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The Strong Interaction and the Quark-Gluon Plasma

The Strong Interaction and the Quark-Gluon Plasma. Marco van Leeuwen. Elementary particles. Standard Model: elementary particles. Quarks: Electrical charge Strong charge (color). up charm top down strange bottom. +anti-particles. Leptons: Electrical charge.

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The Strong Interaction and the Quark-Gluon Plasma

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  1. The Strong Interactionand the Quark-Gluon Plasma Marco van Leeuwen

  2. 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

  3. 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

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

  5. 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

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

  7. 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 String breaks, generate qq pair to reduce field energy

  8. 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)

  9. 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

  10. 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

  11. 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

  12. Heavy ion Collision in ALICE

  13. Nuclear geometry: Npart, Nbin, L, e b y L 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 Transverse view Density profile r: rpart or rcoll Eccentricity x Path length L, mean <L>

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

  15. 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

  16. Selected topics in Heavy Ions • Elliptic flow • Bulk physics, low pT, expansion driven by pressure gradients • Parton energy loss • High-energy parton ‘probes’ the quark gluon plasma • Light/heavy flavour

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

  18. Forming a system and thermalizing Animation: Mike Lisa 1) Superposition of independent p+p: momenta pointed at random relative to reaction plane b

  19. Forming a system and thermalizing Animation: Mike Lisa 1) Superposition of independent p+p: high density / pressure at center momenta pointed at random relative to reaction plane 2) Evolution as a bulksystem Pressure gradients (larger in-plane) push bulk “out” “flow” “zero” pressure in surrounding vacuum more, faster particles seen in-plane b

  20. How does the system evolve? N N   0 0 /4 /4 /2 /2 3/4 3/4 -RP (rad) -RP (rad) 1) Superposition of independent p+p: momenta pointed at random relative to reaction plane 2) Evolution as a bulksystem Pressure gradients (larger in-plane) push bulk “out” “flow” more, faster particles seen in-plane Animation: Mike Lisa

  21. Elliptic flow Hydrodynamical simulation Reaction plane Elliptic flow: Yield modulation in-out reaction plane j reaction plane Anisotropy reduces during evolution v2 more sensitive to early times b

  22. Elliptic flow Mass-dependence of v2 measures flow velocity Good agreement between data and hydro

  23. Higher harmonics Schenke and Jeon, Phys.Rev.Lett.106:042301 In general: initial state may be ‘lumpy’ (not a smooth ellipse) h/s = 0 h/s = 0.16 How much of this is visible in the final state, depends on shear viscosity h

  24. Higher harmonics Alver and Roland, PRC81, 054905 3rd harmonic ‘triangularity’ v3 is large (in central events) Mass ordering also seen for v3 indicates collective flow Dominant effect in azimuthal correlations at pT = 1-3 GeV

  25. Viscosity Viscous liquid dissipate energy For a dilute gas: Liquid gas h increases with T Liquid, densely packed, so: Viscosity minimal at liquid-gas transition Evac: activation energy for jumps of vacancies QGP viscosity lower than any atomic matter h decreases with T

  26. 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

  27. 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

  28. 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?

  29. 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

  30. Two extreme scenarios Scenario I P(DE) = d(DE0) Scenario II P(DE) = a d(0) + b d(E) 1/Nbin d2N/d2pT ‘Energy loss’ ‘Absorption’ p+p Downward shift Au+Au Shifts spectrum to left pT P(DE) encodes the full energy loss process Need multiple measurements to distentangle processes RAA gives limited information

  31. 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)

  32. 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

  33. 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

  34. 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

  35. Extra slides

  36. 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

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