1. Jet Quenching in Heavy Ion Collisions at RHIC and LHC Urs Achim Wiedemann CERN PH-TH Erice, 2008

2. From elementary interactions to collective phenomena How do collective phenomena and macroscopic properties of matter emerge from fundamental interactions ? 1973: asymptotic freedom QCD = quark model +gauge invariance Today: mature theory with a precision frontier background in search for new physics TH laboratory for non-abelian gauge theories QCD much richer than QED: non-abelian theory degrees of freedom change with

3. Question: Why do we need collider energies to test properties of dense QCD matter which arise on typical scales ?

4. Answer 1: Large quantitative gains Increasing the center of mass energy implies Denser initial system, higher initial temperature Longer lifetime Bigger spatial extension Stronger collective phenomena A large body of experimental data from RHIC supports this argument.

5. Answer 2: Qualitatively novel access to properties of dense matter To test properties of QCD matter, large- processes provide well- controlled tools (example: DIS). Heavy Ion Collisions produce auto-generated probes at high Q: How sensitive are such ‘hard probes’?

6. Bjorken’s original estimate and its correction Bjorken 1982: consider jet in p+p collision, hard parton interacts with underlying event collisional energy loss Bjorken conjectured monojet phenomenon in proton-proton Today we know (th): radiative energy loss dominates Baier Dokshitzer Mueller Peigne Schiff 1995 p+p: A+A: Negligible ! Monojet phenomenon! observed at RHIC (Bjorken realized later that this estimate was numerically erroneous.)

7. Parton energy loss - a simple estimate Medium characterized by transport coefficient: ● How much energy is lost ? Number of coherent scatterings:, where Gluon energy distribution: Average energy loss Phase accumulated in medium: Characteristic gluon energy

8. High p T Hadron Spectra Centrality dependence: 0-5%70-90% L largeL small

9. Discovery at RHIC: Au+Au vs. d+Au ● Final state suppression ● Initial state enhancement partonic energy loss

10. The fragility of leading hadrons ? The quenching is anomalously large (I.e. exceeds the perturbative estimate by ~ 5) Why is R AA = 0.2 natural ? Surface emission limits sensitivity to Eskola, Honkanen, Salgado, Wiedemann NPA747 (2005) 511, hep-ph/0406319

11. Coherent effort of community to arrive at quantitative statements

12. ‘True jet’ quenching Where does this associated radiation go to? How does this parton thermalize? What is the dependence on parton identity? Characterize Recoil: What is kicked in the medium? Jet multiparticle final states provide qualitatively novel characterizations of the medium.

13. Multi-particle final states above background Borghini,Wiedemann, hep-ph/0506218 above ~1 ~4 ~7 Exp. measurement ~10 GeV~2 GeV ~100 GeV~4 GeV ~7 GeV~200 GeV R AA only R AA + multi-particle final states Novel challenge for LHC, high luminosity RHIC & TH

14. Jet modifications in dense QCD matter Armesto, Salgado, Wiedemann, Phys. Rev. Lett. 93 (2004) 242301 Jets ‘blown with the wind’ Hard partons are not produced in the rest frame comoving with the medium ‘Longitudinal Jet heating’: The entire longitudinal jet multiplicity distribution softens due to medium effects. Borghini,Wiedemann, hep-ph/0506218

15. Going beyond R AA sharpens our understanding of jet quenching Standard open questions, e.g. are addressed by being sensitive to or E’ E E 1.) ? 2.) What is ? E’ E E This instead of only this

16. Going beyond R AA – implications for theory  Benchmarking: Ideally, models account for p-p baseline, before being extended to medium modifications. a)Single inclusives: b)Jet energy flows: a)Multi-hadron (multi-parton) final states: TH task: multi-dim integral, MC event generator optional MLLA: resums ln[1/x]-contributions for intrajet single- (and multi-) parton distributions Alternative: MC event generator IR safe jet shapes, such as thrust, are perturbatively calculable. Preferred TH tool: MC event generators Motivation for developing MC event generator models of jet quenching

17. JEWEL- Jet Evolution With Energy Loss K. Zapp, G. Ingelman, J. Rathsman, J. Stachel, U.A. Wiedemann, arXiv:0804.3568 [hep-ph]  Baseline: stand-alone Q 2 -ordered final state parton shower without keeping track of color flow ( since this would complicate medium interaction ) Hadronization models: string fragmentation ( associating strings between nearest neighbors in momentum space )  Medium effects : Q 2 -ordering used to embed parton shower in nuclear geometry. Lifetime of virtual state: determines probability of no scattering with probability 1-S, parton undergoes elastic scattering radiative e-loss modeled by f med -enhanced splitting functions (so far).

18. JEWEL gets the vacuum baseline For these jet shape observables, results are insensitive to details of hadronization.

19. JEWEL vacuum baseline for n-jet fraction K. Zapp, G. Ingelman, J. Rathsman, J. Stachel, U.A. Wiedemann, arXiv:0804.3568 [hep-ph] Durham cluster algorithm: define distance between particles particle belong to same jet if

20. JEWEL baseline for jet multiplicity distribution A sensitive test of hadronization model

21. JEWEL - modeling “collisional” e-loss K. Zapp, G. Ingelman, J. Rathsman, J. Stachel, U.A. Wiedemann, arXiv:0804.3568 [hep-ph] JEWEL reproduces standard features of collisional e-loss models.

22. JEWEL can follow the recoil dynamically K. Zapp, G. Ingelman, J. Rathsman, J. Stachel, U.A. Wiedemann, arXiv:0804.3568 [hep-ph]  At which angle and which pt does recoil parton emerge?  Recoil partons can be further showered, scattered and hadronized  thermal mass dependence of recoil can be studied. Caveat: prior to hadronization

23. JEWEL: disentangling elas / inelas processes K. Zapp, G. Ingelman, J. Rathsman, J. Stachel, U.A. Wiedemann, arXiv:0804.3568 [hep-ph]

24. JEWEL: disentangling elas / inelas processes K. Zapp, G. Ingelman, J. Rathsman, J. Stachel, U.A. Wiedemann, arXiv:0804.3568 [hep-ph]

25. JEWEL: e-loss with minor pt-broadening K. Zapp, G. Ingelman, J. Rathsman, J. Stachel, U.A. Wiedemann, arXiv:0805.4759 [hep-ph] Almost no broadening despite extreme choices: E=100 GeV, T=500 MeV, L=5fm, f med = 3

26. Back to MLLA + LPHD: ( controlling the p+p baseline ) argued to give support to particular picture of hadronization, namely LPHD = local parton hadron duality which assumes that at scale Q 0 =M hadron, there is a one-to-one correspondence between partons and hadrons But how accurate is the MLLA approximation to the coherent branching formalism? S.Sapeta and U.A. Wiedemann, Eur. Phys. J. C in press

27. Coherent branching formalism Defines evolution equations for inclusive single-parton distributions inside quarks and gluon jets (single log accuracy, angular ordering) Variables:

28. Coherent branching formalism – initial conditions denotes hadronic scale 1.For the initial conditions 1.To count only gluons in a quark or gluon jet, start from 2.To count all partons in a quark or gluon jet, start from the function counts all quarks in a quark jet the function counts all quarks in a gluon jet S.Sapeta and U.A. Wiedemann, in preparation

29. Coherent branching formalism vs. MLLA For small Y, significant deviations from MLLA -> agreement of MLLA with data should not be taken as strong support for LPHD S.Sapeta and U.A. Wiedemann, in preparation

30. Coherent branching formalism vs. MLLA For very large Y, MLLA shape is approached (but Y=10 is for jets well above TeV scale!) S.Sapeta and U.A. Wiedemann, in preparation

31. Coherent branching formalism Disentangling quark and gluon contributions to single-parton distributions points to origin of small-Y deviations: (kinematics of parent parton biases distribution) S.Sapeta and U.A. Wiedemann, in preparation

32. Instead of a Conclusion: The physics: ‘True’ jet rates are abundant at LHC. ‘True’ jets not (yet) in kinematical reach of RHIC. The challenge: characterize medium-modifications of jets in high multiplicity background. - jet-like particle correlations The jet as a thermometer: jets as a far out-of-equilibrium probe participating in equilibration processes. Sensitive jet features: - jet shapes (i.e. calorimetry) - jet multiplicity distributions (in trans. and long. momentum) - jet composition (i.e. hadrochemistry) Prerequisite: determine E T -distribution of final state hadrons. I walked you through recent work on how to use jets in HICs@ LHC

33. And where this will be tested