Bulk Dynamics in Heavy Ion Collisions

1. Bulk Dynamics in Heavy Ion Collisions Peter Steinberg Brookhaven National Laboratory INPC2004 June 27-July 2, 2004 Göteborg, Sweden

2. Strongly-Interacting Matter On the lattice, onlyreach 75-80% ofStefan-Boltzmann limit In context of N=4 SUSY QCDthis is the signature ofa strongly-interacting plasma(Klebanov et al, 1996) Shift of paradigm:QCD does not predict weakly interacting QGP for accessible T Can we study strongly-interacting matter in A+A collisions?

3. The Bulk of Particles: dN/dh Comprehensive study of particle production in A+A Energy Centrality Rapidity 130 GeV 200 GeV 19.6 GeV dN/dh Most Central h h h PHOBOS Participant Spectator

4. The Bulk of Particles: dN/dpT STAR

5. Dynamical Models Nuclear Geometry Parton distributionsNuclear shadowing 0 fm/c Parton production& reinteraction Chemical Freezeout &Quark Recombination 2 fm/c Jet FragmentationFunctions 7 fm/c Hadron Rescattering Thermal Freezeout &Hadron decays >7 fm/c Independent stages: Bulk physics integrates time history

6. Dynamical Models vs. Data HIJING Hard + Soft ParticleDensitynear y=0 Large variation inpredictions RHIC Data Hadron Transport pp Data

7. The Big Surprise Contrary to popular belief… Bulk observables are “simple” …just not necessarily at h=0!

8. Participant Scaling Participant scaling = Long-range rapidity correlations

9. “Limiting Fragmentation” in A+A peripheral central Yield depends on h’=h-ybeam Logarithmic increase at h=0  centrality-dependent “universal curve”

10. Difference between p+p vs. A+A NA49 In a head-on p+p collision… …half of energy emerges as “leading particles” flat in xF In a typical A+A collision… …“leading particles” can be struck again! May make sense to consider A+A as havingall the available energy for particle production

11. Universality of Total Multiplicity LEP200 GeV A+A per participant pair ~ p+p @s/2 ~ e+e- @s A real surprise at RHIC – not predicted by SPS extrapolations

12. Essential Features of A+A • Npart scaling • Factorization of Energy & Geometry • Universal multiplicity / Npart/2 • Connections between e+e- & p+p • “Limiting fragmentation” How can we understand this in a simple way? Dynamical models have too many independent stages

13. Hydrodynamic Evolution A new canonical image for heavy-ion physics Strongly-interacting 6Li released from an asymmetric trap O’Hara, et al, Science 298 2179 (2002) Hydro useful for strongly interacting matter:buildup of pressure gradients due to geometry How does it work for A+A?

14. Longitudinal Dynamics y z Early thermalization Blackbody EOS Multiplicity formula: Npart scaling Gaussian dN/dy Landau Assumes matterstops briefly andexplodes longitudinally Ultra-high energy density Rapid thermalization

15. Universal Multiplicity Formula Nch=2.2s1/4 Multiplicity formula impliesp=e/3 in early times Npart scaling impliesEntropy~Volume

16. dN/dy: Longitudinal Dynamics M. Murray, BRAHMS dN/dy may be consequence of hydrodynamic evolutionof Lorentz contracted early stage = DYNAMICS

17. Limiting Fragmentation Approximate scalingin y’ y-yT

18. Transverse Dynamics • Assume boost-invariance • dN/dy is initial condition • Non-trivial EOS • Pressure gradients • Radial & elliptic flow • Cooper-Frye Freezeout

19. Radial Flow Au+Au 200 GeV 20% normalizationat 2 GeV Subm. to PRL nucl-ex/0401006 A clear mass effect on top of “thermal” spectrum Kolb & Rapp

20. Elliptic Flow vs. Geometry Hydrodynamic limit STAR PHOBOS Compilation and Figure from M. Kaneta Peripheral collisions show “elliptic flow” Reasonable agreement with hydro

21. pT Dependence STAR “fine structure” (mass dependence)  Described by hydro (and hydro-inspired) fits

22. Pseudorapidity Dependence T. Hirano - CGC+Hydro 19.6, 62,130,200 GeV PHOBOS PHOBOS130 GeV T. Hirano, Nov ‘03 Limiting fragmentation works almost too well for v2(h) (Is there really a hydro “limit”?) Challenge even for 3D hydro calculations, even if rule sounds simple!

23. Hydro approach appears to bewarranted by a wide range of data (but no single model gets everything right!) Joining longitudinal & tranverse stages is arbitrary for now(when does evolution begin?) Serious conceptual issues regardingapplicability of hydro to small systems…

24. More similarities: AA & e+e- Similar “longitudinaldynamics”? Similar “energy density”?

25. Limiting Fragmentation DELPHI PLB 459 (1999) e+e- p+p

26. Similar Freezeout Properties From Braun-Munzinger, Stachel, Redlich (2003) Becattini (1995) A+A e+e- Relative particle yields described usingthermal-statistical modelsin both e+e- and A+A

27. Radial Expansion in p+p? R. Witt, STAR Collaboration

28. Radial Expansion in p+p? Rout / Rout(pp) Rside / Rside(pp) Rlong / Rlong(pp) STAR T. Gutierrez, QM04 HBT radii have similar relativemomentum dependence: Similar “expansion dynamics”?

29. Soft Physics = Difficult Physics? Dynamical models have few constraints although global constraints seem to matter

30. Soft Physics = Hydrodynamics? t = 0.0 fm/c Incoming nuclei: Npart, Lorentz contraction t = 0.1 fm/c Rapid thermalization: Entropy production t < 0.6 fm/c 1D expansion stage: Rapidity distributions t = 0.6 fm/c 3D expansion stage:Elliptic & radial flow t = 6-10 fm/c Freezeout into hadrons: Statistical phenomenology System is strongly interacting throughout(conserving entropy the whole time!)

31. Paths to Progress • How do we understand differences and similarities between A+A and p+p, e+e-? • Is hydrodynamics in conflict with Color Glass Condensate? • How does this system thermalize so rapidly? • Which degrees of freedom thermalize and when? • Partons, hadrons, or something else (G. Brown)? • How can we integrate the longitudinal and transverse physics? • For now, study systematics of initial state, EOS, final state • Ultimately, need 3D hydrodynamic calculations starting at the earliest times  no parameters! • Data over a broad rapidity range with PID is essential • Soft physics is global physics: y = 0 may not be special

32. Extra Slides

33. Longitudinal Transverse y y x z pT pz Longitudinal dynamics provide initial conditionsfor transverse dynamics

34. Energy Density Energy densityrelated toenergy creatednear h=0

35. Centrality Dependence 200/19.6 200/130 Changes in one rapidity region are correlatedwith particles in distant regions Evolution of particle density with centrality is energy-independent

36. Available Energy BRAHMS data suggests only75% “available energy” Contradiction? Possibly. SLD: Leading K± inss jets ~1.5 units fromend of rapidity range Do we consider this to NOT bepart of the jet?

37. Even More Similarities: p+p, e+e- Limiting fragmentation is a general feature of strong radiationMay explain similarity of multiplicity & energy dependence

38. Sizes & Shapes Reactionplane studiesshow ellipticalshape STARnucl-ex/0312009PRL in press HBT correlations in A+Ashow similar informationat all beam energies Rs ~ Ro ~ 6 fm

39. Expansion Dynamics Ro kT(mT)-dependence probesradial expansion Difficult for boost-invarianthydrodynamic calculations Rs

40. Total Multiplicity vs. Models

43. Total Multiplicity in A+A

44. Coalescence at Moderate pT Molnar

45. Two-Particle Correlations 200GeV Au-Au data pt 0.15-2 GeV/c peripheral Trainor central STAR preliminary pT correlations  “Neck Formation” from minijets:Coupling of “hard” with “soft”, perhaps via energy loss

46. Is the QGP Strongly Interacting? McLerran

47. Becattini

48. Becattini

49. Transport & Coalescence Molnar

50. Global Fits to Heavy Ion Data Renk T (GeV) t (fm/c) Even simple modelscan be extended to covera large variety of data