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Parton Energy Loss: The Discovery of Jet Quenching

This presentation discusses the discovery of jet quenching in Au-Au collisions at RHIC and the calculation of parton energy loss using the BDMPS framework. It compares the Parton Quenching Model with RHIC data and explores the limitations of the leading-particle approach. Other topics covered include high-pt particle production in nucleus-nucleus collisions, energy loss for heavy quarks, and the final-state effects in high-energy collisions.

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Parton Energy Loss: The Discovery of Jet Quenching

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  1. I° Convegno Italiano sulla Fisica di ALICE Parton Energy Loss Andrea Dainese Padova – Università e INFN ALICE - Catania, 11.01.2005 Andrea Dainese

  2. Layout • The discovery of jet quenching in Au-Au coll. at RHIC • High-pt particle production in nucleus-nucleus collisions, according to perturbative QCD PARTON ENERGY LOSS • Calculating parton energy loss (BDMPS framework) • Application: the Parton Quenching Model vs. RHIC data • Energy loss for heavy quarks • Limitations of the leading-particle appraoch • Summary ALICE - Catania, 11.01.2005 Andrea Dainese

  3. … “Jet Quenching” – J.D.Bjorken 1982 ALICE - Catania, 11.01.2005 Andrea Dainese

  4. PHENIX p0 (0-10%) Discoveries at RHIC:high-pt suppression • Nuclear modification factor of pt distributions: factor 5 suppression! ALICE - Catania, 11.01.2005 Andrea Dainese

  5. What about jets? Jets via di-hadron correlations: • trigger: highest pt track, pt > 4 GeV • Df distribution: 2 GeV < pt < pttrigger • normalize to number of triggers p+p  di-jet near side away side STAR Coll., Phys.Rev.Lett. 90 (2003) 082302 ALICE - Catania, 11.01.2005 Andrea Dainese

  6. Discoveries at RHIC:no away-side jet in central Au-Au • Quantified via ratio of integrals: pedestal and flow subtracted ~1 in peripheral AA ~0 in central AA STAR Coll., Phys.Rev.Lett. 90 (2003) 082302 ALICE - Catania, 11.01.2005 Andrea Dainese

  7. Final-state effect Is it final-state energy loss? • … or saturation of the parton densities in the initial nuclei? Control experiment, without medium: d-Au no high-pt suppression the away-side jet is there ALICE - Catania, 11.01.2005 Andrea Dainese

  8. h RAB Au 1 medium formed in the collision q q Au q h q ~2-4 GeV/c pT kLkT fgPb / fgp Q2 = 5 GeV2 Cronin enhancement h p pp  h+X LHCRHICSPS RAA<1~1>1 q p h x High-pt particle prod. in AA collisions • A schematic view: • Ingredients: • pp baseline (pQCD) • initial-state effects: • PDF (anti)shadowing • kT broadening (Cronin) • final-state effects: • energy loss • in-medium hadronization ( V.Greco) ALICE - Catania, 11.01.2005 Andrea Dainese

  9. path length L hard parton path length L hard parton Parton Energy Loss • Partons travel ~4 fm in the high colour-density medium • Bjorken (`82): energy loss due to elastic scattering • Successive calculations (`92 ): a QCD mechanism dominates, medium-inducedgluon radiation • Coherent wave-function gluon acummulates kT due to multiple inelastic scatterings in the medium; it decoheres and is radiated Bjorken, Gyulassy, Pluemer, Wang, Baier, Dokshitzer, Mueller, Pegne, Schiff, Levai, Vitev, Zhakarov, Wang, Salgado, Wiedemann,… ALICE - Catania, 11.01.2005 Andrea Dainese

  10. path length L path length L path length L Calculating Parton Energy Loss • To analyse RHIC data, need more than mean DE • Quenching weights: energy loss probability distributions • Main limitation: calculated in E approx. uncertainties (BDMPS) Medium transport coefficient gluon density and momenta Casimir coupling factor: 4/3 for quarks 3 for gluons Probe the medium R.Baier, Yu.L.Dokshitzer, A.H.Mueller, S.Peigne' and D.Schiff, (BDMPS), Nucl. Phys. B483 (1997) 291. C.A.Salgado and U.A.Wiedemann, Phys. Rev. D68 (2003) 014008 [hep-ph/0302184]. ALICE - Catania, 11.01.2005 Andrea Dainese

  11. pt PYTHIA Hadronization pt – DE pt Application: Parton Quenching Model • Quenching weights + Glauber-model-based medium geometry + PYTHIA for parton generation and fragmentation • The procedure in short: • generate parton (q or g) with PYTHIA (or back-to-back pair) • calculate its L and average along the path • use quenching weights to get energy loss • quench parton and then hadronize it (independent fragm. (KKP)) A.D., C.Loizides and G.Paic, Eur.Phys.J. Cin press, hep-ph/0406201. ALICE - Catania, 11.01.2005 Andrea Dainese

  12. naturally matches pt-indepence of suppression at high pt Model vs RHIC data (1) • Density ( ) “tuned” to match RAA in central Au-Au at 200 GeV • Extrapolations • in centrality: according to Glauber-model collision geometry • in s: assuming  Ngluons/volume (s)0.6 (saturation model) Caveat: • No initial-state effects and in-medium hadronization: results given for pt > 5 GeV • Band represents theoretical uncertainty Saturation model: K.J.Eskola, K.Kajantie, P.V.Ruuskanen and K.Tuominen, Nucl. Phys. B570 (2000) 379 [hep-ph/9909456]. ALICE - Catania, 11.01.2005 Andrea Dainese

  13. Model vs RHIC data (2)centrality dependence of RAA pt > 4.5 GeV ALICE - Catania, 11.01.2005 Andrea Dainese

  14. Model vs RHIC data (3)disappearence of the away-side jet ALICE - Catania, 11.01.2005 Andrea Dainese

  15. Model vs RHIC data (4)intermediate RHIC energy s = 62 GeV energy extrapolation works reasonably well ALICE - Catania, 11.01.2005 Andrea Dainese

  16. Model prediction for LHC • Extrapolation to LHC according to saturation model gives: • Most partons are absorbed • Only those from the surface can escape the medium … surface emission dominates ALICE - Catania, 11.01.2005 Andrea Dainese

  17. Yu.Dokshitzer Q Heavy Quarks Lower E loss for ? • In vacuum, gluon radiation suppressed at q < mQ/EQ  “dead cone” effect • Dead cone implies lower energy loss(Dokshitzer-Kharzeev, 2001): • Energy distribution w dI/dw of radiated gluons suppressed by angle-dependent factor • suppress high-energy tail Yu.L.Dokshitzer and D.E.Kharzeev, Phys. Lett. B519 (2001) 199 [hep-ph/0106202]. ALICE - Catania, 11.01.2005 Andrea Dainese

  18. c  e PHENIX Charm Energy Loss at RHIC • Detailed calculation confirms this qualitative feature, although effect is small and uncertainties significant • Very small mass effect for charm at RHIC • Data not yet conclusive (better stats expected within 2005) RHIC N.Armesto, C.A.Salgado and U.A.Wiedemann, Phys. Rev. D69 (2004) 114003 [hep-ph/0312106]. N.Armesto, A.D., C.A.Salgado and U.A.Wiedemann, in preparation. ALICE - Catania, 11.01.2005 Andrea Dainese

  19. LHC LHC Heavy Quark E Loss at LHC • Exploit abundant production of charm and beauty quarks at LHC & study the mass dependence of E loss by measuring RAA for D and B and for their decay leptons ALICE - Catania, 11.01.2005 Andrea Dainese

  20. Open points: the opacity problem • Can we really probe the medium? • Need to relate extracted to an energy density e • QCD estimate for ideal QGP: • A recent analysis* of RHIC data, similar to that presented, extracts energy density 5 larger than that estimated from produced transverse energy dET/dy (Bjorken estimate) • Opacity problem: the interaction of the hard parton with the medium is much stronger than expected (Baier) R.Baier, Nucl. Phys. A715 (2003) 209. * K.J.Eskola, H.Honkanen, C.A.Salgado and U.A.Wiedemann, Nucl.Phys.Ain press, hep-ph/0406319. ALICE - Catania, 11.01.2005 Andrea Dainese

  21. ? Open points: limited sensitivity of RAA • Strong suppression requires very large density • Surface emission scenario • RAA determined by geometry rather than by density itself • Limited sensitivity to • Need more differential observables: • massive partons • RAA vs reaction plane • study of jet shapes • … large  RAA indep. of ALICE - Catania, 11.01.2005 Andrea Dainese

  22. Summary • One of the most exciting discoveries at RHIC ! • We are dealing with energy loss in an extremely opaque medium, but … • theoretically, we are just starting to learn how to probe the medium; still large uncertainties, but improving • experimentally, need more differential studies (vs reaction plane, vs parton species and quark mass) • need to go beyond pt spectra studies, towards ‘real’ jets • The LHC will be a `hard probes machine’ and quenching studies, including heavy quarks and jets, will play a central role ALICE - Catania, 11.01.2005 Andrea Dainese

  23. EXTRA SLIDES ALICE - Catania, 11.01.2005 Andrea Dainese

  24. ALICE - Catania, 11.01.2005 Andrea Dainese

  25. path length L Re-analysis: D.d’Enterria, nucl-ex/0403055 Jet quenching at the SPS (s  17 GeV)? • SPS: only p0 RAA using parameterized pp reference • Need more systematics: other experiments, more particles • WA97/NA57: can provide RAA (Pb-Pb/p-Be) for K0, L and charged hadrons • First step: RCP RAA ~ 1: Cronin enhancement compensated by E loss ? ALICE - Catania, 11.01.2005 Andrea Dainese

  26. NA57 preliminary RCP at the SPS from NA57 RCP = central/peripheral normalized to 1 NN collision K0 and L similar trend at SPS as at RHIC hint for recombination? 1 ~ RCP(K0) > RCP(p0) but… no Cronin enhancement! ALICE - Catania, 11.01.2005 Andrea Dainese

  27. NA57 preliminary RCP at the SPS from NA57 • NA57 K0 data compared to • pQCD calculation by X.N.Wang: • enhancement, expected if no energy loss, not observed • data vs theory suggest significant quenching at the SPS ALICE - Catania, 11.01.2005 Andrea Dainese

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