The Future of Heavy Ion Physics

1. The Future of Heavy Ion Physics Jamie Nagle, University of Colorado at Boulder

2. What have we learned experimentally in the first five years at RHIC? • What is the fundamental science of heavy ion physics in the next decade?

3. Color Glass Condensate When protons are viewed at short wavelength, there is a large increase in low x gluons. At high enough gluon density, we may find a universal description of the matter as a Color Glass Condensate. One can study this universal state of matter via Deep Inelastic Scattering, just one virtual parton at a time. One can do this via electron scattering or proton-nucleus reactions.

4. d d d Au Au Au Au Au Au Experimental Data Suppression of forward hadrons consistent with saturation of low-x gluons. Suppression Factor Centrality dependence and x dependence are being studied to test CGC. x ~ 10-3 x ~ 10-1 x ~ 10-2

5. p0 “Mono-jet” PT is balanced by many gluons Dilute parton system (deuteron) Dense gluon field (Au) Mono-Jets? Tagged photons and jets at forward angles will give precise information on x dependence of saturation effect. Complementary physics to e-A collider and LHC. What if instead of studying one virtual parton at a time, we hit the Color Glass with a hammer?

6. The Hammer 10,000 gluons, quarks, and antiquarks from the nuclear wavefunctions are made physical in the laboratory ! What is the nature of this ensemble of partons?

7. 26 TeV of Available Energy ! Out of a maximum energy of 39.4 TeV in central Au-Au reactions, 26 TeV is made available for heating the system. Measures of transverse energy production indicate initial energy density far above lattice QCD transition value.

8. What Happens to All That Energy? p, p0, K, K*0(892), Ks0, h, p, d, r0, f, D, L, S*(1385), L*(1520), X± , W, D0, D±, J/y’s, (+ anti-particles) in equilibrium at T > 170 MeV

9. How Does the Matter Behave? Simple answer is with a very high degree of collectivity.

10. Like a Perfect Fluid? First time hydrodynamics without any viscosity describes heavy ion reactions. v2 pT (GeV) Thermalization time t=0.6 fm/c and e=20 GeV/fm3 *viscosity = resistance of liquid to shear forces (and hence to flow)

11. Analogy in Atomic System Same phenomena observed in gases of strongly interacting atom The RHIC fluid behaves like this, that is, a strongly coupled fluid.

12. String Theory and Black Hole Physics What could this have to do with quark gluon plasma physics? The Maldacena duality, know also as AdS/CFT correspondence, has opened a way to study the strong coupling limit using classical gravity where it is difficult even with lattice Quantum Chromodynamics. It has been postulated that there is a universal lower viscosity bound for all strongly coupled systems, as determined in this dual gravitational system.

13. Universal Viscosity Bound ? Critical future goal to put the QCD data point on this plot

14. How Do We Put the Point on the Plot? We put a heavier and heavier pebble into the stream. This gives us better access to drag and viscosity of the system. Multi-Strange Baryons Charm and Beauty Charm and beauty measures with full kinematics gives us perfect fluid constraints.

15. Are quarks the degrees of freedom coalescing into hadrons? Key test with W with small hadronic cross section and baryon jet like correlations. Quark Recombination

16. Produced pions Produced photons Calibrating Our Probes High Energy Probes are well described in Proton-Proton reactions by NLO Perturbative QCD.

17. Probes of the Medium Sometimes a high energy photon is created in the collision. We expect it to pass through the plasma without pause.

18. Probes of the Medium Sometimes we produce a high energy quark or gluon. If the plasma is dense enough we expect the quark or gluon to be swallowed up.

19. (from quark and gluon jets) Experimental Results Scaling of photons shows excellent calibrated probe. Quarks and gluons disappear into medium, except consistent with surface emission. Survival Probability Size of Medium

20. Jet correlations in proton-proton reactions. Strong back-to-back peaks. Jet correlations in central Gold-Gold. Away side jet disappears for particles pT > 2 GeV Jet correlations in central Gold-Gold. Away side jet reappears for particles pT>200 MeV Jet Quenching ! Azimuthal Angular Correlations

21. Consistent with speed of sound from lattice QCD. Mach Cone measures the speed of sound. Reaction of the Medium How does the near-perfect liquid react to this large energy deposition? Color shock wave?

22. Measured Reflected Just a preview of what is to come in the future. Changing the Conditions High statistics correlations are crucial as a function of path through the medium and medium temperature.

23. Deconfinement Lattice QCD calculation V(r)/ r Lattice QCD makes a clear prediction for the onset of deconfinement. Different Quarkonia states test the degree of color screening and measure the temperature.

24. Example from lower energies reveals the importance of comparing different states and systems. J/y y’ Example of current statistics from very successful Copper-Copper run. Luminosity upgrade is critical to this program. Upsilon states also are a key control. Extensive p-A running is necessary. Suppression Factor Velocity Quarkonia Probes Understanding deconfinement requires very high statistics J/y and cc as a function of path length through the medium and velocity of probe.

25. Measurement via p-p channel have already been made, but only probes medium near surface. Measurement via dileptonchannels are the next step, and they probe the hottest part of the medium. p e- p e+ p-p invariant mass (GeV) Probing In-Medium Interactions Low mass vector mesons and continuum are extremely sensitive to in-medium interactions between the relevant degrees of freedom.

26. Summary Excitation Function RHIC Heavy Ion Program is already very successful. LHC Heavy Ion Program is likely to be very successful. The accelerating productivity of the RHIC program at this dedicated facility will continue to set the standard, against which other programs will be measured, for world class investigations into hot QCD.

27. Backup Slides

28. Conclusions RHIC program is operating very successfully. Gluon density well above lattice QCD predicted transition level and behaving as zero viscosity “perfect” liquid. This is not the traditionally thought of weakly-interacting gas of quarks and gluons (“the QGP”). This is the creation of a strongly-interacting Quark-Gluon Plasma (or Quark-Gluon Liquid). • The next decade should be very exciting. • 1. Understanding perfect fluid behaviour. • Understanding the nature of deconfinement and the • degrees of freedom

29. Full phase space particle correlations and with particle identification including charm and beauty with upgrades. Gamma-Jet gives important information on modified fragmentation and precision studies of how the energy affects the medium.

30. Thermalization of Energy If we look outside the traditional jet features, we see an extremely broadened set of particles with momentum not so different from <pT> of the bulk medium !

31. Factorization assumption of jet fragmentation completely breaks down. Color recombination ? Baryon Puzzle More (anti) baryons than pions at moderate pT. Does not look like vacuum jet fragmentation. Central Au-Au (anti)proton/pion Proton-Proton

34. 1/NtriggerdN/d() near side away side

36. Degrees of Freedom Values for entropy, energy density, pressure, and temperature will over constrain DOF. Critical measurement of thermal radiation to constrain temperature.

37. Quark Gluon Plasma 20 Year Old Plot: Very little know about matter beyond the boundary. Generic moniker of Quark-Gluon Plasma for this region.

38. We now know a lot more…. • Rich Color Superconducting phases at high density. • Lattice evidence for critical point. • High temperature matter behaves as near-perfect fluid. • Is it the Quark-Gluon Plasma? As defined by experimental evidence.