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

2. 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

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
strangeness S
I3 (u,d content)
I3 (u,d content)
Baryons: 3 quarks
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:
as decreases with Q2
Pole at m = L
LQCD ~ 200 MeV ~ 1 fm-1
Hadronic scale

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

7. Low Q2: confinement a large, perturbative techniques not suitable
Bali, hep-lat/
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/
Tc ~ 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 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

10. 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

11. 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

12. Heavy ion Collision in ALICE

13. Nuclear geometry: Npart, Nbin, L, e
Npart: nA + nB (ex: = 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>

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

15. Heavy ion collisions
Heavy-ion collisions produce ‘quasi-thermal’ QCD matter
Dominated by soft partons p ~ T ~ 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 bulk system
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
-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

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. 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

24. Higher harmonics 3rd harmonic ‘triangularity’ v3 is large
Alver and Roland, PRC81,
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
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

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/split in 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
Hadrons: energy loss
RAA = 1
RAA < 1
Hard partons lose energy in the hot matter

30. 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

31. 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)

32. 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

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