1. Gary D. Westfall 1 New Experimental Results from the Relativistic Heavy Ion Collider - The Perfect Fluid Gary D. Westfall Michigan State University Fermilab Colloquium July 19, 2006

2. Gary D. Westfall 2 RHIC Collisions RHIC is the Relativistic Heavy Ion Collider located at Brookhaven National Laboratory on Long Island, New York Collisions of heavy nuclei at very high energies offer the possibility of transforming the protons and neutrons in the nuclei into a fluid consisting of quarks and gluons We will call this state of matter the quark-gluon plasma (QGP) It is thought that the universe existed as a QGP a few  s after the Big Bang

3. Gary D. Westfall 3 Protons, Neutrons, and Quarks

4. Gary D. Westfall 4 Relation to the Big Bang Benchmarks Energy density 1 GeV/fm 3  1.8  10 15 g/cm 3 Temperature 170 MeV  2.0  10 12 K Conditions that prevailed  10  s after the Big Bang NSCL-RIA

5. Gary D. Westfall 5 StrategyStrategy Look for a novel state of matter created in central Au+Au collisions at top RHIC energies Compare to situations in which the novel state of matter should not be created Lower incident energies Peripheral Au+Au collisions p+p collisions d+Au collisions

6. Gary D. Westfall 6 Central Au+Au Collision at RHIC Study state of matter created early in central Au+Au collisions Equilibrium and bulk properties Elliptic flow and hydrodynamics Jet suppression initial state pre-equilibrium QGP and Hydro. expansion Hadronization Elastic scattering and kinetic freeze-out Hadronic interaction and chemical freeze-out 3000 particles created in a central Au+Au collisions at RHIC 14 fm = 14  10 -15 m  = 100  t = 10 fm/c = 3.3  10 -23 s

7. Gary D. Westfall 7 Lattice QCD O. Kaczmarel et al., Phys. Rev. D 62, 034021 (2000) T c =170 MeV e C =0.5 GeV/fm 3 F. Karsch, Nucl. Phys. A698, 199c (2002).

8. Gary D. Westfall 8 The Relativistic Heavy Ion Collider

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13. Gary D. Westfall 13 The White Papers All four groups concurrently published "white paper" summaries of their work in the journal Nuclear Physics A These papers describe the RHIC experimental groups’ perspectives on discoveries at RHIC based on the first three years of RHIC running Quark-gluon plasma and color glass condensate at RHIC? The perspective from the BRAHMS experiment Nucl. Phys. A757, 1 (2005). The PHOBOS perspective on discoveries at RHIC Nucl. Phys. A757, 28 (2005). Experimental and theoretical challenges in the search for the quark-gluon plasma: The STAR Collaboration’s critical assessment of the evidence from RHIC Collisions Nucl. Phys. A757, 102 (2005). Formation of dense partonic matter in relativistic nucleus-nucleus collisions at RHIC: Experimental evaluation by the PHENIX Collaboration Nucl. Phys. A757, 184 (2005).

14. Gary D. Westfall 14 Equilibrium in RHIC Collisions All of our ideas about the QGP rest on the idea that the system is in equilibrium Chemical equilibrium Represented by the relative yield of particles Temperature, chemical potential, partition function Kinetic equilibrium Represented by p t spectra Temperature, expansion velocity “Blast Wave” Colliding nuclei together at RHIC energies produces very hot, dense, and expanding matter

15. Gary D. Westfall 15 Chemical Equilibrium at RHIC Assume thermally (constant T) and chemically (constant density n i ) equilibrated system at chemical freeze-out Assume that the system is composed of non- interacting hadrons and resonances The assumption of constant n i leads to chemical potentials  Given T and  ’s and the system size, n i ’s can be calculated as a grand canonical ensemble

16. Gary D. Westfall 16 Chemical Equilibrium 200 GeV p+p 200 GeV Au+Au  In p+p, particle ratios are well described  In Au+Au, only stable particle ratios are well described Strangeness enhancement Resonance suppression Strangeness suppression

17. Gary D. Westfall 17 Chemical Freeze-out @ 200 GeV ,K,p ,K,p,  Close to chemical equilibrium ! Close to net-baryon free

18. Gary D. Westfall 18 Blast Wave E.Schnedermann et al, PRC48 (1993) 2462. Blast-wave model: , K, p  T= 90MeV,   T=160MeV, 

19. Gary D. Westfall 19  Sudden Single Freeze-out ?* Kinetic Freeze-out @ 200 GeV Radial flow velocity Kinetic FO temperature ,K,p: T kin decreases with centrality  T kin = const ,  and  flow

20. Gary D. Westfall 20 Temperature and Energy Density Very high temperatures are created in RHIC collisions Very high energy densities are created in RHIC collisions

21. Gary D. Westfall 21 The QGP Shines The QGP can be thought of as a blackbody radiating photons However, there is a huge background of photons from the decay of particles like the  0 With some work, PHENIX has extracted a spectrum of direct photons from central Au+Au collisions at 200 GeV To get a feeling for what we are seeing, let’s go to the Wien Displacement Law

22. Gary D. Westfall 22 Equilibrium Conclusions We reach temperatures and densities consistent with QGP formation We see chemical equilibration with some exceptions Multi-strange baryons seem to freeze-out a a different time We see strong, bulk flow characterized by a kinetic temperature and flow velocity Even multi-strange baryons flow Suggestive of hydrodynamic-like behavior

23. Gary D. Westfall 23 Hydrodynamic Flow Now we will try to quantify the hydrodynamic behavior suggested by the observed blast wave We will study hydrodynamic behavior using elliptic flow, also called the azimuthal anisotropy We start by defining a reaction plane and an impact parameter

24. Gary D. Westfall 24 Reaction Plane and Impact Parameter Reaction Plane Impact Parameter

25. Gary D. Westfall 25 Elliptic Flow x y z pxpx pypy y x Look at non-central collisions Overlap region is not symmetric in coordinate space Almond shaped overlap region Larger pressure gradient in x-z plane than in y direction Spatial anisotropy -> momentum anistropy Process quenches itself -> sensitive to early time in the evolution of the system Sensitive to the equation of state Perform a Fourier decomposition of the momentum space particle distributions in the x-y plane v n is the n th harmonic Fourier coefficient of the distribution of particles with respect to the reaction plane v 1 : directed flow v 2 : elliptic flow

26. Gary D. Westfall 26 Hydrodynamic Expansion In this hydrodynamic calculation, the initial coordinate space is transformed into momentum space anisotropy (arrows) Because the hydro calculations have no viscosity, the resulting azimuthal anisotopy is as large as it can be x (fm) y (fm)

27. Gary D. Westfall 27 Elliptic Flow Note agreement of hydro model absolute value scaling with mass of particle

28. Gary D. Westfall 28 The Perfect Liquid The hydro model reproduces v 2 for the bulk of the produced particles Hydro overpredicts v 2 at lower incident energies The hydro model incorporates zero viscosity Zero mean free path Evidence for the production of a perfect liquid in RHIC collisions

29. Gary D. Westfall 29 How Perfect? Calculations of viscosity of hadron gas and quark gluon plasma compared with water Hadron Gas QGP Csernai, Kapusta, McLerran, nucl-th/0604032

30. Gary D. Westfall 30 Hydro Over-Predicts v 2 at Lower Energies NA49, Phys. Rev. C68, 034903 (2003)  is the eccentricity of the overlap region S is the area of the overlap region Color percolation point (based on mean-free path arguments) Pb+Pb 17 GeV

31. Gary D. Westfall 31 Quark Coalescence in Flow Evidence that v 2 arises at the quark/gluon level

32. Gary D. Westfall 32 Elliptic Flow From M. Gyulassy

33. Gary D. Westfall 33 Conclusions from Elliptic Flow We see hydrodynamic flow for low p t particles Agreement with hydrodynamics with no viscosity implies we see a perfect fluid at RHIC At RHIC energies, hydrodynamics predicts the magnitude of elliptic flow At intermediate p t, we seem to see quark coalescence in elliptic flow Evidence for partonic origin of flow Hydrodynamic behavior not observed at high p t and away from mid-rapidity New calculations are coming out using color glass condensate to predict initial conditions for elliptic flow Initial eccentricity is higher May need some viscosity to reproduce experimental results Stay tuned….

34. Gary D. Westfall 34 Jets in p+p Collisions In p+p collisions, hard quark/gluon scattering can produce back-to-back jets A jet from a p+p collision at 200 GeV in STAR

35. Gary D. Westfall 35 Jets in Au+Au Collisions In Au+Au collisions, hard scattering can also produce jets These jets become an internal probe for the newly created state of matter The produced particles must now traverse the matter produced in the collision to be observed In case the presence of the gold nucleus causes strong effects, we will compare with d+Au

36. Gary D. Westfall 36 Jet Suppression - Nuclear Modification Factor We can study jet suppression using leading hadrons We define a nuclear modification factor, R AA, in terms of the ratio of the p t spectra in nucleus-nucleus collisions divided by the p t spectra in p+p collisions We also define a nuclear modification factor, R CP, in terms of the ratio of the p t spectra in central nucleus-nucleus collisions divided by the p t spectra in peripheral nucleus- nucleus collisions If naïve binary scaling applies, we should get 1 for these factors as a function of p t

37. Gary D. Westfall 37 Suppression of Leading Hadrons The combined data from Runs 1-3 at RHIC on p+p, Au+Au and d+Au collisions establish that a new effect (a new state of matter?) is produced in central Au-Au collisions Au + Au Experimentd + Au Control Experiment Preliminary DataFinal Data

38. Gary D. Westfall 38 Detecting charm/beauty via semileptonic D/B decays Hadronic decay channels: D 0  K  D *  D 0  D +/-  K  Non-photonic electrons: Semileptonic channels: c  e + + anything (B.R.: 9.6%) D 0  e + + anything(B.R.: 6.87%) D   e  + anything(B.R.: 17.2%) b  e + + anything(B.R.: 10.9%) B   e  + anything(B.R.: 10.2%) Drell-Yan (small contribution for p T < 10 GeV/c) Photonic electron background :  conversions (  0   e  e   )  0,  ’ Dalitz decays ,  … decays (small) K e3 decays (small)

39. Gary D. Westfall 39 Charm: Electron suppression Suppression is approximately the same as for hadrons! Indicates necessity of collisional energy loss mechanism? Charged Hadrons Electrons STAR, H. Zhang @ SQM06 STAR, Phys Rev Lett 91 (2003) 072304

40. Gary D. Westfall 40 Jet Suppression - Azimuthal Correlations We can study jet suppression using azimuthal correlations We start with a high p t trigger particle We look at the azimuthal correlations of particles in coincidence with the trigger particles Jets will show up as correlated particles at an azimuthal angle of 180  from the trigger particle “back-to-back” The jets particles on the away-side must travel through the produced matter Can provide information about the produced matter

41. Gary D. Westfall 41 Suppression of Away-Side Jets

42. Gary D. Westfall 42 Azimuthal Dependence of Jet Suppression out-of-plane in-plane

43. Gary D. Westfall 43 Emergence of Dijets with Increasing p T (assoc) Emergence of Dijets with Increasing p T (assoc)  correlations (not background subtracted) Narrow peak emerges cleanly above vanishing background 8 < p T (trig) < 15 GeV/c p T (assoc) > 2 GeV/cp T (assoc) > 3 GeV/cp T (assoc) > 4 GeV/cp T (assoc) > 5 GeV/cp T (assoc) > 6 GeV/cp T (assoc) > 7 GeV/cp T (assoc) > 8 GeV/c

44. Gary D. Westfall 44 Hadron-triggered fragmentation functions Away-side D(z T ) suppressed, but shape unchanged ~0.54 ~0.25 Scaling factors 8 < p T (trig) < 15 GeV/c

45. Gary D. Westfall 45 ConclusionsConclusions The four RHIC experiments have produced strong, consistent results, when combined with theoretical understanding, provide overwhelming evidence new state of matter based on Equilibrium Hydrodynamic behavior Jet suppression This new state of matter is clearly not what we expected when we started our journey, a weakly interacting QGP This new state of matter seems to be a strongly interacting, nearly-perfect fluid, a strongly interacting quark gluon plasma, something qualitatively new, totally unexpected The study of this new state may lead us in new and unexpected directions such string theory applications to RHIC collisions

46. Gary D. Westfall 46 Links for Further Information Today’s Colloquium http://www.nscl.msu.edu/~westfall/Westfall_FNAL.pdf RHIC http://www.bnl.gov/rhic STAR http://www.star.bnl.gov PHENIX http://www.phenix.bnl.gov PHOBOS http://www.phobos.bnl.gov BRAHMS http://www4.rcf.bnl.gov/brahms/WWW/ Quark Matter 2005 http://qm2005.kfki.hu/ My home page http://www.nscl.msu.edu/~westfall

47. Gary D. Westfall 47 Extra Slides

48. Gary D. Westfall 48 R CP at Lower Energies Suppression much less at lower energies

49. Gary D. Westfall 49 R AA Direct Photons

50. Gary D. Westfall 50 Au+Au 0-40% central Elliptic Flow vs Pseudorapidity and Energy  v2v2 PHOBOS nucl-ex/0406021(PRL in press)

51. Gary D. Westfall 51 AdS/CFT=Maldecena conjectured duality between weak gravity in 5D anti-de Sitter space and Infinite coupled conformal N=4 Supersymmetry field theory in 4D Miklos Gyulassy NSAC Presentation 4/5/05

52. Gary D. Westfall 52 J/  Suppression Compare SPS and RHIC results Very similar No model can reproduce both energies

53. Gary D. Westfall 53 Elliptic Flow of Heavy Quarks v 2 for non-photonic electrons

54. Gary D. Westfall 54 R dAu at Forward Rapidities PRL 93 242303 (2004) Cronin-like enhancement at  = 0 Clear suppression as  changes up to 3.2 Consistent with gluon saturation models

55. Gary D. Westfall 55 Heavy Quark Suppression STAR High-p T electron suppression is comparable to inclusive charged hadron suppression Gluons and light quarks Heavy quarks