New Experimental Results from the Relativistic Heavy Ion Collider - The Perfect Fluid

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. 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 thequark-gluon plasma (QGP) • It is thought that the universe existed as a QGP a few s after the Big Bang

3. Protons, Neutrons, and Quarks

4. NSCL-RIA Conditions that prevailed  10 s after the Big Bang Relation to the Big Bang • Benchmarks • Energy density • 1 GeV/fm3 • 1.81015 g/cm3 • Temperature • 170 MeV  • 2.01012 K

5. Strategy • 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. Elastic scattering and kinetic freeze-out Hadronic interactionand chemical freeze-out QGP and Hydro. expansion initial state Hadronization pre-equilibrium 3000 particles created in a central Au+Au collisions at RHIC 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  = 100 14 fm = 1410-15 m t = 10 fm/c = 3.310-23 s

7. Lattice QCD Tc=170 MeV eC=0.5 GeV/fm3 F. Karsch, Nucl. Phys. A698, 199c (2002). O. Kaczmarel et al., Phys. Rev. D 62, 034021 (2000)

8. The Relativistic Heavy Ion Collider

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. 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 pt spectra • Temperature, expansion velocity • “Blast Wave” • Colliding nuclei together at RHIC energies produces very hot, dense, and expanding matter

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

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

17. Chemical Freeze-out @ 200 GeV 200 GeV Au+Au Close to net-baryon free p,K,p Close to chemical equilibrium ! p,K,p,L,X

18. Blast Wave Blast-wave model: E.Schnedermann et al, PRC48 (1993) 2462. , K, p  T= 90MeV, b=0.6 X,   T=160MeV, b=0.45

19. Kinetic Freeze-out @ 200 GeV Kinetic FO temperature • Sudden Single Freeze-out ?* Radial flow velocity • p,K,p: Tkindecreases with centrality • X: Tkin = const • , X and W flow

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

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. 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. 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. Reaction Plane and Impact Parameter Impact Parameter Reaction Plane

25. y z x y py px x Elliptic Flow • 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 • vn is the nth harmonic Fourier coefficient of the distribution of particles with respect to the reaction plane • v1: directed flow • v2: elliptic flow

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 y (fm) x (fm)

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

28. The Perfect Liquid • The hydro model reproduces v2 for the bulk of the produced particles • Hydro overpredicts v2 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. How Perfect? • Calculations of viscosity of hadron gas and quark gluon plasma compared with water Csernai, Kapusta, McLerran, nucl-th/0604032 Hadron Gas QGP

30. Hydro Over-Predicts v2 at Lower Energies Pb+Pb 17 GeV • is the eccentricity of the overlap region S is the area of the overlap region (based on mean-free path arguments) Colorpercolationpoint NA49, Phys. Rev. C68, 034903 (2003)

31. Quark Coalescence in Flow Evidence that v2 arises at the quark/gluon level

32. Elliptic Flow From M. Gyulassy

33. Conclusions from Elliptic Flow • We see hydrodynamic flow for low pt 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 pt, we seem to see quark coalescence in elliptic flow • Evidence for partonic origin of flow • Hydrodynamic behavior not observed at high pt 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. A jet from a p+p collision at 200 GeV in STAR Jets in p+p Collisions • In p+p collisions, hard quark/gluon scattering can produce back-to-back jets

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

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 Experiment d + Au Control Experiment Final Data Preliminary Data

38. Detecting charm/beauty via semileptonic D/B decays • Hadronic decay channels:D0Kp, D*D0p, D+/-Kpp • Non-photonic electrons: • Semileptonic channels: • c  e+ + anything (B.R.: 9.6%) • D0  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 pT < 10 GeV/c) • Photonic electron background: • g conversions (p0 gg; g  e+e-) • p0, h, h’ Dalitz decays • r, f… decays (small) • Ke3 decays (small)

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

40. Jet Suppression - Azimuthal Correlations • We can study jet suppression using azimuthal correlations • We start with a high pt 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. Suppression of Away-Side Jets

42. Azimuthal Dependence of Jet Suppression in-plane out-of-plane

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

44. Hadron-triggered fragmentation functions • Away-side D(zT) suppressed, but shape unchanged Scaling factors ~0.54 ~0.25 8 < pT(trig) < 15 GeV/c

45. Conclusions • 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. 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. Extra Slides

48. RCP at Lower Energies Suppression much less at lower energies

49. RAA Direct Photons

50. Elliptic Flow vs Pseudorapidity and Energy Au+Au 0-40% central PHOBOS nucl-ex/0406021(PRL in press) v2 h