Presentation for NFR - October 19, 2005 - Trine S.Tveter

1. t Recent results from BRAHMS @ RHIC Systems studied so far at RHIC: - Au+Au @ sNN1/2= 62, 130, 200 GeV - Cu+Cu @ sNN1/2= 62, 200 GeV (heavy-ion collisions, possible QGP formation!) - p+p @ s1/2 = 200GeV (elementary collisions) - d+Au @ sNN1/2 = 200 GeV (cold nuclear matter) Stages of a heavy-ion collision at RHIC: - Initial stage with parton distributions inside Lorentz- contracted nuclei - Preequilibrium stage with hard parton-parton collisions - Phase with deconfined (but strongly interacting) partons - sQGP? - Hadronization with subsequent chemical and thermal freezeout Presentation for NFR - October 19, 2005 - Trine S.Tveter

2. The BRAHMS spectrometer Presentation for NFR - October 19, 2005 - Trine S.Tveter

3. Are we creating a QGP? Lattice QCD: Critical energy density eC ~ 0.7 GeV/fm3 for deconfinement. Bjorken's energy estimate formula: indicates energy density e >~ 5 GeV/fm3 at RHIC ! Charged particle pseudorapidity density in Au+Au collisions at h=0 for 3 CM energies gives: - sNN1/2 = 200 GeV: e >~ 5.0 GeV/fm3 - sNN1/2 = 130 GeV: e >~ 4.3 GeV/fm3 - sNN1/2 = 62 GeV: e >~ 3.7 GeV/fm3 Sufficient for QGP formation! Presentation for NFR - October 19, 2005 - Trine S.Tveter

4. Bulk hadron production 0 - 5% central Au+Au collisions at sNN1/2 = 200 GeV: Near-Gaussian rapidity distributions for mesons (and antiprotons). <pT> increasing with mass. 0 - 5% central Au+Au @ sNN1/2 = 5 GeV, Pb+Pb @ sNN1/2 = 17 GeV and Au+Au @ sNN1/2 = 200 GeV: From full stopping to increasing transparency. Presentation for NFR - October 19, 2005 - Trine S.Tveter

5. Antiparticle - particle ratios Evolution of particle ratios with y and sNN1/2 : p / p andK-/K+ at y=0 lower at sNN1/2 =62 GeV. They also fall off faster at higher rapidity. Behaviour of ratios well described assuming thermal / chemical equilibrium at quark level: mq, mS light , strange quark chemical potential. Expectation formS= 0: K-/ K+= (p / p)1/3 K-/K+ and p/p correlated over large range of y and sNN1/2 . Fit to data: K-/K+~ (p / p) 0.25 -> mS ~ 0.25 mq ? Good agreement with statistical- thermal model assuming chemical freezeout temperature T =170 MeV. Presentation for NFR - October 19, 2005 - Trine S.Tveter

6. Strangeness production - K/p ratios K+/p+ K+/p+ sNN1/2 = 200 GeV K-/p- K-/p- BRAHMS prelim. K+/p+and K-/p-: Different dependence on CM energy and on rapidity (mq- driven?) K+/p+(y=0)peaksat lower SPS energies. RHIC energies, high y: SPS-like chemistry. Investigating K/p ratios at sNN1/2 = 62 GeV, y ~ 3 - close to fragmentation region! sNN1/2 =62 GeV Presentation for NFR - October 19, 2005 - Trine S.Tveter

7. Radial flow, kinetic freezeout Kinetic freezeout temperature Tfo Expansion velocity bT Exploding fireball: <pT> mass dependent. Blast wave fits with parameters Tfo and bT. Kinetic freezeout Tfo lower than for chemical freezeout (~ 120 - 140 MeV). bT higher for midrapidity, central collisions, higher sNN1/2. Kinetic freezeout temperature Tfo Expansion velocity bT Presentation for NFR - October 19, 2005 - Trine S.Tveter

8. Elliptic flow 10- 20% central p++p- Elliptic flow v2 in non - central collisions: f is azimuthal angle rel. to reaction plane. v2 (pT) scales with number of valence quarks. Small decrease with rapidity. v2 magnitude close to hydrodynamical limit. Requires thermalization early in partonic phase -> strongly interacting QGP! Au+Au @ 200 GeV Charged hadrons 10 - 30% central Presentation for NFR - October 19, 2005 - Trine S.Tveter

9. Hard scattering and jets Hard scattering in vacuum STAR data: p+p, d+Au, peripheral Au+Au: BTB jet correlations Central Au+Au: Extended medium, far-side jet quenched Deconfined medium: Jet energy loss from gluon bremsstrahlung Presentation for NFR - October 19, 2005 - Trine S.Tveter

10. Nuclear modification of high- pT hadrons Nuclear modification factor from comparison to p+p reference spectra: Should be 1 for superposition of NN collisions. - Strong suppression in central Au+Au collisions, both at y ~ 0 and forward y - Suppression strongly increasing with centrality Presentation for NFR - October 19, 2005 - Trine S.Tveter

11. Suppression - initial- or final- state effect? Compare suppression in hot (Au+Au) and cold (d+Au) nuclear matter. d+Au @ sNN1/2 = 200GeV ath = 0: Cronin-like enhancement, increasing with centrality. Indicates suppression in Au+Au to be final-state effect -> QGP? Presentation for NFR - October 19, 2005 - Trine S.Tveter

12. Nuclear modification - dependence on energy and system size Au+Au Nuclear modification for charged hadrons at h = 0, h = 1: - Au+Au: Same centrality dependence but less suppression at sNN1/2 = 62 GeV. Enhancement in semi- peripheral collisions. - Cu+Cu at sNN1/2 = 62 GeV: Enhancement already in central collisions. General trend: Suppression increases with system size, centrality and energy. Presentation for NFR - October 19, 2005 - Trine S.Tveter

13. Nuclear modification - dependence on particle species Pions and other mesons clearly suppressed. Baryons not suppressed, even enhanced! p/p ratios strongly enhanced rel. to p+p collisions (effect peaks at pT ~ 2 - 3 GeV/c). p/p enhancement scales with Npart. Parton recombination involved in hadron formation? Presentation for NFR - October 19, 2005 - Trine S.Tveter

14. Nuclear modification - Dependence on rapidity BRAHMS preliminary - y = 0, 1, 2 for 0 - 10% central No rapidity dependence of RAA observed in central collisions, regardless of particle species. Centrality dependence of RAA stronger at forward y than at midrapidity. Dense partonic medium at y ~ 0 -> surface emission? More diluted medium at forward y? Presentation for NFR - October 19, 2005 - Trine S.Tveter

15. Initial state effects - dAu collisions Initial- state gluon distributions d+Au collisions: Probing initial parton distribution functions in Au nuclei. High energy: Gluon density grows at low x, limited by shadowing / saturation? At rapidity y one probes xAu values of: xAu ~ pT/ sNN1/2 exp(- y) Experimental RdA (h): Enhancement at h ~ 0 turns into suppression at forward h! Gluon saturation - > Color Glass Condensate? Presentation for NFR - October 19, 2005 - Trine S.Tveter

16. Initial state effects - dAu collisions h ~ 0: Increasing enhancement with centrality h >1: Increasing suppression with centrality Data qualitatively described by "extreme" CGC model, "normal" shadowing, ++ Particle species dependence at h ~ 3.2: Mesons suppressed, baryons enhanced. Not well understood for cold nuclear matter. Experimental situation needs clarification! Presentation for NFR - October 19, 2005 - Trine S.Tveter

17. Summary and plans Heavy-ion collisions at RHIC - observations so far: Bulk properties and suppression / enhancement pattern of high- pT particles reveal new form of matter: - Very high energy density - Strongly interacting, most likely partonic (sQGP?) - Behaviour governed by complex interplay between initial- and final- state effects and by various competing processes Further investigations (-> 2008): - Analysis of high-statistics data sets from Au+Au and Cu+Cu @ sNN1/2 = 200 GeV: Detailed mapping of RAA as function of y, pT, system size, for identified hadrons - Run 2006: p+p @ s1/2 = 200 (+ hopefully, 62) GeV - improve elementary reference spectra - Run 2007: Possible d+Au run Presentation for NFR - October 19, 2005 - Trine S.Tveter

18. Norwegian BRAHMS participation Present BRAHMS contingent in Norway: - Dieter Röhrich (professor, UiB) - Gunnar Løvhøiden (professor, UiO) - Trine S. Tveter (professor, UiO) - Ionut C. Arsene (Ph.D. Student, UiO) - Bjørn H. Samset (Ph.D. Student, UiO) - Hongyan Yang (Ph.D. Student, UiB) - Svein Lindal (master student, UiO) - Atle Qviller (master student, UiO) Collaborating institutions in Denmark, France, Poland, Romania, New York, Kansas, Texas. ~ 50 participants. Presentation for NFR - October 19, 2005 - Trine S.Tveter

19. Presentation for NFR - October 19, 2005 - Trine S.Tveter