The Quark-Gluon Plasma Marco van Leeuwen

2 Elementary particles Atom Electron elementary, point-particle Protons, neutrons Composite particle  quarks up charm top down strange bottom Quarks: Electrical charge Strong charge (color) electron Muon Tau    Leptons: Electrical charge 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

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

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:  s decreases with Q 2 Pole at  =   QCD ~ 200 MeV ~ 1 fm -1 Hadronic scale

6 Asymptotic freedom and pQCD At large Q 2, hard processes: calculate ‘free parton scattering’ At high energies, quarks and gluons are manifest + more subprocesses

7 Low Q 2 : confinement Lattice QCD potential  large, perturbative techniques not suitable Lattice QCD: solve equations of motion (of the fields) on a space-time lattice by MC String breaks, generate qq pair to reduce field energy Bali, hep-lat/

8 QCD matter Bernard et al. hep-lat/ T c ~ MeV Energy density from Lattice QCD Deconfinement transition: sharp rise of energy density at T c Increase in degrees of freedom: hadrons (3 pions) -> quarks+gluons (37)  c ~ 1 GeV/fm 3 g : deg of freedom Nuclear matter Quark Gluon Plasma

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

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

11 Nuclear geometry: N part, N bin, L,  b N part : n A + n B (ex: = 9 + …) N bin : n A x n B (ex: 4 x 5 = 20 + …) Two limits: - Complete shadowing, each nucleon only interacts once,   N part - No shadowing, each nucleon interact with all nucleons it encounters,  N bin Soft processes: long timescale, large   tot  N part Hard processes: short timescale, small ,  tot  N bin Transverse view Eccentricity Path length L, mean Density profile  :  part or  coll x y L

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

13 Centrality dependence of hard processes d  /dN ch 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 Binary collisions weight towards small impact parameter Total multiplicity: soft processes

14 Selected topics in Heavy Ions Elliptic flow –Bulk physics, low p T, expansion driven by pressure gradients Parton energy loss –High-energy parton ‘probes’ the quark gluon plasma –Light/heavy flavour

15 Collective Motion Only type of collective transverse motion in central collision (b=0) is radial flow. Integrates pressure history over complete expansion phase Elliptic flow, caused by anisotropic initial overlap region (b > 0) More weight towards early stage of expansion (the QGP phase)

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

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

18 How does the system evolve? 1) Superposition of independent p+p: 2) Evolution as a bulk system momenta pointed at random relative to reaction plane Pressure gradients (larger in-plane) push bulk “out”  “flow” more, faster particles seen in-plane N  -  RP (rad) 0  /2   /43  /4 N  -  RP (rad) 0  /2   /43  /4 Animation: Mike Lisa

19 Energy dependence of flow Flow at RHIC consistent with ideal hydrodynamics!! … so what will we get at LHC ? NA49, PRC68,

20 Hard probes of QCD matter Use ‘quasi-free’ partons from hard scatterings to probe ‘quasi-thermal’ QCD matter Interactions between parton and medium: -Radiative energy loss -Collisional energy loss -Hadronisation: fragmentation and coalescence Sensitive to medium density, transport properties Calculable with pQCD Quasi-thermal matter: dominated by soft (few 100 MeV) partons

21 Energy loss in QCD matter radiated gluon propagating parton 22 QCD bremsstrahlung (+ LPM coherence effects) Density of scattering centers: Nature of scattering centers, e.g. mass: radiative vs elastic loss Or no scattering centers, but fields  synchrotron radiation? Transport coefficient Energy loss Energy loss probes:

22  0 R AA – high-p T suppression Hard partons lose energy in the hot matter  : no interactions Hadrons: energy loss R AA = 1 R AA < 1  0 : R AA ≈ 0.2  : R AA = 1

23 Two extreme scenarios p+p Au+Au pTpT 1/N bin d 2 N/d 2 p T Scenario I P(  E) =  (  E 0 ) ‘Energy loss’ Shifts spectrum to left Scenario II P(  E) = a  (0) + b  (E) ‘Absorption’ Downward shift P(  E) encodes the full energy loss process Need multiple measurements to distentangle processes R AA gives limited information

24 R AA at LHC S. Wicks, W. Horowitz, QM2006 T. Renk, QM2006 Expected rise of R AA with p T depends on energy loss formalism Nuclear modification factor R AA at LHC sensitive to radiation spectrum P(  E) LHC: typical parton energy > typical  E GLVBDMPS RHIC

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