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Rencontres de Moriond La Thuile , Aosta valley (Italy) March 20-27, 2011. First measurements in Pb—Pb collisions at  s NN =2.76 TeV with ALICE at the LHC. M. Nicassio ( University and INFN Bari) for the ALICE Collaboration. Contents. Introduction

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slide1
Rencontres de Moriond

La Thuile, Aosta valley (Italy)

March 20-27, 2011

First measurements in Pb—Pbcollisions

at sNN=2.76 TeV with ALICE at the LHC

M. Nicassio (University and INFN Bari)

for the ALICE Collaboration

slide2
Contents
  • Introduction
    • ALICE experiment goal and apparatus
    • data takingconditions and eventselection
    • centralitydetermination
  • First measurements
    • multiplicity density ofprimarychargedparticles
    • Phys. Rev. Lett. 105, 252301 (2010), arXiv:1011.3916
    • Phys. Rev. Lett. 106, 032301 (2011), arXiv:1012.1657
    • elliptic flow ofprimarychargedparticles
    • Phys. Rev. Lett. 105, 252302 (2010), arXiv:1011.3914
    • Bose—Einsteincorrelations
    • Phys. Lett .B 696 (2011) 328-337, arXiv:1012.4035
    • ptspectraofprimarychargedparticles and RAA
    • Phys. Lett .B 696 (2011) 30-39, arXiv:1012.1004
  • Summary and outlook

SeeJochen talk

Rencontres de Moriond - 23/03/2011

slide3
The ALICE experiment
  • Main goal
    • study the propertiesofQuark—Gluon Plasma formed in
    • heavy-ioncollisions
  • Maindetectorsusedfor first analyses
    • InnerTrackingSystem
      • SiliconPixel Detector
      • r=3.9 cm |h|<2.0 innerlayer
      • r=7.6 cm |h|<1.4 outerlayer
      • 10M pixels,1200 read-out chips
    • TimeProjectionChamber
      • 85
    • V0scintillators
      • at 0.9 and 3.3 m on either side of IP
      • 2.8<|h|<5.1 and -3.7<|h|<-1.7
    • Zero DegreeCalorimeters
      • at 114 m on either side of IP

ITS

V0C

ZDC

V0A

TPC

ZDC

Rencontres de Moriond - 23/03/2011

slide4
Trigger and eventselection
  • First runs with Pb ions @ sNN=2.76 TeV
    • 4 or 66 bunches
    • 1 to 7107 ions of 208Pb per bunch
    • 0.5 to 81023 cm-2s-1 estimated luminosity
    • interaction rate 50 Hz = 4Hz+45 Hz+1Hz
    • luminousregion: 5.9 cm in z, 50 mm in transverse direction
  • Triggeringwith SPD and both V0 arrays
    • two out ofthree trigger signals
      • two SPD pixel chips hit in the outerlayer
      • a signal in V0A
      • a signal in V0C
    • coincidencebetweenboth V0 arrays more efficient for
    • electromagnetic interaction suppression
  • Background removal
    • electromagneticinteractions: neutron ZDC minimal energydeposit
    • beam-background: V0 timing information, correlationbetween TPC
    • tracks andSPD hits

Hadroniccollisions

Electromagneticinteractions

Beam-background

Rencontres de Moriond - 23/03/2011

slide5
Centralitydetermination
  • Twocomponentmodelofparticle production based on Glauberdescription
  • ofnuclearcollisions
    • Nancestors=fNpart+(1-f)Ncoll
    • particleproducedbyeach source describedby NBD
  • Assumption in Glaubermodelcalculation
    • Woods-Saxondistributionfornuclear density
      • r = 6.620.06 fm
      • skin depth a = 0.5460.010 fm
    • minimum inter-nucleon distance = 0.40.4 fm
    • interaction distance given by sinelNN = 645 mb at sNN=2.76 TeV
    • (frominterpolation of data)

GlauberfitofV0 amplitudedistributionaboveananchorpoint

correspondingto 88% of the hadroniccross-section, where trigger

isfullyefficient and background negligible

Rencontres de Moriond - 23/03/2011

slide6
Centralitydetermination

GlauberfitofV0 amplitudedistributionaboveananchorpoint

correspondingto 88% of the hadroniccross-section, where trigger

isfullyefficient and background negligible

Rencontres de Moriond - 23/03/2011

slide7
Results I: multiplicity density ofprimary

chargedparticles

  • Energy dependence
              • Increase relative to RHIC and ppfactor 2

Analysis on 3600 events:

- 5% mostcentral

- |zvtx|<7 cm

- primaryvertex and tracklets

reconstructed in the SPD

- TPC tracksas a cross check

(dNch/dh)/(0.5)= 8.3  0.4 (sys.)

Rencontres de Moriond - 23/03/2011

slide8
Results I: multiplicity density ofprimary

chargedparticles

  • Centralitydependence

Analysis on 49k events:

-|zvtx|<7 cm

- primaryvertex and tracklets

reconstructed in the SPD

- TPC tracksas a cross check

  • Same trend as at RHIC
  • Increasefrom peripheral to central
  •  factor 2

Rencontres de Moriond - 23/03/2011

slide9
Results I: multiplicity density ofprimary

chargedparticles

  • Comparisonwithmodels
    • two-componentmodels (pQCDprocesses + soft interactions)
    • saturationmodels

Rencontres de Moriond - 23/03/2011

slide10
Results II: elliptic flow ofprimary

chargedparticles

  • Transversemomentumdependenceof v2
  • No changes in v2(pt)
  • comparedto RHIC
  • ptdependece in centrality
  • classessimilarto RHIC
  • Analysis on 45k events:
  • centralityselectioncutting on TPC trackmultiplicity
  • - |zvtx|<10 cm
  • ITS and TPC tracks (TPC tracksas a cross check)
  • variousanalysistechniques

Rencontres de Moriond - 23/03/2011

slide11
Results II: elliptic flow ofprimary

chargedparticles

  • Centralitydependenceofintegratedelliptic flow
      • Comparisonwith RHIC results30 % higher
      • Hydrodynamicmodelpredictionswithviscous
  • corrections in agreement withobservedincrease

|h|<0.8 0.2

Rencontres de Moriond - 23/03/2011

slide12
Results II: elliptic flow ofprimary

chargedparticles

  • Energy dependenceofintegratedelliptic flow
  • Increase in the magnitudeofelliptic flow withenergy

20-30% centralityclass

Rencontres de Moriond - 23/03/2011

slide13
Results III: two-pionBose—Einstein

correlations

  • HBT radii and decouplingtimemultiplicitydependence
  • Volume ofhomogeneityregion:
  • lineardependence on dNch/dh,
  • twotimeslargerthan at RHIC
  • tf:lineardependence on
  • (dNch/dh)1/3, 1.4 times
  • largerthan at RHIC
  • Analysis on 16k events:
  • 5% mostcentralcollisions
  • - |zvtx|<12 cm
  • TPC tracks

Rencontres de Moriond - 23/03/2011

slide14
Results IV: ptspectraofprimary

chargedparticles and RAA

|h|<0.8 0.3

Exponential

  • Analysis on 2.3M events:
  • - |zvtx|<10 cm
  • ITS+TPCtracks

Powerlaw

Peripheralevents and scaled

ppreferencehavesame

pt dependence

  • ppreference at 2.76 TeV: mainapproach
    • interpolationof ALICE measurements in ptbins (at 0.9 and 7TeV),
    • assuming a power law for the increase of the yield with s

Rencontres de Moriond - 23/03/2011

slide15
Results IV: ptspectraofprimary

chargedparticles and RAA

|h|<0.8 0.3

  • Alternativesfor
  • ppreference:
  • CDF data in
  • interpolation
  • NLO scalingof
  • pp data at 0.9
  • TeV
  • In centralcollisions strong suppression
  • and clearchange in shape
  • In peripheralcollisions no ptdependence
  • above2GeV/c
  • Comparisonwith RHIC in centralcollisions:
  • strongersuppressionof high pt particles
  • very dense medium formed at LHC

Rencontres de Moriond - 23/03/2011

slide16
Summary and outlook
  • First ALICE measurements in PbPbcollisions
    • multiplicity a factor2higherthan at RHIC, samecentrality
    • dependence
    • elliptic flow 30% higherthan at RHIC
    • HBT radii and decouplingtimein centralcollisions
    • 10-35% and 30% higherthan at RHIC
  • Ongoingstudies and outlook
    • measurementsforidentifiedparticles
    • soft physicsstudies (resonances, baryons and strangeness)
    • hard physicsstudies (open heavy flavours, energy loss, jets)
    • lookingforward at PbPbcollisions at the end of 2011
      • target forintegratedluminosityfactor3 (or more) higher
      • first injectionsofpA

Rencontres de Moriond - 23/03/2011

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