1. Penetrating Probes of the Quark-Gluon Plasma in Nuclear Collisions at RHIC and the LHC Prof. Brian A. Cole. Columbia University PHENIX and ATLAS

2. Fundamental Interactions Consider: Only “matter” that we can study in the lab has properties determined almost entirely by EM “force”. Caveat: nuclei – but the force is not truly fundamental (e.g. no single gauge boson)

3. RHIC Physics - Background Size scales Atom – 10 -10 m Nucleus – 10 -14 m Nucleon – 10 -15 m (1 fm) Quarks, electrons –Have no “size” per se Mass Scales (MeV/c 2 ) Electron – 0.511 Quark (up) ~ 5 Nucleon – 939 Au Nucleus – 1.8 x 10 5 Physical Constants ħ c  0.2 GeV fm p CuAu

4. Strong Interactions at a Glance Hadrons are composed of quarks Quarks carry “color” charge. –Gluons mediate interaction. gluons also carry “color”.  Gluons couple to gluons @ large r gluon field between quarks collapses into “flux tube”  “Confining” potential What about other combinations? –e.g. penta-quark –Or, how about chilioi-quark ….

5. QCD Thermodynamics (on Lattice) Rapid cross-over from “hadronic matter” to “Quark-Gluon Plasma” at T  170 MeV (  ~ 1 GeV/fm 3 ). Energy density / T 4 saturates rapidly –Except in real-world case of 2+1 flavor ?? Pressure / T 4 saturates less rapidly.  Strongly coupled medium @ RHIC ? Energy Density / T 4 Pressure / T 4

6. “Primordial” Quark Gluon Plasma The Early Universe, Kolb and Turner Thermodynamic degeneracy factor QCD Transition

7. Relativistic Heavy Ion Collider  Run 1 (2000): Au-Au  S NN = 130 GeV  Run 2 (2001): Au-Au, p-p  S NN = 200 GeV  Run 3 (2003): d-Au, p-p  S NN = 200 GeV  Run 4 (2004): Au-Au  S NN = 200, 64 GeV, p-p  S NN = 200 GeV  Run 5 (2005): Cu-Cu  S NN = 200, 64 GeV, p-p  S NN = 200 GeV STAR

8. RHIC Initial Conditions Au+Au @ 200 GeV per nucleon,  = E/m  100. –Au diameter, d  14 fm, contracted d/   0.2 fm  Crossing time < 0.2 fm/c. Add QM:  E  ħ c /  t –Fluctuations with  E > 1 GeV are “on shell”  These are primarily gluons (~ 200)  RHIC is a gluon collider! (10 GeV/fm 3 )

9. RHIC: Au+Au Collision Simulation

10. Use self-generated quarks/gluons/photons as probes of the medium (classic physics technique!) Penetrating Probes of Created Matter z t Collisions between partons

11. Perturbative quantum chromo-dynamics Factorization: separation of  into –Short-distance physics: – calculable using perturbation expansion ** –Long-distance physics:  ’s – universal, can be measured separately. Valid @ large momentum transfer – high p T outgoing particles p-p di-jet Event STAR From Collins, Soper, Sterman Phys. Lett. B438:184-192, 1998

12. pQCD – Single Hadron Production Add fragmentation to hadrons D(z) – fractional momentum dist. of particles created by outgoing quark or gluon (i.e. in a jet) KKP Kretzer data vs pQCD Phys. Rev. Lett. 91, 241803 (2003)

13. How to directly probe medium ? Use quarks & gluons from high-Q 2 scattering –“Created” at very early times (~ 0.1 fm). –Propagate through earliest, highest  matter. (QCD) Energy loss of (color) charged particle –~ Entirely due to radiation –Virtual gluon(s) of quark multiply scatter. e.g. GLV (Gyulassy, Levai, Vitev) formalism Experimentally measure using:  (Leading) high-p  hadrons  Di-jet correlations

14. Central Arms PHENIX Experiment @ RHIC Two spectrometers measuring at 90° Two (forward/backward muon) spectrometers Optimized for rare/penetrating probes. Drift Chamber Pad Chambers RICH EM Calorimeter Time of flight

15. Central Arms In Action  0  The pion is the lightest and most copiously produced meson.  0 :

16. Au-Au  0 Spectra From PHENIX Calculations with no energy loss Calculations with energy loss Observe only 20% of expected yield @ high p T  Energy density ~15 Gev/fm 3  100 x normal nuclear energy density!!  Reminder: critical energy density ~ 1 GeV/fm 3 Transverse Momentum spectrum Expected R AA  Observed/Expected Using p-p data as baseline

17. PHENIX: Au-Au High-p T  0 Suppression  constancy for pT > 4 GeV/c for all centralities? We are now measuring out to truly high p T

18.  0 Suppression: dE/dx Comparisons Quark & gluon dE/dx analysis: Turbide et al (McGill) –Essentially an ab initio calculation –Compared to precision (relatively) data

19. Crucial Control Measurement: Deuteron-Au

20. Prompt Photon Production Prompt photons provide an independent control measurement for jet quenching. –Produced in hard scattering processes –But, no final-state effects (???)

21. PHENIX p-p Prompt  Production Absolute comparison, no fudge factors. pQCD very well reproduces prompt  cross- section. Points: PHENIX Curve: PQCD

22. Au-Au Prompt Photon Production Large background to prompt  measurement –Primarily hadron decays (e.g.  )  total /  background Pions are quenched Photons aren’t Calculations of hard scattering rates in A+A collisions OK. High-p T hadron suppression must be due to jet quenching.

23. STAR Experiment: “Jet” Observations Number of pairs Angle between high energy particles 0º0º180º proton-proton jet event  In Au-Au collisions we see only one “jet” at a time !  How can this happen ?  Jet quenching! q q Analyze by measuring (azimuthal) angle between pairs of particles

24. But, Have We Created “Matter” ? “Pressure” converts spatial anisotropy to momentum anisotropy. Requires early thermalization. Unique to heavy ion collisions Answer: yes  dN/d  x y z

25. “Elliptic Flow” Parameterize azimuthal anisotropy by “v 2 ” parameter – Compare to “eccentricity”: Data consistent w/ hydrodynamic calculations

26. PHENIX QGP “White Paper” Already a year old …

27. “Perfect Fluid?” My view: Perfect fluid is reasonable interpretation of available data but there is room for skepticism.

28. Perfect Fluid? (2) What we do know The matter created in Au+Au collisions must thermalize and generate pressure in time < 1 fm/c. “perturbative” quark/gluon scattering cannot achieve such early thermalization. Thus, “collective” effects are needed. Possibilities: –Residual long-range interactions –Colored bound states (Shuryak) –“Color instabilities” – e.g. runaway growth of local chromo-magnetic fields –Thermalization ab initio ? (Hawking-like radiation from deceleration of initial gluons/fields)

29. (di)Jet Angular Correlations (PHENIX) PHENIX (nucl-ex/0507004): moderate p T

30. Di-jet Distortion vs “Impact Parameter”

31. Origin of di-jet Distortion? Mach cone? Jets may travel faster than the speed of sound in the medium. While depositing energy via gluon radiation. QCD “sonic boom”  Intensity

32. Mach Cone (2) Talk by J. Ruppert at Quark Matter 2005. Suggestive, but there are other possibilities (Cherenkov, coherent gluon radiation, di-jet quenching, …

33. Photon Production off the Plasma (?) The plasma mediates a jet-photon conversion

34. Jet-Conversion Photons: Data There may already be room for jet-conversion contribution in the PHENIX prompt photon data. More detailed studies underway – stay tuned.

35. What I Didn’t Show You Charm quarks also are quenched –And show rapid thermalization! Large charm quark elliptic flow signal –Can only be established at the quark level. Large baryon excess for 2 < p T < 5 GeV/c – Hadron formation by quark recombination We see final state particle flavor distributions consistent with “freeze-out” from chemically equilibrated system. We are rapidly approaching stage where QGP is ONLY viable interpretation of data.  End of the beginning @ RHIC

36. RHIC: Penetrating Probes We are directly observing results of colored probes propagating in colored matter. – Calibrated by photon measurements, d-Au. – ~ 100x energy density of normal nuclei – >> energy density for “phase transition” Matter exhibits strong collective motion – Due to chromo-magnetic instabilities?? – Due to colored bound states?? We are studying first “fundamental” matter that interacts non electromagnetically! We may be seeing collective response of medium to energetic particles (Mach cone?)

37. New Opportunity: Pb+Pb @ LHC w/ ATLAS

38. Why Heavy Ions @ LHC? Low x – Gluon production from saturated initial state Energy density – ~ 50 GeV/fm 3 (?) Rate – “copious” jet production above 100 GeV Jets – Full jet reconstruction Detector – necessary detector “for free”!

39. Why ATLAS? Calorimetery!

40. Simulated Pb+Pb Event in ATLAS (No Jet)

41. Pb+Pb Jet Simulations in ATLAS

42. Heavy Ion Initial Conditions: Modern At very high energies, strong gluon production from gluon fields with large occupation #’s (~ classical) R. Venugopalan, LHC Heavy Ion Workshop @ PANIC

43. Jets as Color Antennas Studies of modified jets in heavy ion collisions may shed light on a “fundamental” problem in (particle) physcs A high-energy quark/gluon acts like a “color antenna” In vacuum, radiation strongly affected by quantum interference. But, in medium thermal gluons “regulate” radiation.

44. LHC Physics Opportunity Create & study quark-gluon plasma at T = 0.8~GeV Study particle production from strong gluon fields. New program with w/ new discoveries ~ guaranteed –If RHIC is any guide … p T reach, rates, detector capabilities at LHC allow for qualitatively different (better!) measurements. Overlap w/ many other sub-fields of physics –Particle physics –Plasma physics –Fluid/hydro dynamics –Thermal field theory, lattice & non-lattice –String theory (!?) – AdS-CFT correspondence –General relativity ???