Global event properties
in Pb+Pb collisions atLHC energies fromALICE
Tatsuya Chujo
for the ALICE collaboration(Univ. of Tsukuba)
ISMD2011, Miyajima, Japan, Sep. 26, 2011

First Pb+Pb collisions at LHC
ü Nov. 8, 2010ü √sNN = 2.76 TeV (14 x RHIC)ü Observed by 3 experimentsü ALICE, ATLAS, CMS

2

ALICE physics publications (as of Sep. 26, 2011)arxiv date

systemenergy (TeV)

observablepublished in1

28/11/09

pp0.9

charged particle dN/dη	

EPJC 65(2010)111

218/04/10

pp0.9, 2.36

charged particle dN/dη, mult. distr.

EPJC 68(2010)89

320/04/10

pp7charged particle dN/dη, mult. distr.

EPJC 68(2010)345

428/06/10

pp0.9, 7

antiproton/proton ratio

PRL 105(2010)072002

503/07/10

pp0.9

pion HBTPRD 82(2010)052001

605/07/10

pp0.9

charged particle pT spectra

PLB 693(2010)53

717/11/10

PbPb

2.76

charged particle dN/dη	

PRL 105(2010)252301

817/11/10

PbPb

2.76

charged particle v2

PRL 105(2010)252302

905/12/10

PbPb

2.76

charged particle RAA

PLB 696(2011)30

1008/12/10

PbPb

2.76

centrality dependence of Nch

PRL 106(2011)032301

1115/12/10

pp

0.9

K0, φ, Λ, cascade

EPJC 71(2011)1594

1217/12/10

PbPb

2.76

pion HBTPLB 696(2011)328

1319/01/11

pp0.9, 7pion HBT

arXiv:1101.3665v1

1421/01/11

pp

0.9

pion, kaon, proton

EPJC 71(2011)1655

1502/05/11

pp

7J/Ψ	

arXiv:1105.0380v1

1619/05/11

PbPb

2.76

charged particle v3, v4, v5

arXiv:1105.3865v1

1712/09/11

PbPb

2.76

angular correlations

arXiv:1109.2501v1
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Space-time evolution of HIC and globalproperties

43

2
1

1.
2.
3.
4.

Frist contact, hard scattering of partons
thermal equilibrium and QGP
Chemical freeze-out (fixed particle ratios)
Kinetic freeze-out(fix particle momentum)
4Global properties ofHeavy Ion Collisions�ü Nch, ETü PID pT spectraü Particle ratios
ü HBTü Flow (vn)à εBjà Tfo, βTà Tch, µBà V, τlifeà η/s, initial condition

Q: How different and/or similar these
global properties at LHC energy
compared to those at RHIC?
5Central Barrel
Tracking, PID
η < 0.9

Muon Arm	-4 < η < -2.5	

Alice numbers:
1300 members
116 institutes
33 countries

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Data samples, statistics
VZERO�

SystemEnergy Trigger Analyzed(TeV)
events

∫Ldt

pp

7

MB
MUON

300M
130M

5 nb-1
16 nb-1

pp

2.76

MB
MUON

65M
~9M

1.1 nb-1
20 nb-1

PbPb

2.76

MB

17M

1.7 mb-1

PRL106 (2011) 032301

ITS (SPD, SDD, SSD)�

• Triggersc MB: based on VZERO (Aand C) and SPD• Centrality selection inPbPbc Amplitudes in the V0scintillatorsc Reproduced by Glaubermodel fit
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Centrality and resolution�
Correlation SPD - VZERO�SPD distribution for
centrality classes
selected by VZERO�

Resolution (RMS)�

Centrality resolution
scales with √Nch
0.5% for central
2% for peripheral�8l Nch, ET give us an insight on
initial energy density �"(t) =E dN
Energy1 dN(t)
1 dE T (t)==mT (t) =VolumeVt A dyt A dydE TdN= mTdydy!

!

Charged particle multiplicity; dNch/dη	! Bj" ! 15 GeV/(fm 2 c)
PRL106 (2011) 032301

PRL105 (2010) 252301••dNch/dη = 1584 ± 76(dNch/dη)/(Npart/2) = 8.3 ± 0.4– ≈ 2.1 x central AuAu at √sNN=0.2 TeV– ≈ 1.9 x pp (NSD) at √s=2.36 TeV••Stronger rise with √s in AA w.r.t. pp
Stronger rise with √s in AA w.r.t. log
extrapolation from lower energies•

Very similar centrality
dependence at LHC & RHIC
c After scaling RHIC results (x2.1)
to the multiplicity of central
collisions at the LHC
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Energy density from ET• From RHIC to LHC:

– increase in dET/dη perparticipant pair by a factor 2.5– Similar centrality dependence• Energy density of the medium
from Bjorken formula! Bj =1 dET
" # R 2 dy

R = 1.12A1/3 fm– Where τ = (unknown) formation
time! Bj" ! 16 GeV/(fm 2 c)
• ≈ 3×εBjτ at RHIC
• RHIC: ετ =5.4±0.6 GeV/(fm2c)

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T

expandingsourcelightT, βheavy
pT

1/pT dN/dpT

pure thermal
source1/pT dN/dpTl To characterize the global
properties on expanding source;
introduce common “symmetric”
velocity field, “radial flow”.�

lightheavypT

Hadron PID in ALICE• PID in the central barrelc dE/dx in TPC
ü Up to 159 samples
ü Resolution ~ 5%c dE/dx in ITSü Low momentum reachc Time of Flight by TOF
ü 3σ separation:
π/K up to 2.5 GeV/cp/K up to 4.0 GeV/c

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Identified hadron spectra• Combined analysis withITS, TPC and TOF• Lines = blast-wave fitscIntegrated YieldscAverage pTcParameters of thesystem at the kinetic
freeze-out (Tf0, βT)
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Identified particle spectra

RHIC	

Hydro
Prediction

RHIC
Blast Wave Fit-

Very significant changes in slope compared to RHIC
Most dramatically change for protons
Very strong radial flow, β ≈ 0.66 (10% higher than RHIC)
Even larger than predicted by most recent hydro15

Baryon to meson ratio: Λ/Ks0• Baryons more abundant atintermediate pT• Baryon/meson ratio increases
with centrality– Consistent withrecombination and/orstrong radial flow.

Enhancement stronger
than at RHIC
" Maximum of Λ/K slightly
pushed towards higher pTthan at RHIC
chigher radial flow?
"16l Particle ratios;
access to the chemical
properties of matter �Multi-strange baryons• Text18π-/π+, K-/K+, pbar/p ratios�

~1.0 forall ratiosat LHC�

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Comparison with thermal model• All yields, except protons, follow thermal model prediction for
grand-canonical ensemble and Tch=164 MeV, µB = 1 MeV.

c Measured proton/pion ratio below thermal model expectationc Strange particles perfectly agree with thermal model expectation
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LHC
Pb-Pb (2.76 TeV)
0-20% centralBraz. J. Phys. vol.37 no.2c São Paulo June 2007	

http://dx.doi.org/10.1590/S0103-97332007000500024 	

N. Xu	

�21

l Extraction of source size “R”
using quantum interferometry,HBT.
How much volume and life time?�
x1	

π	

p1	

X1’	

R	x2	Particle emitting source

p2	

π	

X2’	

P( p1 , p2 )C2 !
P( p1 ) " P( p2 )2~
= 1 + $ eff (q)= 1 + % exp( # R 2 q2 )

HBT correlations•Spatial extent of the particle emitting source extracted frominterferometry of identical bosons– Two-particle momentum correlations in 3 orthogonaldirections à HBT radii (Rlong, Rside, Rout)– Volume: x2 compared to RHIC
– Lifetime from Rlong: 40% higher than that at RHIC

ALICE: PLB696 (2011) 328

ALICE: PLB696 (2011) 328

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System size vs. multiplicity• HBT radii scale linearlywith <N>1/3 in pp andPbPb, but with different
slopes.
• HBT radii in PbPb vs.
trend from lower
energy AA:– Rlong: perfectly agree– Rside: reasonably agree– Rout: clearly below thetrend

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HBT vs. Hydro�
• Behaviour of all 3 radii in qualitative
agreement with hydro expectations• Rout/Rside: some model cannot
describe the data.

ALICE: PLB696 (2011) 328
25l Anisotropic flow;
sensitive to the early time ofcollisions, pressure gradients,and EOS.�

Elliptic flow; v2�

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v2 vs. √sNN�

•PRL 105 (2010) 252302

At LHC, pT integrated v2 increases by 30% compared to RHIC
data at √sNN=200 GeV.
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•

pT dependence of v2
RHIC vs. LHCv2 vs. pT does not change within uncertainties between √sNN=200 GeV
and 2.76 TeV– 30% increase of pT integrated flow explained by higher mean pT due tostronger radial flow at higher energies�

PRL 105 (2010) 252302
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10-20%�

Identified particle v2

40-50%�

• Stronger radial flow à more pronounced massdependence of v2c Hydrodynamics predictions describe well the measured v2(pT) for πand K.
c Disagreement for anti-protons in the more central binü Larger radial flow than in the Hydro modelü Rescatterings in the hadronic phase play an important role (arXiv:1108.5323)
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Higher order harmonics; vn arXiv:1105.3865
ψRP	ψ3	ψ2	

• v3 (“triangular”) harmonic:– v3 has weaker centrality
dependence than v2– When calculated w.r.t. participant
plane, v3 vanishes due tofluctuations.

• pT dependence for v2, v3, v4, v5– v3 sensitive to shear viscosity η/sand to initial conditions (Glauber,CGC)

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Quark number scaling for v2 and v3

v2�

v3�

• v2 scaling with number of constituent quarks.
– Holds for π and K.
– Again, anti-protons do not follow the scaling.
• Breakup more pronounced for more central
collisions
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Fourier decomposition in di-hadron corr.
• Extract 1D Δϕ correlations by integrating theC(Δη,Δϕ) in 0.8<|Δη|<1.8 range and do aFourier decompositionΔη max1C (Δϕ ) =C (Δη , Δϕ ) ∝ 1 + 2∑ vnΔ cos(nΔϕ )∫ΔηΔη max − Δη min min
n =1• 5 components describe completely the correlations at large Δη
and low pTc Strong near-side ridge + double-peaked structure on away side

arXiv:1109.2501
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Summary�ü First data in Pb+Pb collisions at 2.76
TeV by ALICE provides an important
baseline on global properties.
– Smooth transition from RHIC to LHC.
• Hotter, Larger, Longer, less µB, similar v2…

– Many global observables are affected by thestrong radial flow at LHC energies.

ü Enter the new era to study hot anddense matter at LHC energy, i.e. era of
precision measurements by many hardprobes.
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For example…�
Bulk particle production with jets�

Lost energy is used for the low pT bulk particle
production at large angle (by CMS).
Q: reheating the matter?
Need to explore bulk properties with jets (No. of jets, jet axis)
Also jet quenching in high multiplicity events in p+p.
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Thank you for your attention.

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BACKUP�

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• I would like to thank Francesco Prino(ALICE) for his nice set of slides atSQM 2011.

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