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GLOBAL EVENT FEATURES

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Charged multiplicity (central collisions)

• Quantitative Difference from RHIC– dNch/dh ~ 1600 ± 76 (syst)

• on high side of expectations • growth with √s faster in AA than pp : (√s -‘nuclear

amplification’– Energy density ≈ 3 x RHIC (fixed t)

• lower limit, likely t0(LHC) < t0(RHIC)

tmdy

dN

AV

E

0

1)(

PRL105 (2010) 252301PRL105 (2010) 252301

15 GeV/fm3 …….. or more

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Bjorken’s formulaA simple way to estimate the energy density from charged

multiplicity or transverse energy measurements

HP: particle are produced at formation time tf (tf = 1 fm/c or less)

Consider a slice of longitudinal thickness Dz and section A

This slice contains the particles with speed

And such a number can be expressed as:where we have used b ≈ y for b0

With some further calculations we get:

Charged multiplicity: centrality dependence

• dNch/dh as function of centrality (normalised to ‘overlap volume’ ~ Nparticipants)

– DPMJET MC• fails to describe the data

– HIJING MC• strong centr. dependent

gluon shadowing– Others

• saturation models: Color Glass Condensate,‘geometrical scaling’ fromHERA/ photonuclear react. DPMJET

HIJING

Important constraint for modelssensitive to details of saturation

Saturation Models

Published on PRL

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Interferometry - I• Experimentally, the expansion rate and the spatial extent at decoupling are

accessible via intensity interferometry, a technique which exploits the Bose–Einstein enhancement of identical bosons emitted close by in phasespace. This approach, known as Hanbury Brown–Twiss analysis

p

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Interferometry - II

The three-dimensional correlation functions can be fitted with the following expression, accounting for the Bose-Einstein enhancement and for the Coulomb interaction between the two particles:

l= correlation strenght, k(qinv)= squared Coulomb wawefuntion

PLB 696 (2011)

Time at decoupling: t R ̴� out

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System Size from pion interferometry

Spatial extent of the particle emitting source extracted from interferometry of identical bosonsTwo-particle momentum correlations in 3 orth. directions -> HBT radii (Rlong, Rside, Rout) Size: twice w.r.t. RHIC Lifetime: 40% higher w.r.t. RHIC

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Kaon interferometry

Kaon interferometry: complementary to pion due to different mT

Results consisten with those with pions

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Interferometry: pp vs Pb-Pb

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Low-pT particle production

arXiv:1208.1974 [hep-ex]

(low) pT spectra : superposition of collective motion of particles on top of thermal motionCollective motion is due to high pressure arising from compression and heating .

“Blast-Wave” fit to pT spectra [1]: Radial flow velocity <b> ≈ 0.65 (10 % larger than at RHIC)Kinetic freezout temp. TK ≈ 95 MeV (same as RHIC within errors)

Central collisions: radial flow

[1] E. Schnedermann, et al.; Phys. Rev. C48, 2462 (1993)

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Particle yields and ratiosAssuming that the medium created in the collision reaches thermal equilibrium, one can compute particle yield and ratios with thermal models.

Grand canonical ensamble:

Where b =1/T and mi is the chemical potential

Thermal models have been (and are being ) used to fit the measured particles yields (at different c.m. energies) T and the baryochemical potential mb are free parameters to be obtained by the fit.

N.B. T is the chemical freezout temperature……..

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Yield and ratios at RHIC

!

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Particle yields and ratios at LHC - Extracted from pT- integrated identified particle spectra.

- Comparison /Fit with Thermal/ Statistical models work well at RHIC info on chemical freezout temperature and baryochemical potential

Predicted temperature T=164 MeVA.Andronic, P.Braun-Munzinger, J.Stachel NP A772 167Thermal fit (w/o res.): T=152 MeV (c2/ndf = 40/9)X and W significantly higher than statistical model

p/p and L/p ratios at LHC lower than RHIC Hadronic re-interactions ?F.Becattini et al. 1201.6349 J.Steinheimer et al. 1203.5302

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ANISOTROPIC FLOW

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Anisotropic flow: basic idea

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19

Elliptic flow

Px

Py Pz

Reacti

on plane

X

Z

Y

f

Nch

yie

ld

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v2: selected ALICE results

v2 for non-identified particles:

v2 for identified particles:

Large elliptic flow oberved at RHIC consistent with strongly coupled medium with low shear viscosity (ideal fluid)

Stronger ̴mass ̴dependence ̴of ̴the ̴elliptic ̴flow ̴as ̴compared ̴to ̴RHIC: ̴ ̴Due ̴to ̴the ̴larger ̴radial ̴flow? ̴Some ̴deviation ̴from ̴hydrodynamic ̴predictions ̴for ̴(anti) ̴protons ̴in ̴close-to-central ̴collisions: ̴ ̴rescattering?

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V2 scaling at RHIC

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24

25

26

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More on anisotropic flow

v2 and v3 over extended * h interval

V3 sensitive to the fluctuations of the initial nucleon distribution

*Results already published in the central h region: v2 Phys. Rev. Lett. 105, 252302 (2010),v3 Phys. Rev. Lett. 107, 032301 (2011)

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HIGH-PT AND JETS

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Particle spectra at high pT

Parton energy loss:A parton passing through the QCD medium undergoes energy loss which results in the suppression of high-pT

hadron yields

Related observable:nuclear modification factor RAA

Reference: pp collisions

Pb-Pb at different centralities

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RAA for identified particles

• First measurement of (anti-)proton, K and p at high pT (>7 GeV/c) :– The RAA indicates strong suppression, confirming the indications from previous

measurements for non-identified particles– The RAA for (anti-)protons, charged pions and K are compatible above 7 GeV/c ̴� this

suggests that the medium does not affect the fragmentation.

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Charged jet: RAA and RCP

Strong jet suppression observed for jets reconstructed with charged particles– RAA (jet) is smaller than inclusive hadron RAA(h±) at similar parton pT

– data are reasonably well described by JEWEL model K.Zapp, I.Krauss, U.Wiedemann, arXiv:1111.6838

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Near-side (jet-like) structure

N.Armesto et al., PRL 93, 242301

Isolation of near-side peak:Dh–D correlation with triggerLong-range (large Dh) correlation used as proxy for backgroundsh

s

Evolution of near-side-peaksh and s with centrality:Strong sh increase for centralcollisions

Interesting: AMPT describesthe data very well

Influence of flowing medium?