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
R elativistic H eavy I on C ollider 2 O’clock IR June 2000: Startup of RHIC June - September 2000 First Physics Run two energies S NN = 56 and 130 GeV July January 2002 Second Physics Run S NN = 200 GeV (maximal design energy) p+p (reference data) PHOBOS PHENIX STAR BRAHMS
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
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
SiMA Silicon strips TMA Scintillator tiles BBC Čerenkov radiator SiMA TPM1 BBC TMA BBC Global Detectors in BRAHMS Charged Particle Multiplicity Primary Vertex
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) I.Bearden et al, PLB523(2001)227 I.Bearden et al, nucl-ex/ submitted to PRL
EVENT CHARACTERIZATION COLLISION CENTRALITY 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)
dN ch /d measurements in BRAHMS % 5 -10% 10-20%20-30% 30-40%40-50% TPM1 BBCSiMA TMA I.Bearden et al, Phys.Lett.B523(2001)227 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
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 370
=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
Comparison to Model Predictions HIJING – Jet quenching HIJING – No Jet quenching EKRT (Gluon Saturation) Wang & Gyulassy, PRL86(2001)3496 BRAHMS | | | | 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
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
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)
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) Lin et al, PRC64(2001) Parton scattering models give a good description of the data AMPT wider distributions (includes hadronic rescattering) PLB523(2001)227
dN ch /d - Comparison to Model Predictions S NN =200GeV AMPT Zhang et al, PRC61(2001) Lin et al, PRC64(2001) High density QCD gluon saturation Kharzeev and Levin, PLB523(2001)79 Differences for Peripheral Collisions but Small effect ! dN ch /d
Stronger Constraints on the models... Important to use different observables to constrain models S NN =130GeV