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Charged particle production in Pb-Pb collisions at the LHC with the ALICE detector

Charged particle production in Pb-Pb collisions at the LHC with the ALICE detector. M Floris (CERN) for the ALICE Collaboration HP2012 – May 28, 2012. ALICE Results on Charged Particles. The ALICE Experiment Main detectors and Trigger Tracking Performance

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Charged particle production in Pb-Pb collisions at the LHC with the ALICE detector

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  1. Charged particle production in Pb-Pb collisions at the LHC with the ALICE detector • M Floris (CERN) • for the ALICE Collaboration • HP2012 – May 28, 2012

  2. ALICE Results on Charged Particles • The ALICE Experiment • Main detectors and Trigger • Tracking Performance • ptresolution, secondary contamination • Centrality and multiplicity • Event characterization • Basic constraints for theoretical models • High pt particle suppression • Experimentally well defined • Direct comparison to RHIC • Complementary approach to full jet reconstruction M Floris - HP 2012

  3. Trigger detectors and h Coverage h • PbPb data taking • Interaction trigger, combination of: • SPD (Pixels): # of hit chips • V0 (scintillators): hits • 2010: “minimum bias” • 2011: + central/semicentral (V0 Amplitude) • Rare triggers: EMCAL/PHOS/MUON SPD VZEROA VZEROC • 3-out-of-3: VZEROA & VZEROC & SPD • V0AND: (VZEROA & VZEROC) • 2-out-of-3: (VZEROA & V0C) • | (VZEROA & SPD) • | (VZEROC & SPD) • Offline timing cuts (VZERO + ZDC): • remove beam background M Floris - HP 2012

  4. Tracklets and tracks SPD Tracklets: Combination of 2 hits in the 2 SPD layers, within a Dq-Djwindow, pointing to the vertex Less dependent on calibration pTcut-off ~ 50 MeV/c |h| < 1.4  Choice for multiplicity • Tracks: • Global tracks (TPC + ITS) • At least 1 SPD •  secondaries rejection • Compatibility ITS/TPC •  fake rejection • pTcut of ~ 150 MeV/c • |h| < 0.8 M Floris - HP 2012

  5. Tracking performance DCAxy: Transverse distance-of-closest-approach pt resolution DCAxy ~10% at 50 GeV/c Small multiplicity dependence Estimate from track residuals Verified using cosmics & K0s invariant mass distribution  systematic uncertainty: 20% • Good DCAxy resolution • Tool to control contamination from secondaries • Strict DCAxy cut (< 7s), small contamination • Residual contamination: MC + DCAxy fits • Less than 1% for pt > 4 GeV/c M Floris - HP 2012

  6. Centrality Spectators Participants Participants Spectators • Fraction of cross section, 2 approaches: • Fit withGlauberMonte Carlo • Correct: subtract BG, efficiency and integrate multiplicity distributions • Npart, Ncoll, Nspect: require Glauber fit (computed using cuts on impact parameter) • Estimators: V0, SPD clusters, TPC tracks, ZDCs, … • ZDC measures Nspect: test of Glauber picture • Glauber fit ingredients • Woods-Saxon (constrained by low energy electron-nucleus scattering) • Inelastic pp cross section (measured by ALICE) • Nucleons follow straight line trajectories, interact based on their distance • Compute (fit) observables assuming: Central detector ZDC ZDC M Floris - HP 2012

  7. Background & Purity Pure hadronic sample down to ~ 90% centrality • QED pair production (~100 kb) • e+e-, soft • Photonuclear (single/double, ~ 10b) • Kinematics, ~ pA • Eg > 100 GeV • Starlight generator (SLIGHT) • EM dissociation (~100 b) • Few neutrons in ZDC, No central particles • Signal simulated with Hijing • Trigger efficiency: 97% (3-out-of-3) – 99% (2-out-of-3) M Floris - HP 2012

  8. dN/dhvs centrality Plot: arXiv:1202.3233 Plot: arXiv:1202.3233 • Scaling similar to RHIC: • Contribution of hard processes (Ncoll scaling)? • Classes of models • Saturation • 2 components (hard/soft) • models incorporating moderation of multiplicity (shadowing/saturation) favoured • dN/dh scales faster than pp • Trend predicted by some saturation model • Excellentagreement with LHC experiments • Energy density × t0≈ 3 × RHIC M Floris - HP 2012

  9. Nuclear modification factor (RAA) • Suppression of high ptparticles studied through “nuclear modification factor” RAA • pp reference: crucial ingredient • Default: pp 2.76 TeV measurement + Hagedorn fit • Crosschecks: • Interpolation of 0.9 and 7 TeV • NLO scaling of 7 TeV/2.76 TeV • Pythia 8 pp Reference default reference P.Luettig, poster M Floris - HP 2012

  10. RAA: Results • Strong suppression (max at pt~ 6 GeV/c) • Peak at pt ~ 2 GeV/c (Hydro?) • Rise and saturation at higher pt • Models describe rise • Consistent with CMS (but syst lower) VISH2+1: Heinz et al, arXiv:1105.3226 CMS: Eur. Phys. J. C 72 (2012) 1945 Needed: simultaneous description of different observables (IAA, jets), constraints on initial conditions • Energy loss calculations depend on: • Initial conditions • Initial production spectrum • Medium density profile • Space-time evolution • Energy loss model M Floris - HP 2012

  11. Centrality dependence Centrality Multiplicity • Integrated RAA between ptmin and ptmax • Suppression increases with centrality • Comparison to RHIC: • stronger for the same Npart • similar for the same multiplicity M Floris - HP 2012

  12. Summary & Outlook • ALICE characterized charged particle production in PbPb collisions • dNch/dh: centrality dependence similar RHIC, ~× 2 higher (e × t0 ≈ 3 × RHIC) • Stronger high-ptsuppression than at RHIC • In the works: • Forward Nch measurements (-4 < h < 5), total particle production • Reduce uncertainty on initial conditions: upcoming p-Pb run at the LHC p-Pb expectations Plots: arXiv:1111.3646 M Floris - HP 2012

  13. Alice Collaboration 35 Countries, 120 Institutes, over 1300 members M Floris - HP 2012

  14. Backup Slides

  15. Tracking performance – 2 Fraction of primaries (secondariescontamination from material + weak decays of strange particles) • Strict DCAxycut (< 7s), small contamination • Residual contamination: MC + DCAxy fits • Less than 1% for pt > 4 GeV/c M Floris - HP 2012

  16. Tracking performance – 3 TPC track prolongation efficiency to ITS Similar in data and MC Small residual differences systematics on efficiency (~ 4%) M Floris - HP 2012

  17. dN/dhvs centrality Plot: arXiv:1202.3233 • Scaling similar to RHIC: • Contribution of hard processes (Ncoll scaling)? • Multiplicity scaling with centrality: • Stronger than Npart • Different possible scalings(2 component, power laws)reproduce data • Glauberfits not sensitive to choice of parameterization M Floris - HP 2012

  18. M Floris - HP 2012

  19. Centrality and ZDC • ZDCs and ZEMs far from the IP (ZDCs ~114m, ZEM~7.5m) • Response weakly dependent on vertex position • Analyses that do not use vertex cut NPART≃ 2A – NSPECT = 2A – EZDC/EBEAM Consistent picture with forward/central detectors M Floris - HP 2012

  20. M Floris - HP 2012

  21. Tracklet analysis M Floris - HP 2012

  22. M Floris - HP 2012

  23. M Floris - HP 2012

  24. ALICE vs RHIC M Floris - HP 2012

  25. dN/dh for central events Plot: arXiv:1202.3233 Multiplicity and Energy density e: dNch/dh = 1601 ± 60 (syst) on high side of expectations growth with √s faster in AA than pp Energy density ≈ 3 x RHIC Excellent agreement with LHC experiments M Floris - HP 2012

  26. Multiplicity with tracklets Main multiplicity estimator: tracklets, combinatorial background! 3 techniques for subtraction: Shape of BG from Monte Carlo Injection (add few “fake” clusters) Rotation: rotate one layer Crosscheck with full tracks (secondaries via DCA fits)  Fully consistent Normalization: Enlarge Df cut to find a pure BG region M Floris - HP 2012

  27. Centrality dependence M Floris - HP 2012

  28. Reaction plande dependence of RAA M Floris - HP 2012

  29. Beam Background Event Time  Vertex  IP V0C V0A M Floris - HP 2012

  30. Forward Multiplicity h M Floris - HP 2012

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