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Jan Fiete Grosse-Oetringhaus, CERN for the ALICE collaboration

Azimuthal Correlations in Pb+Pb Collisions at Ö s NN = 2.76 TeV measured with ALICE First Measurement of I CP and I AA at LHC. Jan Fiete Grosse-Oetringhaus, CERN for the ALICE collaboration Rencontres de Moriond QCD and High Energy Interactions La Thuile, 2011. Motivation.

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Jan Fiete Grosse-Oetringhaus, CERN for the ALICE collaboration

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  1. Azimuthal Correlations in Pb+Pb Collisions at ÖsNN = 2.76 TeV measured with ALICEFirst Measurement of ICP and IAA at LHC Jan Fiete Grosse-Oetringhaus, CERN for the ALICE collaboration Rencontres de Moriond QCD and High Energy Interactions La Thuile, 2011

  2. Motivation • Ultrarelativistic heavy-ion collisions probe QCD matter at unprecedented energy densities • Characterize the hot and dense medium (quark-gluon plasma) by comparing quenched (with plasma) and unquenched collisions • Use high pT partons as probes that "feel" the medium  jet tomography • Assess modifications of dijet structure by dihadron two-particle azimuthal correlations • STAR (RHIC) found disappearance of away-side peak 4 < pT,trig < 6 2 < pT,assoc < pT,Trig STAR, PRL 91 (2003) 072304 Measurement of ICP and IAA - Jan Fiete Grosse-Oetringhaus

  3. Dihadron Correlations • Study two-particle correlations with per-trigger yields • Lower pT • Ridge • Hydrodynamics, flow • High pT • Quenching/suppression, broadening • Calculate near side (around f = 0) and away side (f = p) yields • Compare central and peripheral  ICP • Compare AA and pp  IAA Pb+Pb 2.76 TeV and trigger particle associated particle Pb+Pb 2.76 TeV Measurement of ICP and IAA - Jan Fiete Grosse-Oetringhaus

  4. EMCAL γ, π0, jets L3 Magnet T0/V0 Trigger ACORDE Cosmic trigger HMPID PID (RICH) @ high pT TRD Electron ID (TR) TOF PID PMD γ multiplicity TPC Tracking, PID (dE/dx) ITS Low pT tracking PID + Vertexing MUON μ-pairs FMD Charged multiplicity PHOS γ, π0, jets ALarge Ion Collider Experiment Dipole Not shown: ZDC (at ±114m) Measurement of ICP and IAA - Jan Fiete Grosse-Oetringhaus

  5. Analysis • 12M Pb+Pb MB collisions used • Tracking with Time Projection Chamber and Inner Tracking System in |h| < 0.8 • Flat f acceptance  No mixed events needed for acceptance correction (in f) • Centrality determination with V0 (forward scintillators) and hits in pixel detector • Corrections applied for efficiency and contamination • Weakly centrality dependent • Two-track effects small but considered Number of tracks ALICE performance, 14.03.11 Pb+Pb 2.76 TeV, TPC tracks f (rad.) ALICE performance14.03.11 MC Pb+Pb 2.76 TeV TPC tracks Tracking efficiency 0-20% 20-40% 40-90% pT (GeV/c) Measurement of ICP and IAA - Jan Fiete Grosse-Oetringhaus

  6. To calculate yields, pedestal needs to be determined Fit in region around p/2 (ZYAM) Different ways to estimate uncertainty Estimate radial flow (v2) contribution using ALICE flow measurement Flow subtraction quite controversial Measure in a region where the signal dominates over pedestal and v2 modulation(8 GeV/c < pT,trig < 15 GeV/c) Indicate difference in measurement if v2 was subtracted Pedestal and Flow different pedestals v2 contribution Measurement of ICP and IAA - Jan Fiete Grosse-Oetringhaus

  7. After pedestal (and optionally v2) subtraction), integrate to obtain yield Y Near side -0.7 < f < 0.7 Away side -0.7 < f – p < 0.7 In bins of associated pT: pT,assoc Divide yields to obtain ICP and IAA Yield Extraction 0-5% 60-90% integration windows Measurement of ICP and IAA - Jan Fiete Grosse-Oetringhaus

  8. Systematic Uncertainties • Detector efficiency and two-track effects • Different detectors for centrality determination • pT resolution • Fold associated pT distribution with momentum resolution • Different pedestal determination schemes • Integration window (between ±0.5 rad. and ±0.9 rad.) Ranges indicate different values for ICP/IAA,Pythia and near/away side Measurement of ICP and IAA - Jan Fiete Grosse-Oetringhaus

  9. ICP • Flat pedestal subtraction  data points • v2 subtracted  line • Difference only at low pT • Statistical and systematic uncertainties (shaded area) shown flat pedestal v2 subtracted Measurement of ICP and IAA - Jan Fiete Grosse-Oetringhaus

  10. ICP (2) • Slightly enhanced near-side: ICP ~ 1.2 … unexpected and interesting • Away side suppressed: ICP ~ 0.6 … expected from in-medium energy loss • v2 contribution small except in lowest bin, there v3 subtraction may be significant Measurement of ICP and IAA - Jan Fiete Grosse-Oetringhaus

  11. Interesting to study yield with respect to unquenched (pp) case No pp data taken at 2.76 TeV, yet Use a MC Pythia6 tune Perugia-0 has been found to describe dihadron correlations at 0.9 and 7 TeV well Using a scaling factor between 0.8 and 1 Interpolate to 2.76 TeV Factor 0.93 ± 13% (stat/syst)  Use scaled Pythia reference to calculate IAA,Pythia IAA Reference 0.9 TeV 7 TeV Data Pythia Measurement of ICP and IAA - Jan Fiete Grosse-Oetringhaus

  12. IAA,Pythia • Central events • Near side enhanced IAA,Pythia ~ 1.5 • Away side suppressed IAA,Pythia ~ 0.5 – 0.7 • Peripheral events • Near side enhanced IAA,Pythia ~ 1.2 • Away side IAA,Pythia consistent with 1 Measurement of ICP and IAA - Jan Fiete Grosse-Oetringhaus

  13. Near side enhancement in ICP and IAA,Pythia in central events Near side is modified  trigger particle sees the medium Possible explanation In the presence of quenching same trigger pT might probe higher parton pT Change of normalization of power law (relative energy loss) leads to constant ratio Different result for different parton spectra  sensitivity to initial parton spectrum Increased parton pT increased yield  increased IAA,Pythia/ICP IAA,Pythia/ICP on away side would be even lower without this effect! Near-Side Enhancement Toy power law example A/(pT – DpT)n pT (GeV/c) Measurement of ICP and IAA - Jan Fiete Grosse-Oetringhaus

  14. 8 < pT,trig < 15 IAA,Pythia: ALICE vs. RHIC PHENIX, PRL 104, 252301 (2010) STAR, PRL97,162301 (2006) • PHENIX subtracts v2 compare ALICE line with PHENIX • STAR measurement in slightly different variable (zT) and d+Au reference Measurement of ICP and IAA - Jan Fiete Grosse-Oetringhaus

  15. Azimuthal dihadron correlations extracted from Pb+Pb collisions pT-region studied where the background (pedestal + v2) contribution is small ICP and IAA,Pythia measured Near side enhanced (slightly in ICP, factor 1.5 in IAA,Pythia) Might be something interesting… Factor 1.5 might be due to the PYTHIA reference  LHC pp run at 2.76 TeV will tell… Away side suppressed (ICP and IAA,Pythia) by about a factor 2 Dihadron correlations will be further studied In more pT regions Characterization of near and away side peak shapes Differentially in Dh Ridge physics These measurements show the presence of a hot and dense medium at LHC and allow to constrain energy-loss models (together with RAA and other measurements) Summary Thank you for your attention! Measurement of ICP and IAA - Jan Fiete Grosse-Oetringhaus

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