1. Rapidity Dependence of Charged Hadron Yields in Central Au+Au Collisions at 200 GeV Djamel Ouerdane Niels Bohr Institute for the BRAHMS Collaboration Quark Matter 2004 Oakland, January 11-18 BRAHMS

2. D. Ouerdane, Quark Matter 2004, Oakland, January 2004 The BRAHMS Collaboration I.G. Bearden 7, D. Beavis 1, C. Besliu 10, Y. Blyakhman 6, 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, E. Enger 12, J. J. Gaardhøje 7, M. Germinario 7, K. Grotowski 4, K. Hagel 8, O. Hansen 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, R. Karabowicz 4, 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. Mikkelsen 12, 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, D. Sandberg 7, S. J. Sanders 11, R.A.Sheetz 1, Z.Sosin 3, P. Staszel 7, T.S. Tveter 12, F.Videbæk 1, R. Wada 8, A.Wieloch 3 and I. S. Zgura 10 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, 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

3. D. Ouerdane, Quark Matter 2004, Oakland, January 2004 Outline of the Talk The BRAHMS experiment Rapidity Dependence of Spectra and Yields From Stopping to Transparency Pions : Landau or Bjorken ? Kaons : Is Strangeness Equilibrated ? Summary & Conclusions

4. The BRAHMS Detector MRS FFS BFS

5. D. Ouerdane, Quark Matter 2004, Oakland, January 2004 Particle Identification TIME-OF-FLIGHT Particle Separation: p max ( 2  cut)= TOFWTOF1 TOF2  / K 2  cut K / p 2 GeV/c 3 GeV/c 4.5 GeV/c 3.5 GeV/c 5.5 GeV/c 7.5 GeV/c RICH: Cherenkov light focused on spherical mirror  ring on image plane Ring radius vs momentum gives PID  / K separation 20 GeV/c Proton ID up to 35 GeV/c CHERENKOV (2 settings)

6. D. Ouerdane, Quark Matter 2004, Oakland, January 2004 Pion invariant differential yields Kaon invariant differential yields By combining all data sets and averaging over the number of collisions, we get the final invariant yields over a broad range of phase-space Particle Spectra Proton invariant differential yields

7. D. Ouerdane, Quark Matter 2004, Oakland, January 2004 Particle Spectra Pions: power lawKaons: exponentialProtons: Gaussian Top 5% central collisions

8. D. Ouerdane, Quark Matter 2004, Oakland, January 2004 Integrated multiplicities (Gaussian fit) N(   ) ~ 1780 N(  + ) ~1760 N(K + ) ~ 290 N(K  ) ~ 240 N(pbar) ~ 85 Rapidity Densities At y ~ 0, dN/dy is ~ 300 (300) for  + (  - ) ~ 47 (44) for K + (K  ) ~ 27 (20) for p (pbar) N(  ) >> N(K) > N(p) N(  + ) = N(   ) N(K + ) > N(K  ) and N(p) > N(pbar) systematically Little decrease of transverse flow

9. D. Ouerdane, Quark Matter 2004, Oakland, January 2004 Nuclear Stopping BRAHMS, submitted to PRL, nucl-ex/0312023 P. Christiansen Ph.D. Thesis Net-proton measured up to y = 3 AGS : high stopping RHIC: more transparent Net-proton rapidity densities (top 5% central collisions)

10. D. Ouerdane, Quark Matter 2004, Oakland, January 2004 Gaussians in p z : 2.03  0.16 Nuclear Stopping Net-baryon after feed-down & neutron corrections 2.00  0.10 6 order polynomial Gaussians in p z :  E = 25.7  2.1 TeV scaling broken empirical scaling Rapidity loss : (N part = 357  10)

11. D. Ouerdane, Quark Matter 2004, Oakland, January 2004 Pion Production: Landau or Bjorken? BRAHMS NA49 AGS Prediction: dN/dy (y) Gaussian of width  given by (simplified model) : Entropy ~ Pion production Expansion ~ isentropic Pion gas = 3D relativistic fluid [L.D. Landau, Izv. Akad. Nauk SSSR 17 (1953) 52 P.Carruthers, M.Duong-van, PRD 8 (1973) 859] Ok as a 1 st approximation BUT systematic difference remains : flatter top, tails broader => more longitudinal flow

12. D. Ouerdane, Quark Matter 2004, Oakland, January 2004 Y < 1 : consistent with Hadron Gas Stat. Model K + /  + : 15.6  0.1 % (stat) K  /   : 14.7  0.1 % (stat) [Phys. Lett. B 518 (2001) 41] Divergence at higher y : Associated K + production No single source with unique T and  B Strangeness with Kaons RAPIDITY DEPENDENCE BRAHMS, PRL90 (2003) 102301 T~constant,  B varies with y Increasing y

13. D. Ouerdane, Quark Matter 2004, Oakland, January 2004 ENERGY DEPENDENCE At y = 0, ratios converge to ~ 15 % Over the full phase space: K + /  + = 16.6  1.5 % (syst) K  /   = 13.7  2.0 % (syst) no change for + Why this behavior ? Net-Kaon distribution evolves like net-proton Strangeness with Kaons

14. D. Ouerdane, Quark Matter 2004, Oakland, January 2004 Summary & Conclusions Transverse momentum spectra of  ’s, K’s and p’s measured in rapidity range |y  | < 3.6, |y K | < 3.4 and |y p | < 3.1 Stopping: dN/dy (p-pbar) ~ 7 at y = 0 => near transparency increases with increasing rapidity rapidity loss ~ 2 units, scaling broken at 200 GeV Pions: N(+) = N(-) over the rapidity range covered Landau picture close to data as a 1 st approximation only Kaons: N(+) ~ N(-) at mid-rapidity (47 and 44) but N(+) > N(-) at y > 2 due to associated K + production K /  : converge to ~ 15% at y ~ 0 (plateau y < 1) same within systematic errors for full phase-space ratios possible indication of strangeness equilibration