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A Critical Review of Jet Quenching:

A Critical Review of Jet Quenching:. Peter Jacobs, LBNL. 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). p T trigger >8 GeV/c.

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A Critical Review of Jet Quenching:

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  1. A Critical Review of Jet Quenching: Peter Jacobs, LBNL Critique of Jet Quenching

  2. 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) Critique of Jet Quenching

  3. pTtrigger>8 GeV/c Jet quenching: large, striking experimental effects… Critique of Jet Quenching

  4. …with some quantitative understanding… Zhang, Owens, Wang and Wang nucl-th/0701045 Df Critique of Jet Quenching

  5. ...and more quantitative understanding... K Reygers, J. Nagle QM08 PHENIX, arXiv:0801.1655 [nucl-ex] Gyulassy, Levai, Vitev Zhang, Owens, Wang, Wang Results (1s range): Critique of Jet Quenching

  6. …and some puzzles… Standard radiative energy loss ignore b-quark contribution Critique of Jet Quenching

  7. …and massive confusion. Critique of Jet Quenching

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

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

  10. Jets as pQCD describes them • pQCD factorization: • parton distribution fn fa/A • + partonic cross section shat • + fragmentation fn Dh/c Jets and Photons

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

  12. Jets in HIC: theory • 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/0702066 • All are based on factorized pQCD: hard parton of defined energy propagating in a medium Assertion: any quantitativeanalysisof jet quenching or medium response to energy lossrequires- at minimum - that factorized pQCD is a credible approximation Critique of Jet Quenching

  13. pQCD at RHIC? p+p inclusive spectra NLO: W. Vogelsang p0 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) Jets and Photons

  14. Fly in the ointment: baryon/meson anomaly L/K0s fragmenting parton: ph = z p, z<1 recombining partons: p1+p2=ph 3-4 GeV: dramatic variation in hadron production mechanisms from p+p → central Au+Au Recombination: what - if any - role do hard processes play at intermediate pT? Jets and Photons

  15. So what? PHENIX state of the art correlations… M.McCumber QM08 “Mach cone” clearly apparent only with low-ish pT triggers Critique of Jet Quenching

  16. So what? STAR state of the art correlations… in-plane fS=0 out-of-plane fS=90o 3<pTtrig<4GeV/c & 1.0<pTasso<1.5GeV/c 20-60% STAR Preliminary STAR Preliminary top 5% Likewise for low-ish pT triggers… Critique of Jet Quenching

  17. Can we turn it off? Check SPS/CERES… S. Kneige, M. Ploskon, H. Appelshauser QM08 • 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) CERES Preliminary hhh h+- and h-+ h++ and h-- preliminary preliminary Critique of Jet Quenching

  18. Could it be simple kinematics? (which is not the same as simple physics…) Cumulants due to momentum conservation N. Borghini, Phys.Rev.C75, 021904 (2007) 2-particle cumulant 3-particle cumulant • 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

  19. Comparison to 2-particle correlations (I) STAR, Phys Rev Lett 95, 152301 Critique of Jet Quenching

  20. Comparison to 2-particle correlations (II) Flow-subtracted 2-part cumulant in- and out-of-plane; ZYAM-like norm N. Borghini arXiv:0710.2588 Comparison is qualitative only - differing kinematic cuts STAR Preliminary top 5% in-plane out-of-plane A. Feng QM08 Critique of Jet Quenching

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

  22. The best way: high (as possible) trigger pT STAR, Phys Rev Lett 95, 152301 A. Adare (PHENIX), QM08 p0-h • Higher trigger momenta: stronger bias towards jets: • → no significant evidence for double-humped structure or Mach cone Critique of Jet Quenching

  23. 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 Critique of Jet Quenching

  24. Jet quenching theory: some open issues • 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++) Critique of Jet Quenching

  25. Jet quenching theory: more open issues Casalderrey-Solana and Wang arXiv:0705.11352 • What is L-dependence of energy loss at strong coupling? • AdS/CFT: dE/dx~L2(A.H. Mueller et al., arXiv0803.3234) • 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? e.g. consider jet quenching as DIS off the QGP Critique of Jet Quenching

  26. Experiment+Theory: quantitative modeling • 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 Critique of Jet Quenching

  27. Quantitative modeling (cont’d) • 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 Critique of Jet Quenching

  28. The TECHQM Initiative Theory-Experiment Collaboration for Hot QCD Matter 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. http://www.bnl.gov/TECHQM/ Critique of Jet Quenching

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