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Jet ‘Quenching’ Status and Perspectives

Jet ‘Quenching’ Status and Perspectives. Urs Achim Wiedemann CERN TH and SUNY Stony Brook. partonic energy loss. Jet Quenching: Au+Au vs. d+Au. Initial state enhancement. Final state suppression. Jets identified hadron specta D-,B-mesons Quarkonia Photons Z-boson tagged jets.

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Jet ‘Quenching’ Status and Perspectives

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

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

  3. Jets • identified hadron specta • D-,B-mesons • Quarkonia • Photons • Z-boson tagged jets The RHICness of Hard Probes 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. in QGP h in vacuum h h Partonic equilibration processes Dynamics of the bulk Dynamics of hadronization 100 fm Jet absorption Jet modification 1 fm 10 GeV 1GeV 100 GeV Time scales: hadronization vs.thermalization

  5. Solve Dirac equation for partonic projectile in external color field of the medium Parton Propagation in Dense Matter Leading order O(E0) scattering determined by eikonal Wilson line During scattering, transverse coordinates are frozen, color rotates 2. Leading energy correction transverse Brownian motion “Furry approximation” Wiedemann, NPB582 (2000) 409

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

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

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

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

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

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

  12. Time-averaged is very large. Dynamical scaling implies • traces energy density RHIC data sQGP WHY? QGP • Interactions in produced matter much stronger than inideal 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” Pion gas Cold nuclear matter The produced matter is opaque - why? R. Baier, NPA 715 (2003) 209

  13. How can we relate to fundamental properties of matter? • defines short-distance behavior of expectation value of two light-like Wilson lines • Related to operators, which measure color field and which may • be calculable in lattice QCD. • Can this be calculated with other modern methods (AdS/CFT?) ? • 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. Where does this associated radiation go to ? How does this parton thermalize ? • Numerous independent tests possible Basis for controlled experimentation with dense matter. What is the dependence on parton identity ? • Tremendous theoretical and experimental activity to further test the microscopic dynamics underlying high-pt hadron suppression. How can we better gauge ‘hard probes’?

  15. 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 Parton energy loss depends on parton identity

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

  17. 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 Tracing heavy quarks with electrons at RHIC

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

  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 If medium shows strong collective flow, what are additional measurable consequences? Armesto, Salgado, Wiedemann, Phys. Rev. Lett. 93 (2004) 242301 Hard partons are not producedin the rest frame comoving withthe medium Flow effect

  22. Instead of a Summarysome 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? • …

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