Studying Hot and Dense QCD Matter
in the LHC era
Urs Achim WiedemannCERN, PH-TH Department
Quark Matter 200930 MarchKnoxville
<<Many in the RHIC community … are interested in the LHC heavy-ion program, but have several questions …
What can we learn at the LHC that is qualitatively new?
Are collisions at LHC similar to RHIC ones, just with a somewhat hotter/denser initial state? If not, why not?
… These questions are asked in good faith.>>
Glenn Young, 25 January e-mail inviting me for a plenary talk
• Large quantitative gains at LHC? YES - bigger - longer - denser
Testing QCD thermodynamics at RHIC and LHCSPS RHIC LHC
• Qualitatively the same system @ LHC “just hotter and denser”? No
QCD thermodynamics indicates characteristic qualitative differences in going from ~1.5 Tc to ~5 Tc.
Which ones? p.t.o.
Karsch, Laermann, Peikert,NPB605 (2001) 579
RBC-Bielefeld,PoS LAT2007:217,2007.
The renormalized Polyakov loop• Trace of Wilson line
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Tr Ωr x ( ) ≡ Tr P exp i dτ A0 τ ,
r x ( )
0
β
∫ ⎡
⎣ ⎢
⎤
⎦ ⎥
⎧ ⎨ ⎪
⎩ ⎪
⎫ ⎬ ⎪
⎭ ⎪
RBC-Bielefeld,PoS LAT2007:217,2007.
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Tr Ωr x ( )⇒ z TrΩ
r x ( )
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Tr Ωr x ( ) = exp −β ΔFq
r x ( )[ ]
- transforms under Z(3)
- interpretation
• Good order parameter for pure SU(3)
A.Kurkela
Limit reached above 3-5 Tc
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Tr Ωr x ( )
Relation to the validity of quasi-particle pictures
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Re A0[ ]
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Im A0[ ]
• The effective potential at T>>Tc
- height of potential barrier
- fluctuations between minima negligible only above 3-5 Tc
Gross, Pisarski, YaffeRev.Mod.Phys.53:43,1981
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Veff
MAX∝T 4
• Quasi-particle models - generally rely on expanding Veff around A0=(0,0)
- this breaks Z(3) invariance
- the complex minima lie outside domain of validity of description
- no sound basis for T< 2-3 Tc
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Veff Tr Ωr x ( )[ ]
Pressure/T4
a quasi-particle model
Lattice data
SB
J.P.Blaizot, QM06
• Quasi-particle models vs. Lattice - significant deviations/uncertainties below
• Strong fluctuations between different Z(3) minima deviates from asymptotic value above
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T < 2 − 3 Tc
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Tr Ωr x ( )
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Tr Ωr x ( )
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T < 2 − 3 Tc
Indicative of qualitative novel physics above 2-3 Tc
QCD thermodynamics is qualitatively different above 2-3 Tc
• Beware: - Lattice shows - naïve quasi-particle models
- LHC reaches well beyond T=400 MeV, where QCD thermodynamics is different
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ε−3p∝ΛQCD2 T 2
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ε−3p∝T 4
QCD vs. AdS/CFT
• conformal
• asymptotic freedom
• superymmetric
• chiral condensate
N=4 SYM in vacuum
Physics near vacuum and at high energy is very different
• confinement
QCD in vacuum
N=4 SYM finite T
QCD at finite T
• degrees of freedom
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ε−3p
• quasi-particles
YESNO
NO YES
NO YES
NOYES
NO melted
NOlessimportant
NO
NObroken
deconfined
YES NO YES approx at T >>Tc
•
Very different, but may be taken care of by normalization
NO
Zero Small for T>2-3 Tc only
Conceivable for T>2-3 Tc only
EXP: Conceptual questions in reach of LHC
TH: Which field theoretical tools are best suited?
Elliptic flowHydro curves atCERN SPS
• To establish hydrodynamic behavior, reliable theoretical baseline needed
- close to perfect liquid ?
- sensitivity to ? - sensitivity to dissipative properties such as ?
- is the prefect liquid more like ketchup or custard?
TECHQM – Collaboration https://wiki.bnl.gov/TECHQM/index.php/Main_Page
NA45: PRL92 (2004) 032301
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ε−3p
€
η /s
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ηdu flow
dx
⎛
⎝ ⎜
⎞
⎠ ⎟
Shear thickening? orShear thickening?
“True” Jets
Et
€
ΔΦ
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Δη
Talks byS. SalurT. Renk,K. Zapp,I. Vitev…
Leading hadron suppression “True” jet quenching
- sufficient for leading fragment?
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E >>ω >> kT ,qTmed >> ΛQCD
Had
ron
s
• branching of leading parton based on
BDMPS-Z-ASW-GLV-WDHG-etc
TECHQM – Collaboration https://wiki.bnl.gov/TECHQM/index.php/Main_Page
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E ≥ ω ≥ kT ,qTmed ≥ ΛQCD
• Exact energy conservation indispensable
=> Monte Carlo needed
• branching of subleading partons not needed
• leading and subleading branchings must be treated on equal footing
=> Monte Carlo
• perturbatrive (vacuum) baseline analytical calculation or MC
• perturbatrive (vacuum) baseline => Monte Carlo needed
Constraining Models of Jet Quenching
Talk byK. Zapp
Monte Carlo Model of Jet Quenching
Requirement: reproduce all analytically known limits
Vacuum Parton Shower
Collisional Energy Loss
Radiative Energy Loss
Perturbative Baseline+ Hadronization Model
Elastic scatteringlimit
BDMPS-limit
Monte Carlo including LPM-effectNeeded: probabilistic implementation of quantum interference
Solution: in vacuum => angular ordering only in medium => formation time constraint only (follows analytically from BDMPS-ASW)
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dI
dω∝
1 ω3 / 2 for ω << ωc
1 ω3 for ω >> ωc
⎧ ⎨ ⎩
⎫ ⎬ ⎭
MC reproduces BDMPS-limits
K. Zapp, J. Stachel, UAWarXiv:0812.3888
Talk byK. Zapp
From limiting cases to full MC• In the BDMPS-limit:
Talk byK. Zapp
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dσ el
dqT2
Θ μ cut2 − qT
2( )⇒
dσ el
dqT2∝
1
qT2 + μ 2
( )2€
ΔE = ωdI
dω0
ωc∫ ∝ qL2
• Relax soft scattering approximation
• Exact E-p-conservation
• Realistic cross sections
H
Extrem
e incohere
nt
K. Zapp, J. Stachel, UAW in prep (lines denote MC results)
Main messages:- more realistic => more incoherent => larger e-loss- quadratic L2 supression due to coherence- linear L-enhancement due to incoherence
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E =100GeV
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Lc ∝1 ˆ q [ ]
Jet shapes, energy flows• Thrust – baseline
• Thrust – medium above baseline
• Jet multiplicity distributions
Recent jet quenching MCs:
JEWEL (K.Zapp et al.)Q-Pythia (Santiago group)YaJEM (T.Renk)
Talks by C.Salgado, T.Renk
Jet Finding Algorithms• Tremendous recent progress on jet finding algorithms - novel class of IR and collinear safe algorithms satisfying SNOWMASS accords kt(FastJet) anti-kt(FastJet) SISCone - new standard for p+p@LHC - fast algorithms, suitable for heavy ions!
M. Cacciari, G. Salam, G. Soyez, JHEP 0804:005,2008
Event multiplicity
Run
time
[sec
]
• Catchment area of a jet - novel tools for separating soft fluctuations from jet remnants
- interplay with MCs of jet quenching needed
60,000 MB eventsdNch/dη ~ 50 !Abundant rate of p+p events with
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dN ch dη ≅ 50 −100- comparable to semi-peripheral Cu+Cu @ RHIC- includes events not dominated by hard jets
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εBj τ 0 =1 fm( ) ≈ 5 −10GeV / fm3
Does multiplicity drive collectivity?
M.A.Lisa et al., Ann.Rev.Nucl.Part.Sci.55:357-402,2005
J. Schukraft, QM06
Jürgen’s mini-serving of the QGP: light ion physics with protons @ LHC?
@ LHC, even proton wave functions are dense
Central issue since Saturation Unitarization of hadronic cross section
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≠
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σ d4 j = σ s
2 j( )
2σ eff
• Double hard cross section
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σ eff
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σ eff ≈1
3σ inel
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σ d4 j ET , jet
min ≥ 20GeV( ) ≅10μb
- scale factor at Tevatron for
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ET , jetmin ≥ 20GeV
- hard partons closer localized/ correlated than average soft ones
- at LHC
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ET , jetmin
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σ d4 j ET , jet
min ≥100GeV( ) ≅ 50 pb
LHC is first collider to give access to scale dependence of scale factor(transverse growth of hard components in hadron wave functions)
Kovner, UAW Phys.Rev.D66:051502,2002.
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s
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5TeV€
10TeV
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14TeV
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50GeV
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100GeV
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σ eff
S.Domdey, H.J.Pirner, UAW, in prep
The LHeC-project
http://www.ep.ph.bham.ac.uk/exp/LHeC/
• 70 GeV electron on 7 TeV proton or 2.75 TeV Pb • Ring-ring or linac-ring
• LHeC physics program
- discovery machine
- precision in the era of sLHC * pdfs at large x and Q2
separating all flavors, valence and sea
* strong coupling at per-mille accuracy to test grand unification * …
- QCD at extreme parton densities
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se− parton ≤1TeV
Summary
LHC heavy ion program is a giant leap into the unexplored regime of high temperature QCD.
We have all reasons to expect that this will allow for major advances on fundamental open questions in QCD.
