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The Quark-Gluon Plasma and Jet Quenching

The Quark-Gluon Plasma and Jet Quenching. Marco van Leeuwen. QCD and hadrons. Quarks and gluons are the fundamental particles of QCD (feature in the Lagrangian). However, in nature, we observe hadrons: Color-neutral combinations of quarks, anti-quarks. Baryon multiplet. Meson multiplet. S

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The Quark-Gluon Plasma and Jet Quenching

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  1. The Quark-Gluon Plasmaand Jet Quenching Marco van Leeuwen

  2. QCD and hadrons Quarks and gluons are the fundamental particles of QCD (feature in the Lagrangian) However, in nature, we observe hadrons: Color-neutral combinations of quarks, anti-quarks Baryon multiplet Meson multiplet S strangeness I3 (u,d content) I3 (u,d content) Mesons: quark-anti-quark Baryons: 3 quarks ‘Red + anti-Red = white’ ‘Red + Green + Blue = white’

  3. Seeing quarks and gluons In high-energy collisions, observe traces of quarks, gluons (‘jets’)

  4. How does it fit together? S. Bethke, J Phys G 26, R27 Running coupling: as decreases with Q2 Pole at m = L LQCD ~ 200 MeV ~ 1 fm-1 Hadronic scale

  5. Asymptotic freedom and pQCD At high energies, quarks and gluons are manifest At large Q2, hard processes: calculate ‘free parton scattering’ + more subprocesses

  6. Low Q2: confinement a large, perturbative techniques not suitable Bali, hep-lat/9311009 Lattice QCD: solve equations of motion (of the fields) on a space-time lattice by MC Lattice QCD potential No free color charges can exist: would take infinite energy field generates quark-anti-quark pairs

  7. QCD matter Energy density from Lattice QCD g: deg of freedom Nuclear matter Quark Gluon Plasma Bernard et al. hep-lat/0610017 Tc ~ 170 -190 MeV ec ~ 1 GeV/fm3 Deconfinement transition: sharp rise of energy density at Tc Increase in degrees of freedom: hadrons (3 pions) -> quarks+gluons (37)

  8. QCD phase diagram Quark Gluon Plasma (Quasi-)free quarks and gluons Temperature Critical Point Early universe Confined hadronic matter High-density phases? Elementary collisions (accelerator physics) Neutron stars Nuclear matter Bulk QCD matter: T and mB drive phases

  9. Heavy ion collisions Lac Leman Lake Geneva Geneva airport CERN Meyrin site Collide large nuclei at high energy to generate high energy density  Quark Gluon PlasmaStudy properties RHIC: Au+Au sNN = 200 GeV LHC: Pb+Pb √sNN≤ 5.5 TeV 27 km circumference

  10. ALICE • Central tracker: • |h| < 0.9 • High resolution • TPC • ITS • EM Calorimeters • EMCal • PHOS • Particle identification • HMPID • TRD • TOF Forward muon arm -4 < h < -2.5 2010: 20M hadronic Pb+Pb events, 300M p+p MB events

  11. Heavy ion Collision in ALICE

  12. Heavy ion collisions Heavy-ion collisions produce‘quasi-thermal’ QCD matter Dominated by soft partons p ~ T ~ 100-300 MeV ‘Bulk observables’ Study hadrons produced by the QGP Typically pT < 1-2 GeV ‘Hard probes’ Hard-scatterings produce ‘quasi-free’ partons  Probe medium through energy loss pT > 5 GeV • Two basic approaches to learn about the QGP • Bulk observables • Hard probes

  13. Centrality examples ... and this is what you see in a presentation central peripheral mid-central This is what you really measure

  14. Centrality Peripheral Central (Almost) Circular Initial shape Elliptic Volume, ‘Number of participants’ Density, Temperature, Pressure Lifetime ‘QGP effects’

  15. Hard Probes of Heavy Ion Collisions To probe this Use this ALICE Pb+Pb event

  16. Participants and Collisions b Npart: nA + nB (ex: 4 + 5 = 9 + …) Nbin: nA x nB (ex: 4 x 5 = 20 + …) • Two limits: • - Complete shadowing, each nucleon only interacts once, s Npart • No shadowing, each nucleon interact with all nucleons it encounters, s  Nbin • Soft processes: long timescale, large s,stot Npart • Hard processes: short timescale, small s, stot Nbin

  17. Testing volume (Ncoll) scaling in Au+Au Direct g spectra PHENIX, PRL 94, 232301 PHENIX Centrality Scaled by Ncoll Direct g in A+A scales with Ncoll A+A initial state is incoherent superposition of p+p for hard probes

  18. Fragmentation and parton showersIn the vacuum (no QGP) mF MC event generators implement ‘parton showers’ Longitudinal and transverse dynamics High-energy parton (from hard scattering) Hadrons Q ~ mH ~ LQCD large Q2 Analytical calculations: Fragmentation Function D(z, m) z=ph/Ejet Only longitudinal dynamics

  19. Medium-induced radiation Radiation sees length ~tf at once Landau-Pomeranchuk-Migdal effect Formation time important Energy loss radiated gluon propagating parton CR: color factor (q, g) : medium density L: path length m: parton mass (dead cone eff) E: parton energy Energy loss depends on density: Path-length dependence Ln n=1: elastic n=2: radiative (LPM regime) n=3: AdS/CFT (strongly coupled) and nature of scattering centers (scattering cross section) Transport coefficient

  20. p0 RAA – high-pT suppression : no interactions RAA = 1 Hadrons: energy loss RAA < 1 : RAA = 1 0: RAA≈ 0.2 Hard partons lose energy in the hot matter

  21. Nuclear modification factor ‘What you plot is what you get’ ‘Absorption’ ‘Energy loss’ 1/Nbin d2N/d2pT p+p Downward shift Shifts spectrum to left Au+Au pT Measured RAA is a ratio of yields at a given pT The physical mechanism is energy loss; shift of yield to lower pT

  22. Nuclear modification factor (pre-QM) ASW: HT: AMY: RHIC √sNN=200 GeV LHC √sNN=2.76 TeV PHENIX run-4 data ALICE: arXiv:1208.2711 CMS: arXiv:1202.2554 RHIC: no pT dependence ? LHC: increase of RAA with pT Model curves: density fit to data Model curves: Density scaled from RHIC Some curves fit well, others don’t  Handle on E-loss mechanism(s)

  23. Di­hadron correlations Combinatorialbackground 8 < pTtrig < 15 GeV associated pTassoc > 3 GeV  trigger Near side Away side Use di-hadron correlations to probe the jet-structure in p+p, d+Au and Au+Au

  24. Di-hadrons at high-pT: recoil suppression d+Au Au+Au 20-40% Au+Au 0-5% pTassoc > 3 GeV pTassoc > 6 GeV High-pT hadron production in Au+Au dominated by (di-)jet fragmentation Suppression of away-side yield in Au+Au collisions: energy loss

  25. Jets in Pb+Pb Main motivation: integrate radiated energy; Determine ‘initial parton energy’ Out-of-cone radiation: suppression of jet yield: RAAjets < 1 In-cone radiation: softening and/or broadening of jet structure First question: is out-of-cone radiation significant?

  26. PbPb jet spectra M. Verweij@HP, QM Charged jets, R=0.3 RCP, charged jets, R=0.3 Jet reconstruction does not‘recover’ much of the radiated energy Jet spectrum in Pb+Pb: charged particle jets Two cone radii, 4 centralities

  27. Pb+Pb jet RAA Jet RAA measured byATLAS, ALICE, CMS Good agreementbetween experiments Despite different methods: ATLAS+CMS: hadron+EM jets ALICE: charged track jets RAA < 1: not all produced jets are seen; out-of-cone radiation and/or ‘absorption’ For jet energies up to ~250 GeV; energy loss is a very large effect

  28. g, hadrons, jets compared Jets g, hadrons Suppression of hadron (leading fragment) and jet yield similar

  29. Model comparison Jet RAA Hadron RAA U. Wiedemann@QM2012 JEWEL: K. Zapp et al, Eur Phys J C69, 617 Schukraft et al, arXiv:1202.3233 M. Verweij@HP, QM2012 At least one model calculation reproduces the observed suppression  Understand mechanism for out-of-cone radiation?

  30. Jet broadening: R dependence Ratio of spectra with different R Larger jet cone: ‘catch’ more radiation  Jet broadening ATLAS, A. Angerami, QM2012 However, R = 0.5 still has RAA < 1 – Hard to see/measure the radiated energy

  31. Jet Quenching • How is does the medium modify parton fragmentation? • Energy-loss: reduced energy of leading hadron – enhancement of yield at low pT? • Broadening of shower? • Path-length dependence • Quark-gluon differences • Final stage of fragmentation outside medium? 2) What does this tell us about the medium ? • Density • Nature of scattering centers? (elastic vs radiative; mass of scatt. centers) • Time-evolution?

  32. The End

  33. Summary • Elementary particles of the strong interaction (QCD): quarks and gluon • Bound states: p, n, p, K (hadrons) • Bulk matter: Quark-Gluon-Plasma • High T~200 MeV • Heavy ion collisions: • Produce and study QGP • Elliptic flow • Parton energy loss

  34. Extra slides

  35. Centrality dependence of hard processes Total multiplicity: soft processes Binary collisions weight towards small impact parameter ds/dNch 200 GeV Au+Au • Rule of thumb for A+A collisions (A>40) • 40% of the hard cross section is contained in the 10% most central collisions

  36. Elementary particles Standard Model: elementary particles Quarks: Electrical charge Strong charge (color) up charm top down strange bottom +anti-particles Leptons: Electrical charge electron Muon Tau nenmnt photon EM force gluon strong force W,Z-boson weak force Force carriers: Atom Electronelementary, point-particle Protons, neutrons Composite particle  quarks EM force binds electronsto nucleus in atom Strong force binds nucleonsin nucleus and quarks in nucleons

  37. Quarks, gluons, jets Jets: Signature of quarks, gluons in high-energy collisions Hadrons High-energy parton large Q2 Q ~ mH ~ LQCD Quarks, gluons radiate/splitin vacuum to hadronise

  38. RAA at LHC Au+Au sNN= 200 GeV Pb+Pb sNN= 2760 GeV Nuclear modificationfactor LHC: RAA rises with pT relative energy loss decreases Larger dynamic range at LHC very important: sensitive to P(DE;E)

  39. Jet broadening: transverse fragment distributions Pb Pb Pb Pb CMS, P. Kurt@QM12 CMS PAS HIN-12-013 Jet broadening: Soft radiation at large angles

  40. Time evolution All observables intregrate over evolution Radial flow integrates over entire ‘push’

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