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First measurements in Pb—Pb collisions at  s NN =2.76 TeV with ALICE at the LHC

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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First measurements in Pb—Pb collisions at  s NN =2.76 TeV with ALICE at the LHC

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  1. 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

  2. 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

  3. 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<r<247 cm, |h|<0.9 • 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

  4. 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

  5. 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

  6. Centralitydetermination GlauberfitofV0 amplitudedistributionaboveananchorpoint correspondingto 88% of the hadroniccross-section, where trigger isfullyefficient and background negligible Rencontres de Moriond - 23/03/2011

  7. 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<Npart>)= 8.3  0.4 (sys.) Rencontres de Moriond - 23/03/2011

  8. 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

  9. Results I: multiplicity density ofprimary chargedparticles • Comparisonwithmodels • two-componentmodels (pQCDprocesses + soft interactions) • saturationmodels Rencontres de Moriond - 23/03/2011

  10. 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

  11. Results II: elliptic flow ofprimary chargedparticles • Centralitydependenceofintegratedelliptic flow • Comparisonwith RHIC results30 % higher • Hydrodynamicmodelpredictionswithviscous • corrections in agreement withobservedincrease |h|<0.8 0.2<pt<5GeV/c Rencontres de Moriond - 23/03/2011

  12. Results II: elliptic flow ofprimary chargedparticles • Energy dependenceofintegratedelliptic flow • Increase in the magnitudeofelliptic flow withenergy 20-30% centralityclass Rencontres de Moriond - 23/03/2011

  13. 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

  14. Results IV: ptspectraofprimary chargedparticles and RAA |h|<0.8 0.3<pt<20GeV/c 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

  15. Results IV: ptspectraofprimary chargedparticles and RAA |h|<0.8 0.3<pt<20GeV/c • 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

  16. 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

  17. Rencontres de Moriond - 23/03/2011

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