Key Results from the BRAHMS Experiment Zhongbao Yin

Key Results from the BRAHMS Experiment Zhongbao Yin for the BRAHMS Collaboration BRAHMS

Outline The BRAHMS experiment ● Baryon stopping ● High p hadron T production ● Summary ● BRAHMS

The BRAHMS Experiment Broad RAnge Hadron Magnetic Spectrometers BRAHMS

Stopping ● ● Energy released for particle production Information on the initial condition for the evolution of relativistic heavy-ion collisions – – ● Landau model: full stopping+isentropic expansion Bjorken scenario: transparency => net baryon poor region at mid-rapidity Information on the baryon transport mechanism BRAHMS

Stopping: pbar/p At Mid-rapidity p-/p+ Au+Au @ 200 Ge. V = 1. 01 ± 0. 04 K-/K+ = 0. 95 ± 0. 05 pbar/p = 0. 75 ± 0. 04 Pair production is predominant ● The ratios, within errors, are constant in the interval y=0 -1 =>Considerable transparency ● Moving away from the central region, the influence of the participant baryons becomes more and more important PRL 90, 102301(2003) BRAHMS

Stopping: Net Protons Au+Au @ 200 Ge. V PRL 93, 102301(2004) HIJING without Baryon Junction is the least unfavoured. BRAHMS

Rapidity and Energy Loss y = 2. 06 0. 16 y = 2. 04 0. 10 E=26+-5 Ge. V/n E= 30+-2 Ge. V/n Rapidity loss saturates while around 75% of initial energy is available for particle production. BRAHMS

Landau or Bjorken ? Au+Au @ 200 Ge. V Simplified Landau model predicts: d. N/dy (y) of Gaussian with width 2 =ln( /2 mp) (Caruthers & Duong-van, PRD 8(1973)859) PRL 94, 162301(2005) BRAHMS

Hard Scattering recombination Study modifications of high p. T production in AA, d. A with respect to pp via Ncoll scaling at different rapidities to disentangle different effects BRAHMS

Nuclear Modification Factors: RAA BRAHMS PRL 91(2003)072305 Rd. Au : enhancement RAA = (yield. AA/<Ncoll>)/yieldpp Rcp = (yieldcentral/<Ncoll>cent)/(yieldperipheral/<Ncoll>peripheral) Large high pt suppression aslo at forward rapidity. BRAHMS

Rd. A in d+Au Collisions BRAHMS PRL 93, 242303 (2004) Enhancement at mid-rapidity, while high pt suppression at h = 3. 2 BRAHMS

Rcp in d+Au collisions ● ● At mid-rapidity, stronger enhancement for more central collision, but reversed centrality dependence at forward rapidity. Consistent with CGC prediction: PLB 599(2004)23 BRAHMS

But… Recombination also works Hwa, Yang & Fries: PRC(2005)024902 with only the recombination of soft and shower partons no multiple scattering, or gluon saturation put in explicitly BRAHMS

RAA for identified hadrons BRAHMS Preliminary ● Strong suppression for pions but no suppression for (anti-)protons at both mid-rapidity and forward rapidity BRAHMS

p/p ratios in Au+Au collisions ● BRAHMS Preliminary ● ● Ratios are large compared with 0. 2 from fragmentation of energetic partons Ratios are smaller at forward rapidity while pion suppression persists. Qualitative agreement with parton recombination with collective flow effect BRAHMS

Summary ● ● RHIC reaches a net-baryon poor region at mid -rapidity In Au+Au collisions at 200 Ge. V: – – ● High pt suppression of charged hadrons and pions at both mid- and forward rapidities No high pt (anti-)proton suppression at both midand forward rapidities In d+Au collisions at 200 Ge. V: – significant reduction of the nuclear modification factor at forward rapidity and this suppression increases with rapidity and centrality BRAHMS

The BRAHMS Collaboration I. Arsene 10, I. G. Bearden 7, D. Beavis 1, C. Besliu 10, B. Budick 6, H. Bøggild 7, C. Chasman 1, C. H. Christensen 7, P. Christiansen 7, J. Cibor 4, R. Debbe 1, E. Enger 12, J. J. Gaardhøje 7, M. Germinario 7, K. Hagel 8, O. Hansen 7, H. Ito 1, 11, A. Jipa 10, F. Jundt 2, J. I. Jørdre 9, C. E. Jørgensen 7, R. Karabowicz 3, E. J. Kim 5, T. Kozik 3, T. M. Larsen 12, J. H. Lee 1, Y. K. Lee 5, S. Lindal 12, R. Lystad 9, G. Løvhøiden 2, Z. Majka 3, A. Makeev 8, B. Mc. Breen 1, M. Mikelsen 12, M. Murray 8, 11, J. Natowitz 8, B. Neumann 11, B. S. Nielsen 7, J. S. Norris 11, D. Ouerdane 7, R. Planeta 4, F. Rami 2, C. Ristea 10, O. Ristea 10, D. Röhrich 9, B. H. Samset 12, D. Sandberg 7, S. J. Sanders 11, R. A. Scheetz 1, P. Staszel 7, T. S. Tveter 12, F. Videbæk 1, R. Wada 8, Z. Yin 9, I. S. Zgura 10 1 Brookhaven 2 IRe. S and Université Louis Pasteur, Strasbourg, France 3 Jagiellonian 4 Institute 5 Johns University, Krakow, Poland of Nuclear Physics, Cracow, Poland Hopkins University, Baltimore, USA 6 New 7 Niels National Laboratory, USA York University, USA Bohr Institute, University of Copenhagen, Denmark 8 Texas A&M University, College Station, USA 9 University 10 University 11 University 12 of Bergen, Norway of Bucharest, Romania of Kansas, Lawrence, USA University of Oslo, Norway BRAHMS