1. What Happened in China ?

2.  Gluons can begin to fuse with high enough gluon density.  Saturation will limit parton production  Final state charged particle yields per collision limited? Gluon Saturation? Gluon Saturation does not appear to set in for peripheral collisions Cannot yet rule out Eskola’s saturation for central collisions Kharzeev’s initial-state saturation picture is consistent with data Eskola, Kajantie, and Tuominen: hep-ph/0009246 Kharzeev, Nandi: nucl-th/0012025 r/  gg  g Phys. Rev. Lett. 86, 3500 (2001)

3. Energy density Bjorken formula for energy density in terms of measured transverse energy assuming a thermalized system at time t 0. PHENIX: Central Au-Au yields 6.88 fm (hard sphere radius) Time to thermalize the system (~ 1.0 - 0.2 fm/c?) Phys. Rev. Lett. 87, 52301 (2001)

4. Is the energy density high enough? The PHENIX EMCal measures transverse energy For the most central events:  Bjorken  ~ 4.6-23 GeV/fm 3  critical  ~ 0.6-1.8 GeV/fm 3 Lattice phase transition: Energy deposition is certainly adequate, but does it create a thermalized new phase of matter while  crit ? Roughly 1.5 to 2 times higher than previous experiments if assume same formation time Lattice  c  Bj  ~ 4.6 GeV/fm 3 J. Nagle  Bj  ~ 23.0 GeV/fm 3 thermalization time?

5. Comparison with pp baseline Central Peripheral consistent with NN scaled by number of collisions Central below scaled NN spectrum p 0 larger deficit than unidentified hadrons p0p0 Scaled NN

6.    ratio’s with pp and peripheral Central/peripheral PTPT PTPT

7. Complications “Ordinary” Nuclear effects:  Cronin Effect and P T broadening  Nuclear shadowing of gluon structure functions  Issues:  Does the parton fragment inside the medium?  Particle composition is a strong function of p T  Other unknown effects? q q

8. Year-1 High P T Conclusions Central collision data shows significant suppression relative to prediction without energy loss. Indicates a novel effect: deviation from point like scaling It is consistent with parton energy loss, but too early to make definitive conclusion. A systematic study including pA and higher P T reach is needed. Stay tuned …

9. “Other” Year-1 PHENIX results HBT Electrons Gamma Distribution Calculation Centrality: 0-5% PHENIX Preliminary (GeV/c) Fluctuations 5.9  0.4 7.9  1.1 6.2 .5 = 0.49 .07 Elliptic Flow PHENIX Preliminary

10. BRAHMS acceptance 00 & 01 FFS BFS

11. N=  dE / P(0)/P(n  1) Background corr.due to secondaries (37-50%) Consistency between 4 independent. detector systems 65 AGeV+65 AGeV:  N(ch)d  = 4050±300 Central 0-5% dN(ch)/d  (  =0) =550 FWHM of distribution  = 7.6  0.7 0-5% 10-20% 20-30% 30-40% 40-50% 5-10% 600 BRAHMS Subm. Phys. Lett. B 7/2001.

12. Charged Particle Mult. at 130 GeV BRAHMS. Subm. Phys Lett. B. 2001 SPS

13. dN ch /d  for 100 AGeV+ 100 AGeV 100 AGeV + 100 AGeV AU+AU  N(ch)d  = 5100±300 Central 0-6% dN(ch)/d  (  =0) =610  50 FWHM of distribution  = 7.9  1.0 BRAHMS 200AGEV

14. Hard and Soft vs. High Density QCD @ 200 AGeV Kharzeev and Levin (nucl-th/0108006) Soft-Hard: dN/d  =(1-X) n pp /2 + X n pp =1049, =339, npp=2.43 =>dN/d  =668 (with X=0.9) High Density QCD-saturation: dN/dy = f( Npart, Q s 2,,  QCD,  s,y) with =0.3 from HERA data => dN/d  =620 (using dN/d  =549 at  s=130GeV) ( ( ( ) ) )

15. Total production of charged particles 130 AGeV 4000 charged part. observed Nch  23.5 pr. part. pair cf. Nch  17 in p+p at  s=130GeV 35-40% increase over p+p Syst BRAHMS 200 AGeV n200 AGeV n5100 charged part. observed nNch  30 pr. part. pair ncf. Nch  20 in p+p at  s=200GeV n50% increase over p+p

16. Bjorken limit reached for Au+Au  s= 130AGeV? ISR R803  s=23  s=63 BRAHMS PRL sept. 2001

17. p-bar/p ratio: Centrality dependence BRAHMS 2k

18. How consistent are the models?

19. Summary RESULTS: 100+100 Nch (0-5%)  5100 dN/d  (y=0)  625.  FWHM  7.8 N(ch)  30 pr. participant-pair dN/d  (y=0)  3.6 pr. part. Pair p-pbar/p 0.48±0.05 (y=2)  0.99±0.01(stat) (y=3) RESULTS: 65+65 Nch (0-5%)  4000 dN/d  (y=0)  550.  FWHM  7.6 N(ch)  23 pr. participant-pair dN/d  (y=0)  3 pr. part. Pair AntiMeson/Meson close to unity p-bar/ p vs y shows increased but still incomplete transparency Midrapidity Plateau? y =0,0.7,2 : pbar/p  0.64, 0.66, 0.41 (±0.05 ± 0.06) Weak pt and centrality dependence Bjorken limit not reached Models inconsistent with data

20. Energy Dependence at  =0 Errors are dominated by systematics AGS/SPS points extracted by measured dN/dy and New data at 200 GeV shows a continuous logarithmic rise at midrapidity f pp (s) =

21. Ratio of dN/d  at 200 & 130 GeV 90% Confidence Level

22. Pseudo-rapidity Distributions Using Octagon and Ring subdetectors Measure out to |  |<5.4 Corrections –Acceptance –Occupancy –Backgrounds (from MC) Systematic errors –10% near  =0 –Higher near rings  Background Corr. HIJING Simulation PRL 87 (2001) forthcoming

23. Centrality Dependence vs.  Total charged multiplicity is about 4200 ± 420 for central events At high  3-4 multiplicity starts to decrease as a function of N part Similar feature seen in pA collisions PRL 87 (2001) forthcoming

24. Comparison to pp and models Peripheral Central Scaled UA5 data HIJING AMPT (rescattering) Y beam   (Y 130 /Y 200 ) dN/d  = f pp (s) PRL 87 (2001) forthcoming DeMarzo et al, 1984 Systematic error not shown

25. Change in dN/d  with energy UA5 looked for ‘limiting fragmentation’ by plotting dN/d  with   - Y beam We can do the same thing with the PHOBOS data –agreement for AA in the fragmentation region 200 GeV 130 GeV UA5 200 GeV (NSD) UA5, Z.Phys.C33, 1 (1986)

26. Saturation model fits to 130 Data m 2 =2Q s m , p T =Q s, ~.3 extracted from HERA F 2 data Kharzeev & Levin, nucl-th/0108006, input from Golec-Biernat & Wüsthoff (1999)

27. Azimuthal Asymmetry at  =0 130 GeV data from PHOBOS Correct for occupancy, resolution of reaction plane estimate Good agreement with hydro calcuations for central events (and STAR…) X =  cos(n  i ), Y =  sin(n  i )  n = atan(Y/X)  v 2 = At midrapidity, using “SymOct” Account for detector response using “weighting matrix”. Pads in  Pads in z

28. Asymmetry vs. Multiplicity Compare –Flow for mid-central events –Multiplicity per participant pair for central events (only 10% variation down to N part =100) Flow (for more peripheral events) seems to scale with particle density 200 GeV data should be interesting! –Saturation or scaling? PRELIMINARY

29. Asymmetry vs. Rapidity Systematic Error ~.007 Preliminary Results – final results coming soon! P. Kolb, Utah proc.

30. Results on particle ratios Measured near midrapidity –0 < y <1 (species-dependent) Final results submitted to PRL (Apr. 2001) Consistent within systematic errors with results presented at QM2001 (Jan. 2001) Smaller systematic errors (10%  6%)

31. Conclusions Systematics of charged particle production have been explored by the PHOBOS experiment –Energy – multiplicity rises approximately logarithmically –Centrality – data shows simple interpolation between pp and central AA –Rapidity – scaling with N part changes in fragmentation region –Azimuthal – elliptic flow appears to scale with multiplicity –Particle ratios are reaching zero net baryon number, small  B Broad features of particle production are consistent with soft nature of strong interactions –pp and pA collisions are very instructive Theoretical models are assimilating new data –None simultaneously describe full set of systematics!

32. Energy dependence of v 2 Logarithmic rise

47. Statistical models Braun-Munzinger et al. (hep-ph/0106066) - Follows curve for / = 1 GeV at freezeout - Uses phenomenological parameterization: J. Cleymans & K. Redlich, PRL 81 (1998) 5284

48. Strangeness production Lines of constant S where: / = 1 GeV I. Increase in strange/non-strange particle ratios II. Maximum is reached III. Ratios decrease (Strange baryons affected more strongly than strange mesons) Braun-Munzinger et al. hep-ph/0106066

49. Implications for ratios (PRELIMINARY) STAR 130 GeV 14% central (    (PRELIMINARY) STAR 130 GeV 14% central (      (*0.2) Braun-Munzinger et al. hep-ph/0106066 Mid-rapidity ratios

50. Sensitivity to multi-strange baryons T (MeV) Ratios Braun-Munzinger et al. hep-ph/0105229 Thermal fit results in T = 174 MeV Model gets K/  correct, but misses on  ratios!!! Statistical errors only  + /h - (Preliminary) STAR 130 GeV 14% central data  - /K - (7% central)

51. Stat. model 200 GeV predictions Becattini et al. PRC 64 (2001) 024901 Use parameterization: Predicts  ~0.8 (Preliminary) STAR 130 GeV minbias data (CAUTION! Really for 4  ratios) Statistical errors only

52. Anti-lambda/lambda Ratio ~ 0.76 +/- 0.02 (stat) 130 GeV Ratio ~ 0.75 minbias GeV/c 2  -> p  - GeV/c 2  -> p  + Preliminary STAR 200 GeV minbias data Uncorrected for absorption, which may be as much as a few percent effect. Ratios at midrapidity, averaged over experimental acceptance in p t

53. Stat. Model Predictions Revisited Becattini et al. PRC 64 (2001) 024901 Use parameterization: (Preliminary) STAR 130 GeV Data (CAUTION! Really for 4  ratios) Pretty close to prediction! (Preliminary) STAR 200 GeV minbias data Statistical errors only

54. Conclusions Statistical model fits do well with most particle/antiparticle ratios. Statistical model fits do not reproduce  /meson ratios very well. –It will be interesting to see how  fits in the picture. Quark coalescence model roughly reproduces particle ratios well. –Losing sensitivity to particle/antiparticle ratios as we approach  B =0. We are still not in a net baryon free region at sqrt(s NN ) = 200 GeV - and perhaps still a long ways away?