Results from the BRAHMS Experiment at RHICF.Rami* for the BRAHMS Collaboration* Institut de Recherches Subatomiques and Universit 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

Relativistic Heavy Ion Collider 2 Oclock IR June 2000: Startup of RHIC June - September 2000 First Physics RunAu+Au @ two energiesSNN = 56 and 130 GeV July 2001- January 2002 Second Physics RunAu+Au @ SNN = 200 GeV (maximal design energy) p+p (reference data)PHOBOSPHENIXSTARBRAHMS

The BRAHMS Collaboration I.G. Bearden7, D. Beavis1, C. Besliu10, Y. Blyakhman6,J. Bondorf7, J.Brzychczyk4, B. Budick6, H. Bggild7, C. Chasman1, C. H.Christensen7, P. Christiansen7, J.Cibor4, R.Debbe1, J. J. Gaardhje7, 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. Jrgensen7, T.Keutgen9, E. J. Kim5, T. Kozik3, T.M.Larsen12, J. H. Lee1, Y. K.Lee5, G. Lvhjden2, 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.Videbk1 R.Wada8 and A.Wieloch3.

1Brookhaven National Laboratory, USA 2IReS and Universit 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

The BRAHMS Experiment at RHICPerspective view of BRAHMS Good Particle Identification over wide range of rapidities (0

SiMA Silicon stripsTMA Scintillator tiles BBC erenkov radiatorSiMATPM1BBCTMABBCGlobal Detectors in BRAHMSChargedParticleMultiplicity Primary Vertex

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)112305I.Bearden et al, PLB523(2001)227I.Bearden et al, nucl-ex/0112001 submitted to PRL

EVENT CHARACTERIZATION COLLISION CENTRALITYAu+Au @ SNN=130GeV

Measured with Multiplicity Detectors (TMA and SiMA) Central Peripheral

Define Event Centrality Classes Slices corresponding to different fractions of the cross section

Central b=0Peripheral b large

For each Centrality Cut Evaluate the corresponding number of participants Npart (Glauber Model)

dNch/d measurements in BRAHMS 0 - 5 % 5 -10%10-20%20-30%30-40%40-50%TPM1BBCSiMATMAI.Bearden et al, Phys.Lett.B523(2001)227Au+Au @ SNN=130GeV

Data from detectors Consistency

By combining all results Cover wide range -4.7 4.7 = -ln (tan(/2))Complete distributionTotal Charged Particle MultiplicitiesdNch/d

dNch/d distributionsAu+AuSNN=130GeVSNN=200GeVI.Bearden et al (BRAHMS)PLB523(2001)227 0-5% 30-40%

Forward s No Centrality Dependence Mid-rapidity (0) Increase with centrality

Centrality Dependence Relative contributions of Soft and Hard processesNch(-4.7

dNch/d - Centrality Dependence

=0 Steady increase =3 Flat dependence (dNch/d scales with Npart) Increase with Npart Onset of hard processesD.Kharzeev and M.Nardi, PLB 507(2001)121dNch/d = ANpart BNcoll Superposition of Soft + Hard

Comparison to Model Predictions Both models HIJING and EKRT reproduce the measured multiplicities Au+Au data much larger than pp Not a simple superposition Medium effects important role in AA collisions It would be interesting to explore the Centrality Dependence in these models Stronger constraints Central CollisionsFor Central Collisions

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) 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

Limiting Fragmentation Central Collisions (5%) SNN=130GeV SNN=200GeV Pb+Pb at SPS

Fragmentation regionAppropriate frame = beam reference frame

No Energy Dependence from SPS to RHIC Consistent with the Hypothesis of Limiting Fragmentation

Observed in several reactions pp, ppbar, p-emulsion, -emulsion

Deines-Jones et al, PRC (2000) 4903 SNN=17.2GeV(Benecke et al, PRC 188(1969)2159)

dNch/d - Comparison to Model PredictionsSNN=130GeV5%5-10%20-30%40-50%UrQMDBass et al,Prog.Part.Nuc.Phys.41(98)255

HIJINGWang and Gyulassy,PRD44(91)3501

AMPTZhang et al,PRC61(2001)067901Lin et al,PRC64(2001)011902 Parton scattering models give a good description of the data AMPT wider distributions (includes hadronic rescattering)PLB523(2001)227

dNch/d - Comparison to Model PredictionsAu+Au @ SNN=200GeVAMPTZhang et al,PRC61(2001)067901Lin et al,PRC64(2001)011902

High density QCDgluon saturationKharzeev and Levin,PLB523(2001)79Differences forPeripheral Collisions but Small effect !dNch/d

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