases with Centrality  Influence of hard scattering processes Two component analysis  Significant contribution at RHIC • dNch/d measured in central collisions can be reproduced by two different models HIJING (“Soft+Hard”) and EKRT (“Gluon Saturation”)  It would be interesting to investigate the Centrality Dependence in these models  Stronger Constraints

  13. Limiting Fragmentation • Fragmentation region Central Collisions (5%) SNN=130GeV Appropriate frame = beam reference frame SNN=200GeV Pb+Pb at SPS SNN=17.2GeV Deines-Jones et al, PRC (2000) 4903 No Energy Dependence from SPS to RHIC Observed in several reactions pp, ppbar, p-emulsion, -emulsion  Consistent with the Hypothesis of Limiting Fragmentation (Benecke et al, PRC 188(1969)2159)

  14. dNch/d- Comparison to Model Predictions SNN=130GeV PLB523(2001)227 UrQMD Bass et al,Prog.Part. Nuc.Phys.41(98)255 HIJING Wang and Gyulassy, PRD44(91)3501 AMPT Zhang et al, PRC61(2001)067901 Lin et al, PRC64(2001)011902 5% 5-10% 20-30% 40-50% • Parton scattering models give a good description of the data • AMPT  wider distributions (includes hadronic rescattering)

  15. dNch/d- Comparison to Model Predictions Au+Au @SNN=200GeV AMPT Zhang et al, PRC61(2001)067901 Lin et al, PRC64(2001)011902 High density QCD gluon saturation Kharzeev and Levin, PLB523(2001)79 dNch/d Differences for Peripheral Collisions but Small effect ! 

  16. Stronger Constraints on the models ... SNN=130GeV Au+Au @  Important to use different observables to constrain models

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