2. 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 dN ch /d  vs. Centrality and  S NN  Comparison to theoretical models  Summary and Conclusion

3. R elativistic H eavy I on C ollider 2 O’clock IR  June 2000: Startup of RHIC  June - September 2000 First Physics Run Au+Au @ two energies  S NN = 56 and 130 GeV  July 2001- January 2002 Second Physics Run Au+Au @  S NN = 200 GeV (maximal design energy) p+p (reference data) PHOBOS PHENIX STAR BRAHMS

4. I.G. Bearden 7, D. Beavis 1, C. Besliu 10, Y. Blyakhman 6,J. Bondorf 7, J.Brzychczyk 4, B. Budick 6, H. Bøggild 7, C. Chasman 1, C. H.Christensen 7, P. Christiansen 7, J.Cibor 4, R.Debbe 1, J. J. Gaardhøje 7, K. Grotowski 4, K. Hagel 8, O. Hansen 7, H. Heiselberg 7, A. Holm 7, A.K. Holme 12, H. Ito 11, E.Jacobsen 7, A.Jipa 10, J. I. Jordre 10, F. Jundt 2, C. E. Jørgensen 7, T.Keutgen 9, E. J. Kim 5, T. Kozik 3, T.M.Larsen 12, J. H. Lee 1, Y. K.Lee 5, G. Løvhøjden 2, Z. Majka 3, A. Makeev 8, B. McBreen 1, M. Murray 8, J.Natowitz 8, B.S.Nielsen 7, K. Olchanski 1, D. Ouerdane 7, R.Planeta 4, F.Rami 2, D.Roehrich 9, B. H. Samset 12, S. J. Sanders 11, I. S. Sgura 10, R.A.Sheetz 1, Z.Sosin 3, P. Staszel 7,T.S. Tveter 12, F.Videbæk 1 R.Wada 8 and A.Wieloch 3. 1 Brookhaven National Laboratory, USA 2 IReS and Université Louis Pasteur, Strasbourg, France 3 Jagiellonian University, Cracow, Poland 4 Institute of Nuclear Physics, Cracow, Poland 5 Johns Hopkins University, Baltimore, USA 6 New York University, USA 7 Niels Bohr Institute, Blegdamsvej 17, University of Copenhagen, Denmark 8 Texas A&M University, College Station. USA 9 University of Bergen, Norway 10 University of Bucharest, Romania 11 University of Kansas, Lawrence,USA 12 University of Oslo Norway The BRAHMS Collaboration ~55 physicists from 12 institutions

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

6. SiMA Silicon strips TMA Scintillator tiles BBC Čerenkov radiator SiMA TPM1 BBC TMA BBC Global Detectors in BRAHMS Charged Particle Multiplicity  Primary Vertex

7. BRAHMS Physics Program Probing Hot and Dense Nuclear Matter by studying:  Reaction Mechanisms and Dynamics Different Observables: dN ch /d , p t spectra  Baryon Stopping (anti-particle/particle ratios)  Strangeness Production  Collective Flow  High p t hadron spectra (Jet Quenching effects) First Results  dN ch /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

8. EVENT CHARACTERIZATION COLLISION CENTRALITY Au+Au @  S NN =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=0   Peripheral b large  For each Centrality Cut  Evaluate the corresponding number of participants N part (Glauber Model)

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

10. dN ch /d  distributions Au+Au  S NN =130GeV  S NN =200GeV I.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 processes N ch (-4.7<  <4.7) I.Bearden et al (BRAHMS) Submitted to PRL 3860  3004630  370

11.   =0  Steady increase   =3  Flat dependence ( dN ch /d  scales with N part )  Increase with N part  Onset of hard processes D.Kharzeev and M.Nardi, PLB 507(2001)121 dN ch /d  = A  N part  B  N coll % of hard   S NN = 130 GeV 20%  7%   S NN = 200 GeV 25%  7% dN ch /d  - Centrality Dependence Superposition of Soft + Hard

12. Comparison to Model Predictions 1 2 3 HIJING – Jet quenching HIJING – No Jet quenching EKRT (Gluon Saturation) Wang & Gyulassy, PRL86(2001)3496  BRAHMS 1 2 3  | |  | |  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 Collisions For Central Collisions

13. SUMMARY BRAHMS has measured dN ch /d  distributions in Au+Au collisions at two energies  S NN =130GeV and 200GeV  Combining different sub-detectors in BRAHMS  “Complete” dN ch /d  distributions  At Forward  ’s  No Centrality Dependence (dN ch /d  scales with N part )  No Energy Dependence (  Limiting Fragmentation)  At Mid-rapidity  dN ch /d  /(0.5 ) increases with Centrality  Influence of hard scattering processes Two component analysis  Significant contribution at RHIC  dN ch /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

14. Limiting Fragmentation Central Collisions (5%)   S NN =130GeV   S NN =200GeV  Pb+Pb at SPS  Fragmentation region Appropriate 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  S NN =17.2GeV (Benecke et al, PRC 188(1969)2159)

15. dN ch /d  - Comparison to Model Predictions  S NN =130GeV 5%5-10% 20-30%40-50% 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  Parton scattering models give a good description of the data  AMPT  wider distributions (includes hadronic rescattering) PLB523(2001)227

16. dN ch /d  - Comparison to Model Predictions Au+Au @  S NN =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 Differences for Peripheral Collisions but Small effect !  dN ch /d 

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