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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 Œ p / D u ŒÆ
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
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