1. Presentation for NFR - October 19, 2005 - Trine S.Tveter Recent results from BRAHMS @ RHIC Systems studied so far at RHIC: - Au+Au @ s NN 1/2 =  GeV - Cu+Cu @ s NN 1/2 =  GeV (heavy-ion collisions, possible QGP formation!) - p+p @ s 1/2 =  GeV (elementary collisions) - d+Au @ s NN 1/2 =  GeV (cold nuclear matter) t 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

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

3. Presentation for NFR - October 19, 2005 - Trine S.Tveter Are we creating a QGP? Lattice QCD: Critical energy density  C ~  GeV/fm 3 for deconfinement. Bjorken's energy estimate formula: indicates energy density  >~  GeV/fm 3 at RHIC ! Charged particle pseudorapidity density in Au+Au collisions at  =0 for  CM energies gives: - s NN 1/2 =  GeV:  >~  GeV/fm 3 - s NN 1/2 =  GeV:  >~  GeV/fm 3 - s NN 1/2 =  GeV:  >~  GeV/fm 3 Sufficient for QGP formation!

4. Presentation for NFR - October 19, 2005 - Trine S.Tveter Bulk hadron production  central Au+Au collisions at s NN 1/2 =  GeV: Near-Gaussian rapidity distributions for mesons (and antiprotons). increasing with mass.  central Au+Au @ s NN 1/2 =  GeV, Pb+Pb @ s NN 1/2 =  GeV and Au+Au @ s NN 1/2 =  GeV: From full stopping to increasing transparency.

5. Presentation for NFR - October 19, 2005 - Trine S.Tveter Antiparticle - particle ratios Evolution of particle ratios with y and s NN 1/2 : p / p and     at y=0 lower at s NN 1/2 =  GeV. They also fall off faster at higher rapidity. Behaviour of ratios well described assuming thermal / chemical equilibrium at quark level:  q,  S light, strange quark chemical potential. Expectation for  S  :      = (p / p) 1/3      and p/p correlated over large range of y and s NN 1/2. Fit to data:      ~  (p / p) 0.25 ->  S ~  q ? Good agreement with statistical- thermal model assuming chemical freezeout temperature T =  MeV.

6. Presentation for NFR - October 19, 2005 - Trine S.Tveter Strangeness production -  ratios s NN 1/2 =  GeV s NN 1/2 =  GeV      and      Different dependence on CM energy and on rapidity (  q - driven?)      (y=0)  peaks  at lower SPS energies. RHIC energies, high y: SPS-like chemistry. Investigating  ratios at s NN 1/2 =  GeV, y ~  - close to fragmentation region! BRAHMS prelim.            

7. Presentation for NFR - October 19, 2005 - Trine S.Tveter Radial flow, kinetic freezeout Exploding fireball: mass dependent. Blast wave fits with parameters T fo and  T. Kinetic freezeout T fo lower than for chemical freezeout (~  MeV).  T higher for midrapidity, central collisions, higher s NN 1/2. Kinetic freezeout temperature T fo Expansion velocity  T

8. Presentation for NFR - October 19, 2005 - Trine S.Tveter Elliptic flow Elliptic flow v 2 in non - central collisions:  is azimuthal angle rel. to reaction plane. v 2 (p T ) scales with number of valence quarks. Small decrease with rapidity. v 2 magnitude close to hydrodynamical limit. Requires thermalization early in partonic phase -> strongly interacting QGP!      central Au+Au @  GeV Charged hadrons  central

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

10. Presentation for NFR - October 19, 2005 - Trine S.Tveter Nuclear modification of high- p T 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

11. Presentation for NFR - October 19, 2005 - Trine S.Tveter Suppression - initial- or final- state effect? Compare suppression in hot (Au+Au) and cold (d+Au) nuclear matter. d+Au @ s NN 1/2 =  GeV at  : Cronin-like enhancement, increasing with centrality. Indicates suppression in Au+Au to be final-state effect -> QGP?

12. Presentation for NFR - October 19, 2005 - Trine S.Tveter Nuclear modification - dependence on energy and system size Au+Au Nuclear modification for charged hadrons at  - Au+Au: Same centrality dependence but less suppression at s NN 1/2 =  GeV. Enhancement in semi- peripheral collisions. - Cu+Cu at s NN 1/2 =  GeV: Enhancement already in central collisions. General trend: Suppression increases with system size, centrality and energy.

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

14. Presentation for NFR - October 19, 2005 - Trine S.Tveter Nuclear modification - Dependence on rapidity No rapidity dependence of R AA observed in central collisions, regardless of particle species. Centrality dependence of R AA stronger at forward y than at midrapidity. Dense partonic medium at y ~  -> surface emission? More diluted medium at forward y? BRAHMS preliminary - y  for  central

15. Presentation for NFR - October 19, 2005 - Trine S.Tveter 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 x Au values of: x Au ~ p T / s NN 1/2 exp(- y) Experimental R dA (  ): Enhancement at  ~  turns into suppression at forward  ! Gluon saturation - > Color Glass Condensate?

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

17. Presentation for NFR - October 19, 2005 - Trine S.Tveter Summary and plans Heavy-ion collisions at RHIC - observations so far: Bulk properties and suppression / enhancement pattern of high- p T 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 @ s NN 1/2 =  GeV: Detailed mapping of RAA as function of y, p T, system size, for identified hadrons - Run 2006: p+p @ s 1/2 =  (+ hopefully,  ) GeV - improve elementary reference spectra - Run 2007: Possible d+Au run

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

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