The Quark-Gluon Plasmaand Jet Quenching

Marco van Leeuwen

2

QCD and hadronsQuarks 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

I3 (u,d content)

S stra

ngen

ess

I3 (u,d content)

Mesons: quark-anti-quark

‘Red + Green + Blue = white’ ‘Red + anti-Red = white’

3

Seeing quarks and gluons

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

4

How does it fit together?

S. Bethke, J Phys G 26, R27

Running coupling:s decreases with Q2

Pole at =

QCD ~ 200 MeV ~ 1 fm-1

Hadronic scale

5

Asymptotic freedom and pQCD

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

At high energies, quarks and gluons are manifest

gqqee

+ more subprocesses

6

Low Q2: 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

Bali, hep-lat/9311009

No free color charges can exist:would take infinite energyfield generates quark-anti-quark pairs

7

QCD matter

Bernard et al. hep-lat/0610017

Tc ~ 170 -190 MeV

Energy density from Lattice QCD

Deconfinement transition: sharp rise of energy density at Tc

Increase in degrees of freedom: hadrons (3 pions) -> quarks+gluons (37)

c ~ 1 GeV/fm3

4gTg: deg of freedom

Nuclear matterQuark Gluon Plasma

8

QCD phase diagram

Tem

per

atu

re

Confined hadronic

matter

Quark Gluon Plasma(Quasi-)free quarks and gluons

Nuclear matter

Neutron stars

Elementary collisions(accelerator physics)

High-density phases?

Ea

rly u

niv

ers

e

Critical

Point

qqB ~

Bulk QCD matter: T and B drive phases

9

Heavy ion collisionsCollide large nuclei at high energy to generate high energy density

Quark Gluon PlasmaStudy properties

STARSTAR

RHIC: Au+Au sNN = 200 GeV

Lac LemanLake Geneva

Geneva airport

CERNMeyrin site

LHC: Pb+Pb √sNN ≤ 5.5 TeV

27 km circumference

10

ALICE

Central tracker:|| < 0.9High resolution• TPC• ITS

Particle identification•HMPID •TRD•TOF

Forward muon arm-4 < < -2.5

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

EM Calorimeters• EMCal• PHOS

11

Heavy ion Collision in ALICE

12

Heavy ion collisions

‘Hard probes’Hard-scatterings produce ‘quasi-free’ partons

Probe medium through energy losspT > 5 GeV

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

Two basic approaches to learn about the QGP1) Bulk observables2) Hard probes

13

Centrality examples

This is what you really measure... and this is what you see in a presentation

centralmid-centralperipheral

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Centrality

Peripheral Central

Density, Temperature, Pressure

(Almost) Circular

Volume, ‘Number of participants’

Initial shape Elliptic

Lifetime

‘QGP effects’

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Hard Probes of Heavy Ion Collisions

Use this

ALICE Pb+Pb event

To probe this

16

Participants and Collisions

b 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, Npart

- No shadowing, each nucleon interact with all nucleons it encounters, Nbin

Soft processes: long timescale, large tot Npart

Hard processes: short timescale, small , tot Nbin

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Testing volume (Ncoll) scaling in Au+Au

PHENIX

Direct spectra

Scaled by Ncoll

PHENIX, PRL 94, 232301

ppTcoll

AuAuTAA dpdNN

dpdNR

/

/

Direct in A+A scales with Ncoll

Centrality

A+A initial state is incoherent superposition of p+p for hard probes

18

Fragmentation and parton showersIn the vacuum (no QGP)

large Q2 Q ~ mH ~ QCDF

Analytical calculations: Fragmentation Function D(z, ) z=ph/Ejet

Only longitudinal dynamics

High-energy

parton(from hard scattering)

Ha

dro

ns

MC event generatorsimplement ‘parton showers’

Longitudinal and transverse dynamics

19

Medium-induced radiation

),(ˆ~ EmFLqCE nRSmed

propagating

parton

radiatedgluon

Landau-Pomeranchuk-Migdal effectFormation time important

Radiation sees length ~f at once

Energy loss depends on density: 1

2

ˆq

q

and nature of scattering centers(scattering cross section)

Transport coefficient

CR: color factor (q, g) : medium densityL: path lengthm: parton mass (dead cone eff)E: parton energy

q̂

2

2

Tf k

Path-length dependence Ln

n=1: elasticn=2: radiative (LPM regime)n=3: AdS/CFT (strongly coupled)

Energy loss

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0 RAA – high-pT suppression

Hard partons lose energy in the hot matter

: no interactions

Hadrons: energy loss

RAA = 1

RAA < 1

0: RAA ≈ 0.2

: RAA = 1

21

Nuclear modification factor

p+p

Au+Au

pT

1/N

bin

d2 N/d

2 pT

‘Energy loss’

Shifts spectrum to left

‘Absorption’

Downward shift

‘What you plot is what you get’

Measured RAA is a ratio of yields at a given pT

The physical mechanism is energy loss; shift of yield to lower pT

ppTcoll

PbPbTAA dpdNN

dpdNR

/

/

22

Nuclear modification factor (pre-QM)

PHENIX run-4 data

RHIC √sNN=200 GeV

ALICE: arXiv:1208.2711CMS: arXiv:1202.2554

LHC √sNN=2.76 TeV

LHC: increase of RAA with pT

RHIC: no pT dependence ?

ASW:

HT:

AMY:

/fmGeV2010ˆ 2q

/fmGeV5.43.2ˆ 2q

/fmGeV4ˆ 2q

Model curves: density fit to data Model curves: Density scaled from RHIC

Some curves fit well, others don’t Handle on E-loss mechanism(s)

23

Di hadron correlations

associated

trigger

8 < pTtrig < 15 GeV

pTassoc > 3 GeV

Use di-hadron correlations to probe the jet-structure in p+p, d+Au

Near side Away side

and Au+Au

Combinatorialbackground

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pT

assoc > 3 G

eVp

Tassoc >

6 GeV

d+Au Au+Au 20-40% Au+Au 0-5%

Suppression of away-side yield in Au+Au collisions: energy loss

High-pT hadron production in Au+Au dominated by (di-)jet fragmentation

Di-hadrons at high-pT: recoil suppression

25

Jets in Pb+Pb

Out-of-cone radiation: suppression of jet yield: RAA

jets < 1

In-cone radiation: softening and/or broadening of jet structure

Main motivation: integrate radiated energy;Determine ‘initial parton energy’

First question: is out-of-cone radiation significant?

26

PbPb jet spectraCharged jets, R=0.3

Jet spectrum in Pb+Pb: charged particle jetsTwo cone radii, 4 centralities

M. Verweij@HP, QM

RCP, charged jets, R=0.3

Jet reconstruction does not‘recover’ much of the radiated energy

27

Pb+Pb jet RAA

Jet RAA measured byATLAS, ALICE, CMS

RAA < 1: not all produced jets are seen; out-of-cone radiation and/or ‘absorption’

For jet energies up to ~250 GeV; energy loss is a very large effect

ATLAS+CMS: hadron+EM jets

ALICE: charged track jets

Good agreementbetween experiments

Despite different methods:

28

, hadrons, jets compared, hadrons Jets

Suppression of hadron (leading fragment) and jet yield similar

29

Model comparison

M. Verweij@HP, QM2012

JEW

EL: K

. Za

pp et al, E

ur Ph

ys J C6

9, 617

U. Wiedemann@QM2012

Hadron RAAJet RAA

Schukraft et al, arX

iv:1202.3233

At least one model calculation reproduces the observed suppression Understand mechanism for out-of-cone radiation?

30

Jet broadening: R dependence

Ratio of spectra with different R

Larger jet cone:‘catch’ more radiation Jet broadening

ATLAS, A. Angerami, QM2012

However, R = 0.5 still has RAA < 1– Hard to see/measure the radiated energy

31

Jet Quenching

1) 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?

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The End

33

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

34

Extra slides

35

Centrality dependence of hard processes

d/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

Binary collisions weight towards small impact parameter

Total multiplicity: soft processes

36

Elementary particles

AtomElectronelementary, point-particle

Protons, neutronsComposite particle quarks

up charm topdown strange bottom

Quarks:Electrical chargeStrong charge (color)

electron Muon Tau

Leptons:Electrical charge

Force carriers:photon EM forcegluon strong forceW,Z-boson weak force

Standard Model: elementary particles

+anti-particles

EM force binds electronsto nucleus in atom

Strong force binds nucleonsin nucleus and quarks in nucleons

37

Quarks, gluons, jetsJets: Signature of quarks, gluons

in high-energy collisions

large Q2

Q ~ mH ~ QCD

High-energy

parton

Hadrons

Quarks, gluons radiate/splitin vacuum to hadronise

38

RAA at LHC

Larger dynamic range at LHC very important: sensitive to P(E;E)

Nuclear modificationfactor

LHC:RAA rises with pT relative energy loss decreases

ppTcoll

PbPbTAA dpdNN

dpdNR

/

/

Au+Au sNN= 200 GeV Pb+Pb sNN= 2760 GeV

39

Jet broadening: transverse fragment distributions

PbPbPbPb PbPbPbPb

CM

S P

AS

HIN

-12-013C

MS

, P. K

urt@Q

M12

Jet broadening: Soft radiation at large angles

40

Time evolution

All observables intregrate over evolution

Radial flow integrates over entire ‘push’