First measurements in Pb—Pb collisionsat sNN=2.76 TeV with ALICE at the LHC

M. Nicassio (University and INFN Bari)for the ALICE Collaboration

Rencontres de MoriondLa Thuile, Aosta valley (Italy)March 20-27, 2011

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Contents Introduction

ALICE experiment goal and apparatus data taking conditions and event selection centrality determination

First measurements multiplicity density of primary charged particles

Phys. Rev. Lett. 105, 252301 (2010), arXiv:1011.3916 Phys. Rev. Lett. 106, 032301 (2011), arXiv:1012.1657

elliptic flow of primary charged particles Phys. Rev. Lett. 105, 252302 (2010), arXiv:1011.3914

Bose—Einstein correlations Phys. Lett .B 696 (2011) 328-337, arXiv:1012.4035

pt spectra of primary charged particles and RAA

Phys. Lett .B 696 (2011) 30-39, arXiv:1012.1004

Summary and outlook

M. Nicassio

See Jochen talk

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The ALICE experiment Main goal

study the properties of Quark—Gluon Plasma formed in heavy-ion collisions

Main detectors used for first analyses Inner Tracking System

• Silicon Pixel Detector r=3.9 cm |h|<2.0 inner layer r=7.6 cm |h|<1.4 outer layer 10M pixels,1200 read-out chips

Time Projection Chamber• 85<r<247 cm, |h|<0.9

V0 scintillators• at 0.9 and 3.3 m on either side of IP• 2.8<|h|<5.1 and -3.7<|h|<-1.7

Zero Degree Calorimeters• at 114 m on either side of IP

ITS

TPC

ZDC

ZDC

V0A

V0C

M. Nicassio

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Trigger and event selection First runs with Pb ions @ sNN=2.76 TeV

4 or 66 bunches 1 to 7107 ions of 208Pb per bunch 0.5 to 81023 cm-2s-1 estimated luminosity interaction rate 50 Hz = 4Hz+45 Hz+1Hz luminous region: 5.9 cm in z, 50 mm in transverse direction

Triggering with SPD and both V0 arrays two out of three trigger signals

two SPD pixel chips hit in the outer layer a signal in V0A a signal in V0C

coincidence between both V0 arrays more efficient for electromagnetic interaction suppression

Background removal electromagnetic interactions: neutron ZDC minimal energy deposit beam-background: V0 timing information, correlation between TPC

tracks and SPD hits

Hadronic collisionsElectromagnetic interactionsBeam-background

M. Nicassio

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Centrality determination Two component model of particle production based on Glauber description of nuclear collisions

Nancestors=fNpart+(1-f)Ncoll

particle produced by each source described by NBD Assumption in Glauber model calculation

Woods-Saxon distribution for nuclear densityr = 6.620.06 fmskin depth a = 0.5460.010 fm

minimum inter-nucleon distance = 0.40.4 fm interaction distance given by sinel

NN = 645 mb at sNN=2.76 TeV (from interpolation of data)

Glauber fit of V0 amplitude distribution above an anchor pointcorresponding to 88% of the hadronic cross-section, where triggeris fully efficient and background negligible

M. Nicassio

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Centrality determination

Glauber fit of V0 amplitude distribution above an anchor pointcorresponding to 88% of the hadronic cross-section, where triggeris fully efficient and background negligible

M. Nicassio

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Results I: multiplicity density of primarycharged particles

Energy dependence

Increase relative to RHIC and pp factor 2

Analysis on 3600 events:- 5% most central - |zvtx|<7 cm- primary vertex and tracklets reconstructed in the SPD- TPC tracks as a cross check (dNch/dh)/(0.5<Npart>)= 8.3 0.4 (sys.)

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Analysis on 49k events:-|zvtx|<7 cm- primary vertex and tracklets reconstructed in the SPD- TPC tracks as a cross check

Centrality dependence

Results I: multiplicity density of primarycharged particles

M. Nicassio

Same trend as at RHIC Increase from peripheral to central factor 2

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Comparison with models two-component models (pQCD processes + soft interactions) saturation models

Results I: multiplicity density of primarycharged particles

M. Nicassio

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Results II: elliptic flow of primary charged particles

Transverse momentum dependence of v2

Analysis on 45k events:- centrality selection cutting on TPC track multiplicity- |zvtx|<10 cm- ITS and TPC tracks (TPC tracks as a cross check)- various analysis techniques

pt dependece in centrality classes similar to RHIC

M. Nicassio

No changes in v2(pt) compared to RHIC

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Centrality dependence of integrated elliptic flow

Comparison with RHIC results 30 % higher Hydrodynamic model predictions with viscous

corrections in agreement with observed increase

|h|<0.8 0.2<pt<5GeV/c

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Results II: elliptic flow of primary charged particles

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Energy dependence of integrated elliptic flow

Increase in the magnitude of elliptic flow with energy

20-30% centrality class

M. Nicassio

Results II: elliptic flow of primary charged particles

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Results III: two-pion Bose—Einsteincorrelations

HBT radii and decoupling time multiplicity dependence

Volume of homogeneity region: linear dependence on dNch/dh, two times larger than at RHIC

Analysis on 16k events:- 5% most central collisions- |zvtx|<12 cm- TPC tracks

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tf: linear dependence on (dNch/dh)1/3, 1.4 times larger than at RHIC

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Results IV: pt spectra of primary charged particles and RAA

pp reference at 2.76 TeV: main approach interpolation of ALICE measurements in pt bins (at 0.9 and 7 TeV),

assuming a power law for the increase of the yield with s

Analysis on 2.3M events:- |zvtx|<10 cm- ITS+TPC tracks

|h|<0.8 0.3<pt<20GeV/c

Power law

Exponential

M. Nicassio

Peripheral events and scaledpp reference have samept dependence

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|h|<0.8 0.3<pt<20GeV/c

Alternatives forpp reference:-CDF data in interpolation-NLO scaling of pp data at 0.9 TeV

M. Nicassio

Results IV: pt spectra of primary charged particles and RAA

Comparison with RHIC in central collisions: stronger suppression of high pt particles very dense medium formed at LHC

In central collisions strong suppression and clear change in shape In peripheral collisions no pt dependence above 2 GeV/c

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Summary and outlook First ALICE measurements in PbPb collisions

multiplicity a factor 2 higher than at RHIC, same centrality dependence

elliptic flow 30% higher than at RHIC HBT radii and decoupling time in central collisions

10-35% and 30% higher than at RHIC

Ongoing studies and outlook measurements for identified particles soft physics studies (resonances, baryons and strangeness) hard physics studies (open heavy flavours, energy loss, jets) looking forward at PbPb collisions at the end of 2011

• target for integrated luminosityfactor 3 (or more) higher• first injections of pA

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