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The Quark-Gluon Plasma

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The Quark-Gluon Plasma

Marco van Leeuwen

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

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

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

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

At high energies, quarks and gluons are manifest

At large Q2, hard processes: calculate ‘free parton scattering’

+ more subprocesses

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

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)

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

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

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>

... and this is what you see in a presentation

central

peripheral

mid-central

This is what you really measure

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

- 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

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)

Animation: Mike Lisa

1) Superposition of independent p+p:

momenta pointed at random

relative to reaction plane

b

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

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

- Flow at RHIC consistent with ideal hydrodynamics!! … so what will we get at LHC ?

NA49, PRC68, 034903

Use ‘quasi-free’ partons from hard scatterings

Calculable with pQCD

to probe ‘quasi-thermal’ QCD matter

Quasi-thermal matter: dominated by soft (few 100 MeV) partons

Interactions between parton and medium:

- Radiative energy loss
- Collisional energy loss
- Hadronisation: fragmentation and coalescence

Sensitive to medium density, transport properties

radiated gluon

propagating parton

m2

QCD bremsstrahlung(+ LPM coherence effects)

Transport coefficient

l

Energy loss

Energy loss probes:

Density of scattering centers:

Nature of scattering centers, e.g. mass: radiative vs elastic loss

Or no scattering centers, but fields synchrotron radiation?

: no interactions

RAA = 1

Hadrons: energy loss

RAA < 1

: RAA = 1

0: RAA≈ 0.2

Hard partons lose energy in the hot matter

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

GLV

BDMPS

T. Renk, QM2006

RHIC

RHIC

S. Wicks, W. Horowitz, QM2006

LHC: typical parton energy > typical E

Expected rise of RAA with pT depends on energy loss formalism

Nuclear modification factor RAA at LHC sensitive to radiation spectrum P(E)

- 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