Exploring Hot QCD Matter with ALICE

PHENO11, Madison WI Hot QCD Matter in ALICE 1

• Heavy Ion Collisions: what are we after?• ALICE Overview• ALICE results from 2010 Pb+Pb run• Putting together RHIC and LHC:

What have we learned about hot QCD matter ?

Peter Jacobs, Lawrence Berkeley National Laboratoryfor the ALICE Collaboration

QCD Phase Diagram: qualitative view

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Tem

pera

ture ~170

MeV

Baryon chemical potential µB

~few hundred MeV

Deconfined Quark-Gluon Plasma

QCD thermodynamics: calculation

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42

30TgDOF

4T

T [MeV]

QCD on the lattice (B=0)

Cross-over, not sharp phase transition (like ionization of atomic plasma)

Slow convergence to non-interacting Steffan-Boltzmann limitWhat are the quasi-particles? “Strongly-coupled” plasma?

RHIC LHC?

4Hot QCD Matter in ALICE

ALICE is the comprehensive heavy ion experiment at the LHC

Design optimized for huge particle multiplicities of nuclear collisions

ALICE

PHENO11, Madison WI

ALICE vs ATLAS/CMS

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Requirements for heavy ion physics:• measure large-scale collective phenomena:

reconstruct complex hadronic events• precise measurements of heavy flavor, photons, leptons, jets and jet fragments• energy scale

→ robust tracking ~ 100 MeV – 100 GeV→ calorimetry ~ 200 GeV

• low material budget near vertex• particle ID: multiple detector technologies

Requirements for Higgs/SUSY searches:• missing energy signatures: hermetic coverage• energy scale 10 GeV – 1 TeV• tiny cross sections: high rate and rejection capabilities

ALICE favors robust tracking, precision, and low mass over large acceptance, high rate, and huge dynamic range

November 7 2010: First Pb+Pb collisions at √sNN=2.76 TeV

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Particle ID: TPC dE/dx

Hot QCD Matter in ALICECopious production of anti-nuclei

Tomography via -conversions

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Inner material understood better than 10%

Compare data and MC

NLO(W. Vogelsang)

M

Charm in Pb+Pb

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J/ψμ+μ-

D+K-

D0K-+

Heavy flavor in p+p: consistency check

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Compare directly measured electrons and electrons calculated from D-decay

good agreement at low pt (charm dominant)

Measuring collision geometry I

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Nuclei are “macroscopic”characterize collisions by impact parameter

Correlate particle yields from ~causally disconnected parts of phase space

correlation arises from common dependence on collision impact parameter

Measuring collision geometry II

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For

war

d ne

utro

ns

Charged hadrons ~3

• Order events by centrality metric• Classify into percentile bins of “centrality”

HI jargon: “0-5% central”

Glauber modeling• Nbin: effective number of binary nucleon collisions (~5-10% precision)• Npart: number of (inelastically) participating nucleons

ALICE Results I: hadron multiplicityPRL, 105, 252301 (2010), arXiv:1011.3916

√sNN=2.76 TeV Pb+Pb, 0-5% central, |η|<0.5

2 dNch/dη / <Npart> = 8.3 ± 0.4 (sys.)PHENO11, Madison WI 13Hot QCD Matter in ALICE

dNch/dη: model comparisons

pp extrapolation

pQCD-based MC

Saturation

PRL, 105, 252301 (2010), arXiv:1011.3916

dNch/dη = 1584 ± 76 (sys.)

√sNN=2.76 TeV Pb+Pb, 0-5% central, |η|<0.5

Energy density estimate (Bjorken):

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dNch/dη: Centrality dependencePRL, 106, 032301 (2011), arXiv:1012.1657

Interpolation between 2.36 and 7 TeV pp

Pb+Pb, √sNN=2.76 TeV

2.5% bins

|η|<0.5

ALICELH

C s

cale

RH

IC scaleRHIC

peripheral central

PHENO11, Madison WI 15Hot QCD Matter in ALICEStriking centrality-independent scaling RHICLHC

dNch/dη vs. centrality: modelsPRL, 106, 032301 (2011), arXiv:1012.1657

Two-component models Soft (~Npart) and hard

(~Ncoll) processes

Saturation-type models Parametrization of the saturation

scale with centrality

Comparison to data DPMJET (incl. string fusion)

stronger rise than data HIJING 2.0 (no quenching)

Strong centrality dependent gluon shadowing

Fine-tuned to 0-5% dN/dη Saturation models [12-14]

Most have too much saturation

Pb+Pb, √sNN=2.76 TeV

Albacete and Dumitru (arXiV:1011.5161):• Most sophisticated saturation model:

evolution, running coupling• Captures full centrality dependence…?

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Collective Flow of QCD Matter

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Initial spatial anisotropy

xy z

py

px

22

22

xy

xy

22

22

2

yx

yx

pp

ppv

Elliptic flow

Final momentum anisotropy

Interaction of constituents

Elliptic flow v2: LHC vs RHIC

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PRL 105, 252302 (2010)PRL 105, 252302 (2010)

Striking similarity of pT-differential v2 at RHIC and LHC

19Hot QCD Matter in ALICE

Shear viscosity in fluids

PHENO11, Madison WI

Shear viscosity characterizes the efficiency of momentum transport

Large quasi-particle interaction cross section Strongly-coupled matterSmall shear viscosity”perfect liquid”

AdS/CFT and kinetic theory: absolute lower bound

Elliptic flow: data vs. viscous hydrodynamic modeling

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e.g. Song, Bass, and Heinz, arXiv:1103.2380

pT-differentialpT-integrated

peripheralcentral

Preferred values: /s(RHIC)=0.16, /s(LHC)=0.20

Shear viscosity: expectations from QCD

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Analytic: Csernai, Kapusta and McClerran PRL 97, 152303 (2006)Lattice: H. Meyer, PR D76, 101701R (2007)

pQCD w/ running coupling

Chiral limit,resonance gas

1/4 Lattice QCD

Temperature (MeV)

If TLHC > TRHIC, expect /s(LHC) > /s(RHIC)

Jet quenching

22

Total medium-induced energy loss:

Plasma transport coefficient:

Apr 4, 2011 LHC News - Sonoma State

Radiative energy loss in QCD (multiple soft scattering):

Jet quenching via leading charged hadron suppression

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Phys. Lett. B 696 (2011)Phys. Lett. B 696 (2011)

peripheral

central

pTpT

p+p reference at 2.76 TeV: interpolated

Jet quenching: RHIC vs. LHC

Apr 4, 2011 LHC News - Sonoma State 24

Phys. Lett. B 696 (2011)Phys. Lett. B 696 (2011)

Qualitatively similar, quantitatively different

Where comparable, LHC quenching is larger

higher color charge density

0 vs charged hadrons/RHIC vs LHC

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RHIC 0, , direct

RHIC/LHC charged hadrons

High pT dependence qualitatively different:• different quenching mechanisms?• consequence of steeper incl spectrum at RHIC? (near phase space limit…)

Jet quenching: comparison to pQCD-based models

Apr 4, 2011 LHC News - Sonoma State 26

X-F Che et al.,arXiv1102.5614 Horowitz and Gyulassy, arXiv1104.4958

Several formalisms different treatments of medium, radiative/elastic e-lossModels calibrated at RHICScale energy density with charged multiplicity (factor~2)

Models systematically predict too much quenching….?• must measure p+p reference at 2.76 TeV (data now on tape)• something missing in the formalism?

Summary and Outlook

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Initial LHC heavy ion run: machine and ALICE worked superbly

First task is to rediscover and compare to the striking heavy ion phenomena found at RHIC

• qualitative similarities but quantitative differences• consistent picture of strongly-coupled (low viscosity) fluid with high color-charge density (opaque to jets)

• discrepancies with models: requires some rethinking

Next for ALICE: qualitative quantitative• quarkonia (deconfinement signature)• charm• full jets (newly commissioned large EMCal)• correlations of many kinds…