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

Explore ALICE results on charged particles in Pb-Pb collisions, tracking performance, centrality, and more from HP2012 reports by M. Floris for ALICE Collaboration. Comprehensive study on triggers, tracking, centrality, background, and purity factors using ALICE detector at LHC, valuable for theoretical models comparison and optimization.

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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

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  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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