1. Probing High-Temperature QCD Matter at 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. 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 nuclear matter p, n

4. ~ 10  s after Big Bang Hadron Synthesis strong force binds quarks and gluons in massive objects: protons, neutrons mass ~ 1 GeV/c 2 ~ 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 T c in high-energy heavy-ion collisions

6. 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. (Year 2000) Pb+Pb collisions at √s NN = 17 GeV at the SPS

7. “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 neutron stars Quark Matter Hadron Resonance Gas Nuclear Matter Color Superconductor SIS AGS SPS RHIC & LHC early universe BB T T C ~170 MeV 940 MeV 1200-1700 MeV baryon chemical potential temperature RHIC & SPS

8. Relativistic Heavy Ion Collider RHIC was proposed in 1983 RHIC began providing collisions in 2000 √s NN = 200 GeV = 10 x Collision-Energy at SPS  New probe available High-p T 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: è 22-500 GeV for p-p è 5-200 GeV for Au-Au (per N-N collision) Luminosity –Au-Au: 5 x 10 27 cm -2 s -1 –p-p : 1.5x10 32 cm -2 s -1 (polarized)

10. The RHIC Experiments STAR

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

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

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

18. Single Particle Spectra (low p T ) Decreasing slope for increasing particle mass and centrality peripheral central

19. 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/d  ~ 1 + 2 v 2 (p T ) cos (2  ) +... Second Fourier coefficient v 2 : Coordinate space: initial asymmetry Momentum space: final asymmetry multiple collisions (pressure) pypy pxpx

20. Data compared to Hydro Reaction Plane (Angle  2 )  Hydrodynamics with 0 viscosity  Thermalization in < 1 fm/c p T [GeV/c] v2v2

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 The matter we have created at RHIC behaves like a strongly coupled fluid, not like “weakly coupled gas of quarks and gluons” K. M. O’Hara et al, Science 298, 2179

23.  /S [1/4  ] AdS/CFT for calculating properties of strongly-coupled gauge theories RHIC “fluid” might be at ~2-3 on this scale (at T~10 12 K) How small can viscosity be? Conjectured lower bound on viscosity/entropy = 1/4  P.K. Kovtun, D.T. Son, and A.O. Starinets, Phys. Rev. Lett. 94:111601, 2005.

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

25. “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  quark or gluon   Jet Fragmentation Function

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

28. Thermally- shaped Soft Production Hard Scattering Good agreement with NLO perturbative QCD calculations High p T particle yields serve as a calibrated probe of the nuclear medium in nucleus+nucleus (A+A) and deuteron+nucleus (d+A) collisions Production cross section of  0 p+p collisions = “baseline”

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

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

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

32. Theoretical Understanding? Understood in an approach that calculates energy loss of hard- scattered parton through gluon radiation in a dense partonic medium (15 GeV/fm 3 ~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 p T probes have been calibrated and are now being used to explore properties of the medium Au-Au d-Au * Note deuteron-gold control experiment with no suppression

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

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

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

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

39. Hard Scattering   + jet increased gluon-radiation within plasma?  If  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 Is there any particle not affected by the opaque medium?

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 Fragmentation Function - Study the particle distribution in a jet  initial  Modified Jet Calculate yields as a function of p T,assoc /p T,trig from correlation function Compare distribution in vacuum to medium to look for medium modification Integrate yields  -rich triggers  0 triggers

42. Direct Measure of Medium Modification to Fragmentation Function  initial  Direct  00 Associated yields per trigger Modified Jet A. Hamed, Hard Probes 2008

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  +jet measurement holds promise to be one of such probes Higher luminosity needed for definitive  +jet measurement Future at RHIC is exciting

45. Extra Slides

46. 0  Extraction of direct  away-side yields R=Y  -rich+h /Y  0+h near Y  +h = (Y  -rich+h - RY  0+h )/(1-R) away Assume no near-side yield for direct  then the away-side yields per trigger obey A. Hamed STAR Experiment ICHEP08 Philadelphia, PA July 29 th -August 5 th. Results: Method of extract direct  associated yield 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 p T. O(α s 2 α(1/α s +g))

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

48. What do we learn from R AA ? Effect of collision medium on hadron p T 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 p T hadrons (jet fragments) Dense medium (QGP) would cause depletion in spectrum of leading hadron at high p T - “jet quenching”

49. High-p T Predictions X-N. Wang, Phys. Rev. C58 (1998) 2321 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 :

51.  0 -   dN / d  Elliptic flow Jet Studies via Correlations p T trig – p T of the trigger particle p T assoc bin – range of p T selected to associate with the trigger particle p T trig > 4 GeV/c p T assoc = 2-4 GeV/c

52. An example of N binary ~ A*B scaling Small cross section processes scale as though scattering occurs incoherently off nucleons in nucleus scale as A 1.0 in  +A scale as N binary ~A*B in A+B 7.2 GeV muons on various targets. M. May et al., Phys. Rev. Lett. 35, 407, (1975)

53. “Binary-Scaling” and R AA Define Nuclear Modification Factor R AA  Effect of nuclear medium on yields The probability for a “hard” collision for any two nucleons is small The total probability in A+A collision is multiplied by the number of times we try, i.e. – the cross-section scales with the number of binary collisions - N binary

54. Yield of  0 measured by PHENIX p+p collisions Au+Au collisions

55. Evolution of Jet Structure At higher trigger p T (6 < p T,trig < 10 GeV/c), away-side yield varies with p T,assoc For lower p T,assoc ( 1.3 < p T,assoc <1.8 GeV/c), away-side correlation has non-gaussian shape  becomes doubly-peaked for lower p T,trig pedestal and flow subtracted 4 < p T,trig < 6 GeV/c, 2 < p T,assoc < p T,trig