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

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, (2010), arXiv: Phys. Rev. Lett. 106, (2011), arXiv: elliptic flow of primary charged particles Phys. Rev. Lett. 105, (2010), arXiv: Bose—Einstein correlations Phys. Lett.B 696 (2011) , arXiv: p t spectra of primary charged particles and R AA Phys. Lett.B 696 (2011) 30-39, arXiv:  Summary and outlook M. Nicassio2Rencontres de Moriond - 23/03/2011 See Jochen talk

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 |  |<2.0 inner layer r=7.6 cm |  |<1.4 outer layer  10M pixels,1200 read-out chips Time Projection Chamber 85<r<247 cm, |  |<0.9 V0 scintillators at 0.9 and 3.3 m on either side of IP 2.8<|  |<5.1 and -3.7<|  |<-1.7 Zero Degree Calorimeters at 114 m on either side of IP ITS TPC ZDC V0A V0C M. Nicassio3Rencontres de Moriond - 23/03/2011

Trigger and event selection  s NN =2.76 TeV  First runs with Pb  s NN =2.76 TeV 4 or 66 bunches 1 to 7  10 7 ions of 208 Pb per bunch 0.5 to 8  cm -2 s -1 estimated luminosity interaction rate 50 Hz = 4Hz+45 Hz+1Hz luminous region: 5.9 cm in z, 50  m 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 collisions Electromagnetic interactions Beam-background M. Nicassio4Rencontres de Moriond - 23/03/2011

Centrality determination  Two component model of particle production based on Glauber description of nuclear collisions N ancestors =fN part +(1-f)N coll particle produced by each source described by NBD  Assumption in Glauber model calculation Woods-Saxon distribution for nuclear density r = 6.62  0.06 fm skin depth a =  fm minimum inter-nucleon distance = 0.4  0.4 fm interaction distance given by  inel NN = 64  5 mb at  s NN =2.76 TeV (from interpolation of data) Glauber fit of V0 amplitude distribution above an anchor point corresponding to 88% of the hadronic cross-section, where trigger is fully efficient and background negligible M. Nicassio5Rencontres de Moriond - 23/03/2011

Centrality determination Glauber fit of V0 amplitude distribution above an anchor point corresponding to 88% of the hadronic cross-section, where trigger is fully efficient and background negligible M. Nicassio6Rencontres de Moriond - 23/03/2011

Results I: multiplicity density of primary charged particles  Energy dependence  Increase relative to RHIC and pp  factor 2 Analysis on 3600 events: - 5% most central - |z vtx |<7 cm - primary vertex and tracklets reconstructed in the SPD - TPC tracks as a cross check (dN ch /d  )/(0.5 )= 8.3  0.4 (sys.) M. Nicassio7Rencontres de Moriond - 23/03/2011

Analysis on 49k events: -|z vtx |<7 cm - primary vertex and tracklets reconstructed in the SPD - TPC tracks as a cross check  Centrality dependence Results I: multiplicity density of primary charged particles M. Nicassio8Rencontres de Moriond - 23/03/2011  Same trend as at RHIC  Increase from peripheral to central  factor 2

 Comparison with models two-component models (pQCD processes + soft interactions) saturation models Results I: multiplicity density of primary charged particles M. Nicassio9Rencontres de Moriond - 23/03/2011

Results II: elliptic flow of primary charged particles  Transverse momentum dependence of v 2 Analysis on 45k events: - centrality selection cutting on TPC track multiplicity - |z vtx |<10 cm - ITS and TPC tracks (TPC tracks as a cross check) - various analysis techniques  p t dependece in centrality classes similar to RHIC M. Nicassio10Rencontres de Moriond - 23/03/2011  No changes in v 2 (p t ) compared to RHIC

 Centrality dependence of integrated elliptic flow  Comparison with RHIC results  30 % higher  Hydrodynamic model predictions with viscous corrections in agreement with observed increase |  |< <p t <5GeV/c M. Nicassio11Rencontres de Moriond - 23/03/2011 Results II: elliptic flow of primary charged particles

 Energy dependence of integrated elliptic flow  Increase in the magnitude of elliptic flow with energy 20-30% centrality class M. Nicassio12Rencontres de Moriond - 23/03/2011 Results II: elliptic flow of primary charged particles

Results III: two-pion Bose—Einstein correlations  HBT radii and decoupling time multiplicity dependence  Volume of homogeneity region: linear dependence on dN ch /d  two times larger than at RHIC Analysis on 16k events: - 5% most central collisions - |z vtx |<12 cm - TPC tracks M. Nicassio13Rencontres de Moriond - 23/03/2011   f : linear dependence on (dN ch /d  ) 1/3, 1.4 times larger than at RHIC

Results IV: p t spectra of primary charged particles and R AA  pp reference at 2.76 TeV: main approach interpolation of ALICE measurements in p t bins (at 0.9 and 7 TeV), assuming a power law for the increase of the yield with  s Analysis on 2.3M events: - |z vtx |<10 cm - ITS+TPC tracks |  |<0.80.3<p t <20GeV/c Power law Exponential M. Nicassio14Rencontres de Moriond - 23/03/2011 Peripheral events and scaled pp reference have same p t dependence

|  |<0.80.3<p t <20GeV/c Alternatives for pp reference: -CDF data in interpolation -NLO scaling of pp data at 0.9 TeV M. Nicassio15Rencontres de Moriond - 23/03/2011 Results IV: p t spectra of primary charged particles and R AA  Comparison with RHIC in central collisions: stronger suppression of high p t particles  very dense medium formed at LHC  In central collisions strong suppression and clear change in shape  In peripheral collisions no p t dependence above 2 GeV/c

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 luminosity  factor 3 (or more) higher first injections of pA M. Nicassio16Rencontres de Moriond - 23/03/2011

M. Nicassio17Rencontres de Moriond - 23/03/2011