Heavy Ion Collisions at the LHC

startup:p+p 2007Pb+Pb 2007/8

Urs Achim WiedemannPhysics Department, CERN

9 October 2004ALICE-Korea Workshop

Kinematics at the LHC

small-x: determines properties of the medium

larg

e Q

: d

eter

min

es p

rob

es

of

the

med

ium

2

Hadronization versus Thermalization of Partons

hh

h

Lhadr ~constQhadr

EQhadr

in vacuumin QGP

EH ­ E =· s C R

4q L therm

2

Lhadr ~E / Q hadr2

L therm~ E / qQ hadr=1 GeV

q =1GeV 2

fm

1GeV 10 GeV 100 GeV

100 fm

1 fm

Lmedium

E

Dynamics ofhadronization

softinter-

mediate hard

Jetabsorption

Jet heating

Dynamics ofthe bulk

Partonic equilibration processes

Setting the Transverse Momentum Scales

pThadr …6 7GeV Ò pT

parton…8 10 GeV

● High Lhadr …L thermpT

parton thermalizationjet tomography

● Intermediate Lhadr ~L therm~L med

dynamics of hadronization

pT

2 GeV „ pThadr „ 6 7GeV

● Soft pT

bulk dynamics

pThadr „ 2 GeV

High pt Partons as Probes of Dense Matter

1. hadronic high-pt spectra

Centrality Dependence: Au+Au vs. d+Au

Preliminary DataFinal Data

● Final state suppression ● Initial state enhancement

The medium-modified Final State Parton Shower

Medium characterized bytransport coefficient:

q =Â2

Á~ n density

Baier, Dokshitzer, Mueller, Peigne, Schiff (1996); Zakharov (1997); Wiedemann (2000); Gyulassy, Levai, Vitev (2000); Wang ...

Salgado,Wiedemann PRD68:014008 (2003)

● energy loss of leading parton ● pt-broadening of shower

Î c =q L 2 / 2

Î c / Î =32

Î c

Î=10

Î c

Î=3.2

Î c / Î =1À2=

k t2

q L

q L

Quenching of Leading Hadroproduction

suppression of leadinghadrons governed by

X.N. Wang, nucl-th/0307036

Centrality dependence testsinterplay of density and geometry

E.Wang, X.N. Wang, PRL 89 (2002) 162301; Gyulassy, Vitev, Wang PRL 89 (2002) 252301; Salgado, Wiedemann PRL 89 (2002) 092303

»#É=0.2fm / c~15GeV

fm 3

suppression implies

ÉQGPlife =4 ±2 ±? fm / c

scaling

q L 2

X.N. Wang, nucl-th/0307036q

GeV 2

fm= const.

.~O 1

»3 / 4 GeVfm 3

Eskola, Honkanen, Salgado, Wiedemann, 2004

Fragility of leading hadron spectra as hard probe

q =0GeV 2

fm

q =1GeV 2

fm

q =5GeV 2

fm

● surface emission limits sensitivity to density q

dÈ medAA Œh ƒ X =

f

dÈ vacAA Œh ƒ X ×P f ­ E , L , q

quenching weight

× D f Œhvac z

fragementation

d È vacAA Œh ƒ X =

ijk

f i / A x1 × f j / A x 2

nuclear modified pdfs

×È ij Œ f ƒ k

Testing the Mechanism: E-Loss of Heavy Quarks

● medium-induced radiation fills the dead-cone

À2= k t2 / q L

mE

= 0.1

massive dI / dÀ2

massless

dead cone

Î c

Î=10

Armesto, Salgado, Wiedemann, hep-ph/0312106

● vacuum radiation suppressed in the dead-cone ¾„ m / E

Dokshitzer, Kharzeev, PLB 519 (2001) 199

● total energy loss comparable but smaller than in the massless case

Armesto, Salgado, Wiedemann, hep-ph/0312106

Uncertainties:- massive quarks are slower

- finite energy corrections large

(hadronization inside medium relevant for higher pt)

1k t

2 Òk t

2

k t2 ƒ Î 2 m 2 / E 2

2

Djordjevic, Gyulassy, nucl-th/0310076B.W. Zhang, E. Wang, X.N. Wang, nucl-th/0309040

Beyond Leading Hadroproduction

Where does thisassociated radiationgo to ?

How does this partonthermalize ?

Jets in Heavy Ion Collisions at the LHC

­ ½­ Ë

E t

● Experiments at LHC will detect jets above background

● How can we quantify their medium modifications above background ? What do we learn from them ?

A. Accardi et al., hep-ph/0310274 CERN TH Yellow Report

'Jet Heating': Jet Shapes and Jet Multiplicities

Çvac R =1

N jetsjets

E t R

E t R =1

Salgado, Wiedemann, hep-ph/0310079, Phys. Rev. Lett. in press

● Energy fraction in fixed jet cone

R= ­ ½2 ƒ ­ Ë 2

weakly dependent on medium

medium

vacuum

● Multiplicity within small jet cone broadens significantly

unaffected by high-multiplicitybackground !

. kt

Armesto, Salgado, Wiedemann, 2004

Asymmetric Jet Shapes test Collective Flow

ETjet =100 GeV

q L = 1GeV 2

q L flow= q L

● Hard partons are not produced in the rest frame comoving with the medium

T ÂÃ= »ƒ p uÂuà p gÂÃ

test this

Theory predicts

● Characteristic L- dependence ­ EC · s Î c =· s

12

q L 2

● Dependence on the identity of the parent parton

● Relation between energy loss and pt-broadening

­ E q L 2 , pT2 q L

● Dependence on collective motion and energy density of medium

Experimental Tests

➔ leading hadron spectra as function of centrality and orientation w.r.t. reaction plane

➔ jets and jet-like correlations

➔ charmed (beauty) hadrons ➔ - ratio at high ptp / p

➔ hadroproduction associated to high-pt trigger particles

➔ jet shapes and jet multiplicity distributions

Intermediate pt

Fragmentation inside the medium:what happens now ?

Breakdown of Independent (pert.) Fragmentation● Jet quenching does not affect ratio in contrast to observationD q Œ p / D q ŒÆ

● How does the bleaching of color charges proceed dynamically ?

abare colored

parton

dressed quarkor

color stringor

diquarkor...

'bleached' hadron

Recombination

String Fragmentation

Diquark Dressing

D u Πp / D u ή

S. Bass

D a Œh z ,Q 2pQCD:

Fragmentation versus Recombination

start from quark spectrum: w · P / z

EdN h

d 3 P=+0

1 dz

z 2 w · P / z D · Œh z

● Fragmentation:

● Recombination

EdN M

d 3 P~C M w · P / 2 2 + d 3 q ËM q

2

EdN B

d 3 PC B w · P /3 3

recombination

fragmentationph= z p q , z „ 1

ph= p1ƒ p2

Exp. spectrum: recombination dominatesPowerlaw tail: fragmentation dominates

Range of applicability: intermediate pt

upper bound on pt

v 2h žP T Ÿ= nv 2

1n

PT

Castillo(STAR)@HIC03

● Quark number scaling of v2Voloshin, QM2002, nucl-ex/0210014

Soft pt

Statistics and Dynamics of the Bulk

Hadronization in the Grand Canonical Ensemble

Which microscopic dynamics turns quarks and gluons into hadrons ?

● Recent refinements: - system size dependence - hadronization of charm

● Hadronization of a QGP in chemical equilibrium at (T, ) ?Â

de Forcrand, Philipsen 1, Fodor, Katz Allton et al. D'Elia, Lombardo

● Hadron yields and ratios fitted by statistical model in terms of (T, ) Â

'Blastwave Model': Snapshot at Freeze-out

● Recent refinements: anisotropy

What is the microscopic dynamics determining these distributions ?

Lisa, Retiere, nucl-th/03122024● Low pt Hadron Spectra fitted by - local thermal equilibrium - collective flow

T ½, buÃ

T fo~100 MeV v trans…0.5 c

space-time geometry: HBT ?, asHBT !

Hydro: Learning from Agreements or Deviations ?

p t ~1 2 GeVHirano, Nara, nucl-th/0307015

Kolb, Rapp PRC67 (2003) 044903

Molnar, Gyulassy, NPA 697 (2002) 495

● How to understand the range of validity of hydrodynamics ?

Opacity Problem !- hadronization may change flow by by a large factor (2-3)

- realistic freeze-out condition (beyond Cooper-Frye) needed for some data, e.g. HBT

● Hydrodynamics + statistical hadronization works up to for

- pt -spectra - rapidity distribution - elliptic flow

Molnar, Voloshin PRL 91 (2003) 092301

Hadronization matters !

Sinyukov, Akkelin, Hama, PRL 89 (2002) 052301Akkelin, Braun-Munzinger, Sinyukov, NPA710 (2002) 439 Tomasik, Wiedemann PRC68 (2003) 034905

Saturation Physicsand its possible tests heavy ion collisions

· s Q s2 j 1È Ç‡ 1

non-linear small-x evolutionperturbatively calculable

High density: but

Balitsky; Kovchegov; Jalilian-Marian,Kovner, Leonidov, Weigert; ...

The projectile: nuclear parton distribution at small x● lepton-nucleus scattering at small x

¹ virtual=q q ƒ q q g ƒ ...

● unintegrated target gluon distribution

at initial rapidity

● Evolution with rapidity: Balitsky 1994, Kovchegov 1999

[

]

*

Geometric Scaling at small x●Solution of BK-equation depends on saturation scale only

● Data on lepton-proton scattering show geometric scaling

Stasto, Golec-Biernat, Kwiecinski, 1999

● Data on lepton-nucleus scatteringArmesto, Salgado, Wiedemann, 2004=/

The Day 1 Observable at RHIC and LHC

signal proportional to multiplicity

PHOBOS, Phys. Rev. Lett. 85 (2000) 3100Au+Au at RHIC

?● Multiplicity important as centrality measure and for estimates of energy density

Bjorken, 1982

A model for the multiplicity in A+A

Ansatz:

Armesto, Salgado, Wiedemann, 2004

provides fair description ofmultiplicity and its centralitydependence at SPS and RHIC

Multiplicity: expectations from naïve extrapolations

Armesto, Salgado, Wiedemann hep-ph/0407018 our modellogarithmicextrapolation

figures from G. Roland

● What determines the initial energy density created in nucleus-nucleus collisions ?● What is the nature of equilibration processes and what is their time scale ?

● What is the dynamics of hadronization and how does it depend on system size ?

● ...

The RHIC-ness of accessible Scales and Questions

1GeV 10 GeV 100 GeV

100 fm

1 fm

dynamics ofhadronization

softinter-

mediate hard

Jetabsorption

Onset of thermalization “DIS on QGP”

dynamics ofthe bulk

+ A-dependence + - dependence + d-Au collisions+ LHC