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Jet Quenching: Energy Loss of Color-Charged Probes

Jet Quenching: Energy Loss of Color-Charged Probes . One of the first discoveries at RHIC!. PRL 88 (2002) 22301. PHENIX. Outline of My Talk. Introduction Probing the QGP Jets and how to measure them Experimental evidence for jet quenching High pt particle suppression in Au-Au

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Jet Quenching: Energy Loss of Color-Charged Probes

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  1. Jet Quenching: Energy Loss of Color-Charged Probes One of the first discoveries at RHIC! PRL 88 (2002) 22301 PHENIX

  2. Outline of My Talk • Introduction • Probing the QGP • Jets and how to measure them • Experimental evidence for jet quenching • High pt particle suppression in Au-Au • d-Au control experiment • Suppression of jet-jet correlations • Look at more detailed analysis • Medium modification of jet-correlations • Baryon puzzle • Azimuthal event anisotropy • Outlook Axel Drees

  3. RHIC Relativistic Heavy Ion Collisions Quark-Gluon Plasma Critical point Color super- conductor Early Universe Temperature Hadron Gas “frozen Quarks” Color-flavor locking nuclei mbaryon or nucleon density Neutron Stars? The Phase Diagram of Nuclear Matter • QGP in Astrophysics • early universe: time < 106 seconds • possibly in the interior of neutron stars • Quest of heavy ion collisions • create QGP as transient state in heavy ion collisions • verify existence of QGP • study properties of QGP 170 MeV 1Gev/fm3 Overwhelming evidence for strongly interacting plasma produced at RHIC Axel Drees

  4. Ideal Experiment to Probe the QGP • Rutherford experiment a atom discovery of nucleus SLAC electron scattering e  proton discovery of quarks QGP penetrating beam (jets or heavy particles) absorption or scattering pattern Nature needs to provide penetrating beams and the QGP in Au-Au collisions • QGP created in Au-Au collisions as transient state for 10-12 s • penetrating beams created by parton scattering before QGP is formed • high transverse momentum particles  jets • Heavy particles  J/ Axel Drees

  5. p L p p K p p jet J/Y Freeze-out Hadronization QGP Thermaliztion Hard Scattering Au Au Space-time Evolution of Collisions time g g e  Expansion  space Axel Drees

  6. schematic view of jet production leading particle hadrons hadrons leading particle q q hadrons leading particle hadrons leading particle Jets: A Penetrating Probe for Dense Matter • What is a jet? • Incoming partons may carry large fraction x of beam momentum • These partons can scatter with large momentum transfer • Results in large pT of scattered partons • appears in laboratory as “jet” of particles • Jet production can be observed as • high pT leading particles • angular correlation • In a gold gold collision • Scattered partons travel through dense matter • Expected to loose a lot of their energy • Energy loss observed as • suppression of high pT leading particles • suppression of angular correlation • Depending on path length, i.e. centrality and angle to reaction plane reaction plane Axel Drees

  7. Hard Scattering of Partons: Jet Production • Incoming quarks and gluons (a,b) • described by Parton Distribution Function • PDF deduced from experimental data • Scatter with large momentum transfer“Hard scattering” and create c,d • Early in the collision (t ~ 1/Q2) • With large momentum (jets) • With large mass (heavy flavor) • Calculable in pQCD • c,d fragment and create hadrons • Fragmentation functions from data • Fragmentation Theorem AB hXfa/A(xa,Q2a)fb/B(xb,Q2b) a b cdDh/c(zc,Q2c) hard Hard scattering  high pT (jets) or high mass Axel Drees

  8. Jet production measured indirectly by transverse momentum (pT) spectrum Identified particles (p0) Charged particles (h = p, K, p, .. ) At RHIC energies different mechanisms are responsible for different regions of particle production Thermally produced “soft” particles “hard” particles from jet production Hard component can be calculated with QCD Data agrees with QCD calculation “calibrated” reference hard Particle Spectra from p-p Collisions p0 from p-p collisions QCD calculation soft Axel Drees

  9. Participants Scaling from p-p to Heavy Ion Collisions • Hard-scattering processes in p-p • quarks and gluons are point-like objects • small probability for scattering in p-p • p-p independent superposition of partons • Minimum bias A-A collision • assume small medium effects on parton density • superposition of independent p,n collisions • collision probability increases by A2 • cross section scales by number of binary collisions • Impact parameter selected A-A collisions • superposition of p,n collisions among participants • calculable analytically by nuclear overlap integral • or by MC simulation of geometry “Glauber Model” Axel Drees

  10. g q g q Binary Scaling in Au-Au tested with Direct Photons • pp collisions: • qg-Compton scattering • Direct g production described by NLO pQCD • Au-Au collisions: • Direct g rates scale with Nbinary • Similar scaling observed for charm quark production Hard processes in Au-Au scale with Nbinary Axel Drees

  11. Investigate Medium Modification of Spectra Compare Au+Au to nucleon-nucleon particle spectra: Nuclear Modification Factor: “Soft” or bulk particle production R < 1 scales with ~Npart “Hard” or jet-like particle production R = 1 expected scaling with Ncoll R > 1 enhancement, e.g. Cronin effect R < 1 suppression or jet quenching Axel Drees

  12. Suppression of p0 in Central AuAu Collisions PRL 91 (2003) 72301 Nuclear modification factor: PHENIX PHENIX preliminary High pT suppressed by factor ~ 5 pp to central AuAu and peripheral to central Au-Au Axel Drees

  13. High-pT Suppression seen by RHIC Experiments High pt region: independent of pt & particle species charged particles PHENIX p0 Jet quenching well established experimentally Axel Drees

  14. xG(x,Q2) x QS What is the Origin of the Jet Suppression? • Energy loss of partons in dense medium • Partons interact strongly with medium • Radiate gluons and loose much of their energy (~GeV/fm) Jets disappear or are “quenched” in dense medium • Initial state scattering • Multiple elastic scatterings (Cronin effect) before “hard” scattering • Increases transverse momentum • Nuclear enhancement observed in pA and AA at lower energies • Initial state gluon saturation • Quantum mechanical effect • Wave function of gluons overlap, gluon density saturates below scale Qs Jets are not quenched, but are a priori made in fewer numbers Axel Drees

  15. Centrality Dependence Au + Au Experiment d + Au Control Experiment Different and opposite centrality evolution of Au+Au experiment from d+Au control nucl-ex/0308006 Preliminary Data Jet suppression is clearly a final state effect Axel Drees

  16. deuteron gold collision gold-gold collision Control Experiment with d-Au • Initial state saturation effect • Gluon density saturated in incoming gold nucleus • Deuteron shows no or little saturation • Expect suppression of jet yield, but with reduced magnitude • Final state “jet quenching” • Medium created in d-Au has small volume • Jets easily penetrate short distance • No suppression of jet yield expected Final state effect: no suppression Initial state effect: suppression Axel Drees

  17. Energy Loss Analysis of 0RAA GLV calculation: dNg/dy ~ 1100 e ~ 15 GeV/fm3 Medium with extremely opaque core Jets biased towards surface emission Axel Drees

  18. Centrality Dependence of Suppression • Hard region: pT > 4.5 GeV/c • Suppression depends on centrality but not on pT • Characteristic features of jet fragmentation independent of centrality • pQCD spectral shape • h/p0 constant • xT scaling PHENIX Convolute jet absorption or energy loss with nuclear geometry (many publications) Centrality dependence characteristic for jet absorption in extremely opaque medium! Insensitive to details of energy loss mechanism Axel Drees

  19. p+p Trigger particle with high pT > pT cut 1 yield/trigger 0 Df to all other particles with pT > pT cut-2  /2  0 Au+Au yield/trigger elliptic flow random background 0  /2  0 statistical background subtraction Au+Au ??? Au-Au yield/trigger suppression? 0  /2  0 Azimuthal Correlations from Jets pp jet+jet STAR Jet correlations in Au-Au via statistical background subtraction Axel Drees

  20. Disappearance of the “Away-Side” Jet Integrate yields in some f window on near and away side pedestal and flow subtracted trigger 6 <pt< 8 GeV partner 2 < pt < 6 GeV Near-side: p+p, d+Au, Au+Au similar Back-to-back: Au+Au strongly suppressed relative to p+p and d+Au Suppression of the away side jet in central Au+Au Axel Drees

  21. Suppression of Back-to-Back Pairs Jet correlation strength: Near side Compared to jet absorption model (J.Jia et al.) Away side Away side jets are suppressed consistent with jet absorption in opaque medium Axel Drees

  22. d+Au Broadening of the Away Side Jet in Azimuth Away side jet suppression or broadening of fragmentation? Axel Drees

  23. Non Gaussian Structure in Away Side Correlation Sonic boom from quenched jets? • Shuryak et al.: Energy loss of jet results in conical shock wave in strongly interacting plasma Axel Drees

  24.    More Detailed Look at Near-Side Correlations d+Au, 40-100% • Near side: small Df • Two components in Dh: • Short range, jet-like • Long range, flat STAR preliminary Au+Au, 0-5% Jet fragments boosted by longitudinally expanding medium? 3 < pT(trig) < 6 GeV2 < pT(assoc) < pT(trig) Axel Drees

  25. Many Other Mysteries Remain if Data Analyzed in Detail • Particle composition in range 2 to 5 GeV • pp data described by jet fragmentation • d-Au data described by adding nuclear enhancement (Cronin effect) • Au-Au data drastically different from jet fragmentation Baryon enhancement at intermediate pt! Axel Drees

  26. Intermediate pT Baryons ARE from Jets • Angular correlations with identified particles: • Trigger (meson/baryon) 2.5-4.0 GeV • Partner (charged) 1.7-2.5GeV/c • Integrate dN/df over 55° near and away side • Jet partner ~ equally likely for trigger baryons & mesons! • Near side: increased compared to pp • Away side: partner rate as in p+p confirms jet source of baryons! • “disappearance” of away-side jet from narrow angle for both baryons and mesons PHENIX Clear evidence for medium modification of jet fragmentation One interpretation: recombination of quarks from plasma with jet fragments Axel Drees

  27. preliminary 20-60% central STAR Reaction Plane Dependence • Energy loss depends on path length • Should depend on angle of jet with respect to reaction plane Qualitatively observe expected reaction plane dependence Typically quantified by 2nd coefficient v2 of Fourier expansion of dN/df At low pt jet quenching competes with hydrodynamic flow Axel Drees

  28. Azimuthal Anisotropy v2 in 200 GeV Au+Au v2 {2-particle} v2 {AuAu – pp} v2 {4-particle} STAR ~15% PHENIX preliminary v2 at high pT exhausts all reasonable geometric limits (Shuryak) too large to be accounted for by energy loss consistent with other high pT observations (Drees et al.) Origin of large v2 unclear! Axel Drees

  29. On tape; analysis ongoing Discovery of jet quenching Most data seen today Outlook Into the “Near” Future Axel Drees

  30. pQCD direct g + jet quenching PHENIX Preliminary AuAu 200 GeV 0-10% pQCD direct g g q g q Outlook into the “Away” Future g-jet: the golden channel for jet tomography Quark gluon Compton scattering: g-energy fixes jet energy g & Jet direction fix kinematics measure DE as function of: E, “L”, flavor 70% of photons are prompt photons Promising measurement at RHIC: every low cross section; pT< 8-10 GeV on tape luminosity and detector upgrades: extend range to pT~25 GeV and |y|<3 Axel Drees

  31. Summary • Jet quenching is firmly established at RHIC • Final state effect • Consistent with energy loss in opaque medium • Clear indication for in medium modifications of jet fragmentation • Many interesting observations promise much more insight into strongly interacting QGP • v2 puzzle • Baryon enhancement • Detailed particle correlation data • Much more data on tape • Future upgrades of RHIC will allow to fully exploit the unique opportunities of hard probes to study the strongly interacting QGP at RHIC • Luminosity and detector upgrades Axel Drees

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