1. Heavy Ions at the LHC Theoretical issues Super-hot QCD matter What have we learned from RHIC & SPS What is different at the LHC ? Goals of HI experiments at the LHC LHC2003 Workshop Fermilab April 30 – May 3, 2003

2. QCD phase diagram LHC initial state RHIC

3. From hadrons to QGP QGP = quark-gluon plasma Hadron gas QCD equation of state from lattice QCD

4. Signatures of a QCD phase change Effects of “latent heat” in (E,T) relation Enhancement of s-quark production Bulk hadronization Disappearance of light hadrons (ρ 0 ) Thermal l + l - and  radiation Dissolution of Ψ, Y bound states Large energy loss of fast partons (jet quenching) Net charge and baryon number fluctuations Collective vacuum excitations (DCC)

5. Thermodynamics near T c Additional energy is used up to liberate new degrees of freedom (color!) in transition region TE

6. Hadrons at the boiling point ? Plateau is an indicator of latent heat of phase transformation. RHIC SPS AGS

7. Strangeness enhancement… Flavor Mass (MeV) (sss) (qss) (qqs) NA57 data …probes chiral symmetry restoration and deconfinement

8. High-energy parton loses energy by rescattering in dense, hot medium. q q Radiative energy loss qq g can be described as medium effect on parton fragmentation: Jet Quenching

9. Scaled   yield in Au+Au vs. p+p collisions Phenix preliminary Peripheral Central

10. Peripheral p /  ratio HIGH-ENERGY PHYSICS: Wayward Particles Collide With Physicists' Expectations Charles Seife EAST LANSING --At a meeting here last week, resear- chers announced results that, so far, nobody can explain. By slamming gold atoms together at nearly the speed of light, the physicists hoped to make gold nuclei melt into a novel phase of matter called a quark-gluon plasma. But al-though the experiment produced encoura- ging evidence that they had succeeded, it also left them struggling to account for the behavior of the particles that shoot away from the tremen- dously energetic smashups.

11. Azimuthal anisotropy v 2 pypy Coordinate space: initial asymmetry Momentum space: final asymmetry pxpx x y Semiperipheral collision Collective flow behavior extends to higher p T for baryons (p,  ) than mesons ( ,K) Indicates early equilibration (< 1 fm/c)

12. Quark recombination ? Fragmentation Recombination

13. Recombination vs. Fragmentation Fragmentation… … never competes with recombination for an exponential spectrum: … but wins out at large p T, where the spectrum is a power law ~ (p T ) -b Quark distribution function at “freeze-out” Recombination: For a thermal distribution:

14. Model fit to hadron spectrum T eff = 350 MeV fitted to spectrum pQCD spectrum shifted by 2.2 GeV R.J. Fries, BM, C. Nonaka, S.A. Bass (PRL in print) Also explains p/   ratio ≥ 1 below 5 GeV/c !

15. Does v 2 reflect parton flow? Recombination model suggests that hadronic flow reflects partonic flow (n = number of valence quarks): Provides measurement of partonic v 2 ! P. Sorensen (UCLA – STAR)

16. What’s different (better) at the LHC ? Higher energy density  0 at earlier time  0. Jet physics can be probed to p T > 100 GeV. b, c quarks are plentiful, good probes. Increased lifetime of QGP phase (10-15 fm/c)  Initial state effects less important. QGP more dominant over final-state hadron interactions. Much larger “dynamic range” compared to RHIC

17. Parton saturation at small x ~ 1/Q 2  After “liberation”, partons equilibrate and screen color force

18. E CM dependence of dN/dy, dE/dy NLO pQCD with geometric parton saturation (Eskola et al. - EKRT) Probably overestimate of growth of dN/dy, dE/dy with E CM RHIC LHC RHIC LHC

19. Quo vadis - J/  ? V qq is screened at scale (gT) -1  heavy quark bound states dissolve above some T d. But deconfined c-quarks can recombine at hadronization and form new J/ . J/  suppression ?J/  enhancement ? Karsch et al. Thews et al.

20. Jet quenching at the LHC I. Vitev, M. Gyulassy, PRL 89 (2002) 252301

21. Hadron production at the LHC  r = 0.75  r = 0.85  r = 0,65 R.J. Fries et al. (unpublished) includes parton energy loss

22. Photon tagged jets High-energy photon defines energy of the jet, but remains unaffected by the hot medium. q g  Parton energy loss is measured by the suppression of the fragmentation function D(z) near z  1.

23. Measuring the density R.J. Fries, BM, D.K. Srivastava, PRL 90 (2003) 132301 g  q q + g  q +  q + q  g +  Backscattering probes the plasma density and initial parton spectrum

24. Summary SPS: First glimpse (“evidence”) of the QGP RHIC: Discovery of the QGP ?! LHC: (Quantitative) exploration of the QGP facilitated by plentiful hard probes ! Specific questions: –How does dE/dx depend on energy density? –How is the fragmentation function modified? –Are c and b quarks thermalized? –Gluon saturation in nuclei at small x –Higher twist effects