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A Critical Review of Jet Quenching: Peter Jacobs, LBNL 1Critique of Jet Quenching WW08 April 6-12, 2008.

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Presentation on theme: "A Critical Review of Jet Quenching: Peter Jacobs, LBNL 1Critique of Jet Quenching WW08 April 6-12, 2008."— Presentation transcript:

1 A Critical Review of Jet Quenching: Peter Jacobs, LBNL 1Critique of Jet Quenching WW08 April 6-12, 2008

2 WW08 ; April 6-12, 2008Critique of Jet Quenching2 Other talks: progress in the understanding jet quenching and its consequences This talk: (my humble opinion of) some open issues (mostly: one simple point about Mach cones)

3 Jet quenching: large, striking experimental effects… p T trigger >8 GeV/c WW08 April 6-12, Critique of Jet Quenching

4 …with some quantitative understanding… 4 Zhang, Owens, Wang and Wang nucl-th/  WW08 April 6-12, 2008 Critique of Jet Quenching

5 ...and more quantitative understanding... Results (1  range): PHENIX, arXiv: [nucl-ex] Gyulassy, Levai, Vitev Zhang, Owens, Wang, Wang K Reygers, J. Nagle QM08 WW08 April 6-12, Critique of Jet Quenching

6 …and some puzzles… WW08 ; April 6-12, 2008 Critique of Jet Quenching 6 Standard radiative energy loss ignore b-quark contribution

7 …and massive confusion. WW08 April 6-12, Critique of Jet Quenching

8 What can we learn from jet quenching? WW08 ; April 6-12, 2008 Critique of Jet Quenching 8 Jet energy loss dominated by momentum transfers ~ gT ~ few hundred MeV even at very high jet energy Two distinct sets of questions: 1.What are the mechanisms of e-loss? What do they tell us about the micro-structure of the medium? 2.What is the response to the lost energy and what does it tell us about the macro-structure of the medium?

9 Jets and Photons9 Beam Remnants Beam Remnants p = (uud) p = Jets as Nature makes them ,K,p,n,…} Jet Initial State Radiation (ISR) Hadronization Final State Radiation (FSR) Detector

10 Jets and Photons 10 Jets as pQCD describes them pQCD factorization: parton distribution fn f a/A + partonic cross section  hat + fragmentation fn D h/c

11 Jets in heavy ion collisions WW08 ; April 6-12, 2008 Critique of Jet Quenching 11 The great benefits of jets and other hard probes: 1.Controlled “beams” with well-calibrated intensity (assumes factorization) 2.Final-state interactions with colored matter are theoretically tractable using controlled approximations → Quantitative probes of hot QCD matter

12 Jets in HIC: theory WW08 ; April 6-12, 2008 Critique of Jet Quenching 12 Four main theoretical approaches: GLV (Gyulassy, Levai, Vitev) Higher Twist (Wang et al.) ASW (Armesto, Salgado, Wiedemann, based on BDMSP) AMY (Arnold, Moore Yaffe) see e.g.: Majumder, nucl-th/ All are based on factorized pQCD: hard parton of defined energy propagating in a medium Assertion: any quantitative analysis of jet quenching or medium response to energy loss requires - at minimum - that factorized pQCD is a credible approximation

13 Jets and Photons13 pQCD at RHIC? p+p inclusive spectra NLO: W. Vogelsang 00 charged hadrons Charged hadrons: ~agreement within large theory uncertainties N.B. region ~3-4 GeV (important later): theory provides only weak test (but good enough for government work)

14 Jets and Photons 14  /K 0 s Fly in the ointment: baryon/meson anomaly 3-4 GeV: dramatic variation in hadron production mechanisms from p+p → central Au+Au fragmenting parton: p h = z p, z<1 recombining partons: p 1 +p 2 =p h Recombination: what - if any - role do hard processes play at intermediate p T ?

15 So what? PHENIX state of the art correlations… WW08 ; April 6-12, 2008 Critique of Jet Quenching 15 “Mach cone” clearly apparent only with low-ish p T triggers M.McCumber QM08

16 So what? STAR state of the art correlations… WW08 ; April 6-12, 2008 Critique of Jet Quenching 16 3


17 Can we turn it off? Check SPS/CERES… WW08 ; April 6-12, 2008 Critique of Jet Quenching 17 S. Kneige, M. Ploskon, H. Appelshauser QM08 CERES Preliminary No, we can’t turn it off… note strong shape variation with seemingly minor variation in centrality not a backround issue: S/B~1/9 (M. Ploskon, private communication) hhh h+- and h-+h++ and h-- prelimina ry

18 Could it be simple kinematics? (which is not the same as simple physics…) Cumulants due to momentum conservation N. Borghini, Phys.Rev.C75, (2007) N= multiplicity of ensemble conserving momentum Not obvious what N should be in a space-time extended system (causality) N certainly less than total event multiplicity → free parameter 2-particle cumulant 3-particle cumulant

19 Comparison to 2-particle correlations (I) WW08 ; April 6-12, 2008 Critique of Jet Quenching 19 STAR, Phys Rev Lett 95,

20 Comparison to 2-particle correlations (II) WW08 ; April 6-12, 2008 Critique of Jet Quenching 20 top 5%STAR Preliminary A. Feng QM08 in-planeout-of-plane Comparison is qualitative only - differing kinematic cuts Flow-subtracted 2-part cumulant in- and out-of-plane; ZYAM-like norm N. Borghini arXiv:

21 Comparison to 3-part correlation data… WW08 ; April 6-12, 2008 Critique of Jet Quenching 21 dAu Central Au+Au 0-12% J. Ulery, QM08 STAR Preliminary Comparison is qualitative only - differing kinematic cuts Momentum conservation

22 The best way: high (as possible) trigger p T WW08 ; April 6-12, 2008 Critique of Jet Quenching 22 A. Adare (PHENIX), QM08  0 -h STAR, Phys Rev Lett 95, Higher trigger momenta: stronger bias towards jets: → no significant evidence for double-humped structure or Mach cone

23 WW08 ; April 6-12, 2008Critique of Jet Quenching23 I find the experimental evidence in favor of Mach Cone formation induced by jet propagation to be weak Jet triggers not obviously dominant in regions where phenomena appear to be strongest Conventional (non-jet) explanations not excluded Systematic check with collision energy fails Systematic check with jet trigger bias fails This may disappoint some of us but the sooner we are clear on the point the better

24 Jet quenching theory: some open issues WW08 April 6-12, Critique of Jet Quenching Static vs. dynamic medium: GLV, BDMPS vs AMY, Djordjevic et al.,… Elastic vs radiative…? is there even a conceptual distinction in a medium? (Wang) experimental sensitivity? (Renk: L-dependence + di-hadrons) Current models assume incoherent induced-gluon emission suitable for inclusive observables (leading hadron suppression) unsuitable for multi-hadron (intra-jet) observables interference essential for modelling model angle-ordered fragmentation Urgent need for jet-quench MC with solid theory basis a la PYTHIA, HERWIG (Vitev, Salgado++, Wiedemann++)

25 Jet quenching theory: more open issues WW08 April 6-12, Critique of Jet Quenching Jets at RHIC+LHC: we are on the threshold of quasi-full jet reconstruction in H.I.C. (talks by Brooke, Sevil, John,…): what will we really learn from this? Casalderrey-Solana and Wang arXiv: What is L-dependence of energy loss at strong coupling? AdS/CFT: dE/dx~L 2 (A.H. Mueller et al., arXiv ) e.g. consider jet quenching as DIS off the QGP

26 Experiment+Theory: quantitative modeling WW08 ; April 6-12, 2008 Critique of Jet Quenching Heavy ion collisions are complex detailed, controlled modeling is essential for quantitative connection between measurement and basic theory Schematic calculations and toy models are essential to explore new ideas are the wrong tools for quantitative interpretation of data The era of toy models for jet quenching is largely past

27 Quantitative modeling (cont’d) WW08 ; April 6-12, 2008 Critique of Jet Quenching 27 Needed: mature, complete dynamical calculations that utilize our full knowledge of heavy ion physics explore the phase space of uncertainties (initial conditions, quenching schemes, viscosity,…) make significant and testable predictions This is not a substitute for doing basic theory but rather its complement – consider it the bridge between theory and experiment

28 The TECHQM Initiative WW08 ; April 6-12, 2008 Critique of Jet Quenching 28 Workshop at BNL May 6-7 This workshop will initiate TECHQM ("Theory-Experiment Collaboration for Hot QCD Matter"), a new joint working group of theorists and experimentalists. The goal of TECHQM is to further the understanding of hot QCD matter through detailed, quantitative analysis of heavy ion collision experimental data and theory, together with the dynamical modeling which connects them. Theory-Experiment Collaboration for Hot QCD Matter


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