1. Jet ‘Quenching’ Status and Perspectives Urs Achim Wiedemann CERN TH and SUNY Stony Brook

2. Jet Quenching: Au+Au vs. d+Au ● Final state suppression ● Initial state enhancement partonic energy loss

3. The RHICness of Hard Probes Jets identified hadron specta D-,B-mesons Quarkonia Photons Z-boson tagged jets The range:,x, luminosity Basic strategy: Abundant yield at collider energies ( allows differential study of exp. signal ) + robust + large signal ( medium effect larger than TH uncertainty ) = Basis for controlled experimentation + controlled TH interpretation.

4. Time scales: hadronization vs.thermalization h h h in vacuum in QGP 1GeV 10 GeV 100 GeV 100 fm 1 fm Dynamics of hadronization Jet absorption Jet modification Dynamics of the bulk Partonic equilibration processes

5. Parton Propagation in Dense Matter Solve Dirac equation for partonic projectile in external color field of the medium 1.Leading order O(E 0 ) scattering determined by eikonal Wilson line During scattering, transverse coordinates are frozen, color rotates 2. Leading energy correction transverse Brownian motion Wiedemann, NPB582 (2000) 409 “Furry approximation”

6. Example: gluon production in q+A Incoming free quark wave function dressed to O(g) Outgoing from target Number of produced gluons Target average: Bertsch Gunion spectrum + Brownian Motion Kovner Wiedemann, PRD 64 (2001) 114002 Kovchegov Mueller 1998

7. The medium-modified Final State Parton Shower Radiation off produced parton Parton undergoes Brownian motion: Baier, Dokshitzer, Mueller, Peigne Schiff(1996), Zakharov (1997) Wiedemann, NPB 588 (2000) 303 Two approximation schemes: 1.Harmonic oscillator approximation:2. Opacity expansion in powers of Measures target average:

8. The medium-modified Final State Parton Shower Medium characterized by transport coefficient: Baier, Dokshitzer, Mueller, Peigne, Schiff (1996); Zakharov (1997); Wiedemann (2000); Gyulassy, Levai, Vitev (2000); Wang... Salgado,Wiedemann PRD68:014008 (2003) ● energy loss of leading parton ● pt-broadening of shower

9. Energy Loss in a Strongly Expanding Medium Salgado, Wiedemann PRL 89, 092303 (2002) ● In A-A collisions, the density of scattering centers is time-dependent: ● Dynamical Scaling Law: same spectrum obtained for equivalent static transport coefficient: ● Calculations for a static medium apply to expanding systems Rescaled spectrum = 1.5, 1.0, 0.5, 0

10. Quenching Weights: probability of energy loss BDMS (2001) Average: Typical: Quenching weight defines medium-modified fragmentation.

11. The fragility of leading hadrons ? Why is R AA ~ p T -independent? Trigger bias more severe for large p T Why is R AA = 0.2 natural ? Surface emission limits sensitivity to Eskola, Honkanen, Salgado, Wiedemann NPA747 (2005) 511, hep-ph/0406319

12. The produced matter is opaque - why? ● traces energy density Pion gas Cold nuclear matter RHIC data sQGP QGP R. Baier, NPA 715 (2003) 209 WHY? Interactions in produced matter much stronger than in ideal QGP. measures combination of energy density and flow (some support from RHIC data) parton energy loss calculations need quantitative improvements (no indication from RHIC that this is dominant effect) for the values favored by RHIC-data “Opacity problem” ● Time-averaged is very large. Dynamical scaling implies

13. How can we relate to fundamental properties of matter? defines short-distance behavior of expectation value of two light-like Wilson lines o Related to operators, which measure color field and which may be calculable in lattice QCD. o Can this be calculated with other modern methods (AdS/CFT?) ? o Even if is not calculable from 1st principles, its energy dependence is, since it satisfies non-linear QCD evolution equation. Well-defined but difficult problem in QCD.

14. How can we better gauge ‘hard probes’? Where does this associated radiation go to ? How does this parton thermalize ? What is the dependence on parton identity ? Numerous independent tests possible Basis for controlled experimentation with dense matter. Tremendous theoretical and experimental activity to further test the microscopic dynamics underlying high-pt hadron suppression.

15. Parton energy loss depends on parton identity Vacuum radiation is suppressed in the `dead cone’ due to quark mass Dramatic Consequence: in jets of ~ 100 GeV, leading hadron carries ~1/4 of jet energy for light quark jets ~ 3/4 of jet energy for b-quark jets Medium-induced gluon radiation is reduced as well for m/E > 10 % Dokshitzer, Kharzeev, PLB 519 (2001) 199 Armesto, Salgado, Wiedemann, PRD69 (2004) 114003 B.W. Zhang, E. Wang, X.N. Wang, PRL93 (2004) 072301 Djordjevic,, Gyulassy, NPA733 (2004) 265 massive massless dead cone

16. Disentangling Color Charge vs. Mass Dependence at the LHC Massless “c/b” Massive c/b Color charge dependence dominates Mass dependence dominates Armesto, Dainese, Salgado, Wiedemann, PRD71:054027, 2005

17. Tracing heavy quarks with electrons at RHIC Armesto, Cacciari, Dainese, Salgado, Wiedemann, work in progress HQ-decays dominate e-spectrum at RHIC pp-benchmark well-described by NLO (FONLL) but large TH uncertainties nuclear modification factor tends to be overpredicted by e-loss calculations (but within current EXP errors) Much more stringent test of energy-loss if b- and c-decay contributions could be disentangled

18. Jets in Heavy Ion Collisions at the LHC A. Accardi et al., hep-ph/0310274 CERN TH Yellow Report Experiments will detect jets above background How can we characterize the medium-modification of these jets above background ?

19. Transverse Jet Heating Salgado, Wiedemann, Phys. Rev. Lett. 93: 042301 (2004) ● Energy fraction in fixed jet cone weakly dependent on medium medium vacuum ● Multiplicity within small jet cone broadens strongly unaffected by high-multiplicity background !. kt

20. Longitudinal Jet Heating Borghini,Wiedemann, hep-ph/0506218 Medium expected to soften and increase the longitudinal multiplicity of ‘true jets’. Softening in qualitative agreement with triggered particle correlations. Awaits detailed test at the LHC.

21. Jets in pionic winds and partonic storms Hard partons are not produced in the rest frame comoving with the medium Armesto, Salgado, Wiedemann, Phys. Rev. Lett. 93 (2004) 242301 Flow effect If medium shows strong collective flow, what are additional measurable consequences?

22. Instead of a Summary some directions, which motivate the next exp. and th. steps WHY study the microscopic mechanism of high-pt hadron suppression? - to characterize properties of dense QCD matter - to understand onset of parton thermalization in a particularly well-controlled EXP + TH setting - to get novel access to the dynamics of hadronization by using the medium as tool to test it HOW study parton propagation and energy loss in the medium? - improve theory (recoil, finite energy corrections, connection of BDMPS transport parameter to fundamental QCD predictions, …) - extend range of applicability (sensitivity to parton identity, to multiplicity distributions, to high-pt particle correlations …) Connect study of medium-modified parton propagation to other collective phenomena in heavy ion collisions? - independent test of collective flow? - hadrochemical composition of jet remnants? - …