ads/cft and heavy ion collisions at rhic and lhc hong liu massachusetts institute of technology
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AdS/CFT and Heavy Ion Collisions at RHIC and LHC
Hong Liu
Massachusetts Institute of Technology
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QCDQCD has presented us many fascinating dynamical phenomena:
Recently, heavy ion collision experiments opened new windows into probing dynamical phenomena in QCD:
Confinement, chiral symmetry breaking, asymptotic freedom,internal structure of nucleons……
largely guided by experiments, great challenges for theorists.
Many body physics, collective phenomena, …….
thermalization, finite temperature, ……
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Smooth crossover at MeV 170~CTSmall baryon density :
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Relativistic heavy ion collisions
Deconfinement crossover in QCD: TC ~ 170 MeV
LHC: Pb + Pb (2009) GeVsNN 500,5
RHIC (2000): Au+Au GeVsNN 200
NNs : center of mass energy per pair of nucleons
Au: 197 nucleons; Total: 39.4 TeV
Temperature (1 fm after collision) ~ 250 MeV
Baryon chemical potential ~ 27 MeV
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QCD Phase diagram
RH
IC
LHC
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Quark-gluon liquid
Experiments at RHIC suggest:
At T ~ 1.5 TC , the quark-gluon plasma (QGP) is so strongly coupled that it is better thought ofas a liquid than a gas.
A B
Evolution well described by ideal hydrodynamics
(very small viscosity)
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)1.0(densityentropy
osityshear viscO
s
)1(~ Os
according to perturbative QCD calculations
~ 10s
Water
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How to calculate properties of a strongly coupled QGP liquid?
dynamical quantities: scarce and indirect
Lattice calculations:
great for static (thermodynamic) properties
Perturbative QCD calculations:
right theory, wrong approximation
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String theory to the rescue!
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AdS/CFT correspondenceMaldacena (1997), Gubser, Klebanov,Polyakov; Witten (1998)
Strongly coupledgauge theories
Classical gravity in a higher dimensional spacetime
Finite temperature
QGPBlack hole
Infinite families of examples are known, including gauge theories which are:
conformal or nonconformalconfining or non-confining supersymmetric or non-supersymmetric
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0z
(3+1)-dim world,
Our (3+1)-dimensional world lies at the boundary of AdS.
AdS z
AdS: anti-de Sitter spacetime (negative cosmological constant)
Finite Temperature
event horizon (black hole)
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it is NOT yet known what is the gravity description of QCD.
However,
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StrategyUnderstand QGP properties in other theories which are analyzable at strong coupling and compare with experiments.
(wrong theory, right approximation)
new discovery machine,
“Ising model’’ for QCD QGP:
N=4 Supersymmetric Yang-Mills at finite T
two parameters: NC , scale invariant
Large NC , large λ limit: “easy’’ to calculate
look for universality
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Purpose of the talk
Describe some of the dynamical insights into properties of strongly coupled plasma obtained from AdS/CFT:
• Shear viscosity
• Jet quenching
• heavy quark diffusion
• Thermodynamic properties
• Quarkonium suppression (a prediction from string theory)
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Q1: ThermodynamicsN=4 SYM:
4
3
000
P
P
S
S
Thermodynamics of very weakly and very strongly coupled plasmas can be similar.
Infinite families of gauge theories (with different gauge groups and matter contents) share similar properties:
2.1.89 ,4
3
0
ffS
S
Gubser, Klebanov, Peet
Nishioka, Takayanagi
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RHIC
QCD: 06.14
38.0
)(
) few a(
CT
Weak dependence on T of in the range TC ~ 4 TC 4/T
approximately scale invariant
Karschhep-lat/0106019
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Q2: Shear viscosityFor all known strongly coupled QGPs with a gravity description:
Conformal or not, supersymmetric or not, chemical potential or not, confining at T=0 or not, having fundamentals or not with varying number of degrees of freedom
)1.0(Os
QCD:
The only other system which comes close: strongly coupled cold atomic gas
Policastro, Son, and Starinets
Kovtun, Son and Starinets Buchel, J. Liu
08.04
1
s
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Universality ?
We now know infinite classes of different QGPs:
similar thermodynamical properties
universality of shear viscosity
To what observables does the universality apply ?
Is QCD at T~ a few TC in this class?
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Q3:Jet Quenching10-20 GeV jet
????
an excess of low energy particles redistributed to rather large angles
The dominant effect of the medium on a high energy parton is medium-induced Bremsstrahlung.
High energy partons lose energy quickly in the QGP
q̂ : reflects the ability of the medium to “quench” jets.
encodes all the soft physics (in the high energy limit )
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Jet Quenching parameter
: 5-15 GeV2/fmq̂
Perturbation theory:
Experimental estimate:
: < 1 GeV2/fmq̂
q̂
From
is NOT proportional to the number of scattering centers. q̂
N=4 SYM:
33
45
432/3
69.26ˆ TNTq cSYMSYM
/fm.GeV 4.5ˆ 2SYMq MeV TC
N s 300 ,2
1 ,3
HL, Rajagopal, Wiedemann
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Universality of ?
A family of conformal field theories (CFT) with a gravity dual: (large N and strong coupling)
ˆ
ˆ
44
N
CFT
N
CFT
s
s
q
qsCFT : entropy density
63.0120
5.47
ˆ
ˆ
44
N
QCD
N
QCD
s
s
q
q
Estimate for QCD:
HL,RajagopalWiedemann,
q̂
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Q4: Heavy quarks moving in a QGP
Considering a very heavy quark moving in a QGP,
an electron moving in the water
a bullet moving in the water
(point-like, bremsstrahlung)
or
(described by hydrodynamics)
is it more like
not known in QCD
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A moving quark in a strongly coupled N=4 plasma
Chesler , Yaffe, 0712.0050 See also Gubser, Pufu and Yarom
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A prediction from string theory
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Heavy quarkonia as probes of QGP
The potential between the quark and anti-quark in a quarkonium bound state is sensitive to the screening of the plasma.
A hallmark of QGP is that it screens color objects.
Heavy ion collisions: color screening in the produced medium
Matsui and Satz (1987)
J/ψ suppression
They dissociate at Td > TC
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Screening of heavy quarks in QCD
O.Kaczmarek, F. Karsch, P.Petreczky,F. Zantow, hep-lat/0309121
Heavy quark potential for T > TC
Screening length:
TLS /5.0~
Heavy ion collisions: Q and Qbar move relative to the plasma
screening at finite velocity ? (not known)
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Screening in N=4 SYM
L
V(L): potential between a Q and Qbar
LLVT
1~)( :0
TTLS
277.0)(
LS(T)
Similar to QCD above TC
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Finite velocity scaling
Finding string shapeof minimal energy
Event horizon
Moving at a finite velocity v
TLL SS
1)v1(~)v1)(0(~v)( 4/124/12
HL,Rajagopal,Wiedemann
Chernicoff, Garcia, Guijosa
Peeters, Sonnenschein, Zamaklar
Dissociation temperature:
)0()v-(1~v)( 4/12dd TT
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I will now assume a similar scaling applies to QCD
TLL SS
1)v1(~)v1)(0(~v)( 4/124/12
Dissociation temperature:
)0()v-(1~v)( 4/12dd TT
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A prediction from string theoryHL,Rajagopal,Wiedemann
This effect may be tested at RHIC II or LHC
Could lead to significant suppression at large PT.
RHIC ~1.5 Tc
)0()v-(1~v)( 4/12dd TT
/ (c c)J
(b b)
: Tdiss ~ 2.1 TC
: Tdiss ~ 3.6 TC
Asakawa, Hatsuda;Datta, Karsch, Petreczky, Wetzorke
zero velocity : (lattice)
J/ψRHIC
LHC?
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Quark Matter 2008, Jaipur, India, Feb. 4-10,
200833Zebo Tang, USTC/BNL
Nuclear modification factor RAA
• Double the pT range to 10GeV/c
• Consistent with no suppression at high pT: RAA(pT>5 GeV/c) = 0.89±0.20
• Indicates RAA increase from low pT to high pT
• Different from expectation of most models: AdS/CFT:
H. Liu, K. Rajagopal and U.A. Wiedemann, PRL 98, 182301(2007) and hep-ph/0607062
Two Component Approach: X. Zhao and R. Rapp, hep-ph/07122407
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Propagation of mesons in QGP
Does the screening affect the propagation of mesons in a QGP?
, Cv k k
0.88 for 0.65 C dv T T
Speed limit :
0.35 for 0.98 C dv T T
Mateos, Myers and Thomson
Ejaz, Faulkner, HL, Rajagopal, Wiedemann
1Cv
dTT 0CvAs ,
This could also lead to observable effects.
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Heavy ion collisions and AdS/CFT
String theory techniques provide qualitative, and semi-quantitative insights and predictions regarding properties of strongly interacting quark-gluon plasma.
“Wrong theory, right approximation” appears to work much better than “right theory, wrong approximation”
Universality?
Thank You
Many other things I did not have time to discuss