The Strong Interaction and the Quark-Gluon Plasma Marco van Leeuwen

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

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 strangeness S I3 (u,d content) I3 (u,d content) Baryons: 3 quarks Mesons: quark-anti-quark

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

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

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

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

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)

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

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

ALICE 2010: 20M hadronic Pb+Pb events, 300M p+p MB events 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

Heavy ion Collision in ALICE

Nuclear geometry: Npart, Nbin, L, e 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 y L Eccentricity x Path length L, mean <L>

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

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

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

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

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

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 bulk system Pressure gradients (larger in-plane) push bulk “out”  “flow” “zero” pressure in surrounding vacuum more, faster particles seen in-plane b

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

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

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

In general: initial state may be ‘lumpy’ 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

Higher harmonics 3rd harmonic ‘triangularity’ v3 is large 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

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

Medium-induced radiation Landau-Pomeranchuk-Migdal effect Formation time important Energy loss radiated gluon Radiation sees length ~tf at once 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

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

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?

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

Two extreme scenarios P(DE) encodes the full energy loss process 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

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

Di­hadron correlations 8 < pTtrig < 15 GeV Combinatorial background 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

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

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

Extra slides

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