Probing High-Temperature QCD Matter at RHIC (Relativistic Heavy-Ion Collider)

1. Probing High-Temperature QCD Matterat the Relativistic Heavy-Ion Collider (RHIC) Saskia Mioduszewski 18 September 2008

2. Group Members Postdocs: Rory F. Clarke Ahmed Hamed Graduate Students: Matthew Cervantes Martin Codrington (Chemistry) Supported by D.O.E. and Sloan Foundation

3. nuclear matter p, n Goal of RHIC: To Study Fundamental Puzzles of Hadrons • Confinement • Quarks do not exist as free particles • Generation of mass • Free quark mass ~ 5-7 MeV • Quarks become “fat” in hadrons, constituent mass ~ 300-400 MeV • Complex structure of hadrons • Sea anti-/quarks • Gluons • Origin of Spin of the nucleon These phenomena must have occurred with formation of hadrons

4. ~ 10 ms after Big Bang Hadron Synthesis strong force binds quarks and gluons in massive objects: protons, neutrons mass ~ 1 GeV/c2 ~ 100 s after Big Bang Nuclear Synthesis strong force binds protons and neutrons in nuclei

5. Expectation from Numerical Simulations of Finite-Temperature QCD Stefan-Boltzmann limit Expectation: create a “weakly coupled gas of quarks and gluons” by reaching Tc in high-energy heavy-ion collisions

6. (Year 2000) New State of Matter created at CERN At a special seminar on 10 February, spokespersons from the experiments on CERN's Heavy Ion programme presented compelling evidence for the existence of a new state of matter in which quarks, instead of being bound up into more complex particles such as protons and neutrons, are liberated to roam freely. Pb+Pb collisions at √sNN = 17 GeV at the SPS

7. early universe T RHIC & LHC Quark Matter SPS TC~170 MeV AGS SIS Hadron Resonance Gas Color Superconductor temperature Nuclear Matter neutron stars baryon chemical potential 1200-1700 MeV 940 MeV mB “Travel” Back in Time Quest of heavy-ion collisions: heat and compress nuclear matter • create Quark Gluon Plasma (QGP) as transient state in heavy ion collisions (e.g. Au+Au collisions) • verify existence of QGP • study properties of QGP • study QCD confinement and how hadrons get their masses RHIC & SPS

8. Relativistic Heavy Ion Collider • RHIC was proposed in 1983 • RHIC began providing collisions in 2000 • √sNN = 200 GeV = 10 x Collision-Energy at SPS • New probe available High-pT particles from “hard” scattering

9. RHIC Specifications • 3.83 km circumference • Two independent rings • 120 bunches/ring • 106 ns crossing time • Capable of colliding ~any nuclear species on ~any other species • Energy: • 22-500 GeV for p-p • 5-200 GeV for Au-Au(per N-N collision) • Luminosity • Au-Au: 5 x 1027 cm-2 s-1 • p-p : 1.5x1032 cm-2 s-1(polarized)

10. STAR The RHIC Experiments

11. PHENIX

12. STAR

14. Characterizing the collisions • Different centralities, i.e. size of overlap region • Asymmetry of reaction zones • How does the matter behave? • Can we probe the matter that exists only for a short time?

15. Not all A+A collisions are the same -- “Centrality” Spectators Participants For a given b, “billiard ball” model predicts Npart (No. participants) and Nbinary (No. binary collisions) 15 fm b 0 fm 0 Npart 394 0 Nbinary ~1000

16. Kinematics for colliders Pseudo-rapidity: Mid-rapidity: η = 0, perpendicular to the incident beams η = 4: Scattering at θ = 2.1o in the CM (or lab) frame Transverse momentum (pT) and pseudorapidity () provide a convenient description

17. Radial Flow – Collective Expansion of system due to pressure – Heavier particles shifted to higher pT – Observable: <bT> from slopes as a function of mass and/or centrality – Spectra can be described by hydrodynamic models for pT< 2-3 GeV/c and mid-peripheral to central events

18. <bT> Single Particle Spectra (low pT) • Decreasing slope for increasing particle mass and centrality central peripheral

19. Momentum space: final asymmetry Coordinate space: initial asymmetry py multiple collisions (pressure) px Elliptic Flow in Non-central Collisions • Early state manifestation of collective behavior: • Asymmetry generated early in collision, quenched by expansion  observed asymmetry emphasizes early time Fourier Expansion: dN/df ~ 1 + 2 v2(pT) cos (2 f) + ... Second Fourier coefficient v2:

20. Data compared to Hydro Hydrodynamics with 0 viscosity f Reaction Plane (Angle Y2) v2 pT [GeV/c]  Thermalization in < 1 fm/c

21. How does the expected “Quark Gluon Plasma” compare with the “Perfect Fluid” that we have found at RHIC? Can we quantify the properties of this new form of matter?

22. Same behavior as observed in gases of strongly coupled Li atoms K. M. O’Hara et al, Science 298, 2179 The matter we have created at RHIC behaves like a strongly coupled fluid, not like “weakly coupled gas of quarks and gluons”

23. How small can viscosity be? Conjectured lower bound on viscosity/entropy = 1/4p AdS/CFT for calculating properties of strongly-coupled gauge theories P.K. Kovtun, D.T. Son, and A.O. Starinets, Phys. Rev. Lett. 94:111601, 2005. h/S [1/4p] RHIC “fluid” mightbe at ~2-3 on this scale (at T~1012 K)

24. Probing the Medium The QCD analogue of x-ray tomography • Need an external calibrated source • Calculate absorption cross sections •  Interpret the results

25.    quark or gluon “Hard” processes to probe the matter • Large momentum transfer – or close distance • Can resolve partons: valence quarks, sea quarks and gluons – scattered parton fragments into a “jet” • Coupling is weak - pQCD applicable Jet Fragmentation Function

26. cone of hadrons “jet” increased gluon-radiation within plasma? p p Hard scattering Jets in heavy ion collisions hard-scattered parton in p+p hard-scattered parton during Au+Au hadron distribution softened, jets broadened?

28. Thermally-shaped Soft Production Hard Scattering Production cross section of p0 p+p collisions = “baseline” • Good agreement with NLO perturbative QCD calculations • High pT particle yields serve as a calibrated probe of the nuclear medium in nucleus+nucleus (A+A) and deuteron+nucleus (d+A) collisions

29. “no effect” Systematizing Our Expectations • Describe in terms of scaled ratio RAA= 1 for “baseline expectations” • Will present most of Au+Au and d+Au data in terms of this ratio

30. peripheral Nbinary = 12.3  4.0 central Nbinary = 975  94 Discovery of Strong Suppression Scaling of calibrated probe works in peripheral Au+Au, but strong suppression in central Au+Au

31. Nuclear Modification Factor RHIC 200 GeV central - Suppression peripheral – Nbinary scaling  Comparison of peripheral to central binary scaling

32. * Note deuteron-gold control experiment with no suppression Theoretical Understanding? Understood in an approach that calculates energy loss of hard-scattered parton through gluon radiation in a dense partonic medium (15 GeV/fm3 ~100 x normal nuclear matter) Au+Au suppression (I. Vitev and M. Gyulassy, hep-ph/0208108) d+Au enhancement(I. Vitev, nucl-th/0302002 ) • Our high pT probeshave been calibratedand are now being used to explore propertiesof the medium d-Au Au-Au

33. What have we learned? • Nuclear matter created at RHIC is very opaque and dense (estimates of 100 x normal nuclear matter density) • Strong collective behavior • Coupling must be strong for v2 to be so large Now we want to characterize this new matter more quantitatively (viscosity, transport coefficients, color charge density)

34. Jet Reconstruction in Au+Au Collisions e+e- q q (OPAL@LEP) pp jet+jet (STAR@RHIC) Au+Au ??? (STAR@RHIC)

35. Jet Studies via Correlations pT,trig > 4 GeV/c dN/d  pT,assoc = 2-4 GeV/c -p/2 0 p  pT,trig – pT of the trigger particle pT,assoc bin – range of pT selected to associate with the trigger particle

36. Escaping Jet -“Near Side” Au+Au peripheral Au+Au central Lost Jet -“Away Side” Azimuthal distributions in Au+Au pedestal and flow subtracted Phys Rev Lett 90, 082302 Near-side: peripheral and central Au+Au similar to p+p Strong suppression of back-to-back correlations in central Au+Au collisions

37. “Reappearance of away-side jet” With increasing trigger pT, away-side jet correlation reappears Increasing pT,trig 4 < pT,trig< 6 GeV/c, 2< pT,assoc< pT,trig Increasing pT,assoc

38. Or just tangential emission ? Punch-through Jet ? Medium Modification to Fragmentation Function Are we probing the medium? Or is it simply too opaque? 8 < pT,trig< 15 GeV/c, pT,assoc > 6 GeV/c Centrality

39. g increased gluon-radiation within plasma? Hard Scattering  g + jet Is there any particle not affected by the opaque medium? • If g is produced in hard scattering, instead of q or g, expect it to escape without interaction  calibrated probe • Then could study jet on opposite side as a function of the energy of photon

40. Effect of Dense Medium on Direct Photons Hadrons are suppressed, photons are not – photons serve as the “control” experiment PHENIX, Phys. Rev. Lett. 96, 202301 (2006)

41.  0 Fragmentation Function FragmentationFunction-Study the particle distribution in a jet • Calculate yields as a function of pT,assoc/pT,trig from correlation function • Compare distribution in vacuum to medium to look for medium modification g (Einitial) g-rich triggers p0 triggers Integrate yields Modified Jet

42. Direct  0 Associated yields per trigger Direct Measure of Medium Modification to Fragmentation Function A. Hamed, Hard Probes 2008 g (Einitial) Modified Jet

43. STAR Preliminary Ratio of Central Au+Au to Peripheral (~ Medium/Vacuum) Jet Yields Within the current uncertainty in the scaling the medium effect on jets associated to a direct  trigger is similar to jets associated to 0 trigger.

44. Summary • RHIC has been successfully operating since 2000 • The expectation of QGP as a weakly coupled gas of quarks and gluons has been challenged by data • Medium created is strongly interacting (liquid-like) and very opaque • Currently experiments are trying to make measurements that can characterize the medium properties more quantitatively • g+jetmeasurement holds promise to be one of such probes • Higher luminosity needed for definitive g+jet measurement • Future at RHIC is exciting

45. Extra Slides

46. Results: Method of extract direct  associated yield O(αs2α(1/αs+g))  0 away away Y+h = (Y-rich+h - RY0+h )/(1-R) near near R=Y-rich+h/Y0+h This procedure removes correlations due to contamination (asymmetric decay photons+fragmentation photons) with assumption that correlation is similar to 0 – triggered correlation at the same pT. Extraction of direct away-side yields • Assume no near-side yield for direct then the away-side yields per trigger obey A. Hamed STAR Experiment ICHEP08 Philadelphia, PA July 29th -August 5th.

47. This atomic system may also be near the bound. T. Schafer, arXiv:0707.1540v1 (2007).

48. What do we learn from RAA? Effect of collision medium on hadron pT spectra: • Parton scattering with large momentum transfer  Hard-scattered partons (jets) present in early stages of collisions • Hot and dense medium  Hard-scattered partons sensitive to hot/dense medium Theory predicts radiative energy loss of parton in QGP • Emission of hadrons  High pT hadrons (jet fragments) Dense medium (QGP) would cause depletion in spectrum of leading hadron at high pT - “jet quenching”

49. X-N. Wang, Phys. Rev. C58 (1998) 2321 High-pT Predictions It has been predicted that jet production will be affected by medium effects due to the production of hot dense matter in high energy relativistic heavy ion collisions

50. Scaling from p+p to A+A • For hard-scattering processes, expect point-like scaling. For inclusive cross sections : • For semi-inclusive yields, expect :