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.

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Presentation transcript:

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 pt resolution, 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 28/05/2012 M Floris - HP 2012

Trigger detectors and h Coverage 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 VZEROA SPD 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 28/05/2012 M Floris - HP 2012

Tracklets and tracks Tracklets: Tracks: SPD Tracklets: Combination of 2 hits in the 2 SPD layers, within a Dq-Dj window, pointing to the vertex Less dependent on calibration pT cut-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 pT cut of ~ 150 MeV/c |h| < 0.8 28/05/2012 M Floris - HP 2012

distance-of-closest-approach 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 28/05/2012 M Floris - HP 2012

Centrality Central detector ZDC ZDC Participants Spectators Fraction of cross section, 2 approaches: Fit with Glauber Monte 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 Spectators Participants ZDC ZDC 28/05/2012 M Floris - HP 2012

Background & Purity Pure hadronic sample down to ~ 90% centrality QED pair production (~100 kb) Few neutrons in ZDC, No central particles e+e-, soft Signal simulated with Hijing Photonuclear (single/double, ~ 10b) Trigger efficiency: Kinematics, ~ pA 97% (3-out-of-3) – 99% (2-out-of-3) Eg > 100 GeV Starlight generator (SLIGHT) EM dissociation (~100 b) 28/05/2012 M Floris - HP 2012

dN/dh vs centrality Scaling similar to RHIC: 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 Excellent agreement with LHC experiments Energy density × t0 ≈ 3 × RHIC 28/05/2012 M Floris - HP 2012

Nuclear modification factor (RAA) Suppression of high pt particles 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 28/05/2012 M Floris - HP 2012

VISH2+1: Heinz et al, arXiv:1105.3226 RAA: Results 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 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) Energy loss calculations depend on: Initial conditions Initial production spectrum Medium density profile Space-time evolution Energy loss model 28/05/2012 M Floris - HP 2012

Centrality dependence Multiplicity Integrated RAA between ptmin and ptmax Suppression increases with centrality Comparison to RHIC: stronger for the same Npart similar for the same multiplicity 28/05/2012 M Floris - HP 2012

Summary & Outlook ALICE characterized charged particle production in PbPb collisions dNch/dh: centrality dependence similar RHIC, ~ × 2 higher (e × t0 ≈ 3 × RHIC) Stronger high-pt suppression 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 28/05/2012 M Floris - HP 2012

35 Countries, 120 Institutes, over 1300 members Alice Collaboration 35 Countries, 120 Institutes, over 1300 members 28/05/2012 M Floris - HP 2012

Backup Slides

Tracking performance – 2 Fraction of primaries (secondaries contamination from material + weak decays of strange particles) Strict DCAxy cut (< 7s), small contamination Residual contamination: MC + DCAxy fits Less than 1% for pt > 4 GeV/c 28/05/2012 M Floris - HP 2012

Tracking performance – 3 TPC track prolongation efficiency to ITS Similar in data and MC Small residual differences systematics on efficiency (~ 4%) 28/05/2012 M Floris - HP 2012

dN/dh vs centrality Scaling similar to RHIC: 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 Glauber fits not sensitive to choice of parameterization 28/05/2012 M Floris - HP 2012

28/05/2012 M Floris - HP 2012

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 28/05/2012 M Floris - HP 2012

28/05/2012 M Floris - HP 2012

Tracklet analysis 28/05/2012 M Floris - HP 2012

28/05/2012 M Floris - HP 2012

28/05/2012 M Floris - HP 2012

ALICE vs RHIC 28/05/2012 M Floris - HP 2012

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 28/05/2012 M Floris - HP 2012

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 28/05/2012 M Floris - HP 2012

Centrality dependence 28/05/2012 M Floris - HP 2012

Reaction plande dependence of RAA 28/05/2012 M Floris - HP 2012

Beam Background Event Time  Vertex  IP V0C V0A 28/05/2012 M Floris - HP 2012

Forward Multiplicity h 28/05/2012 M Floris - HP 2012