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High-p T results from ALICE

High-p T results from ALICE. Marco van Leeuwen, Utrecht University, for the ALICE collaboration. Hard probes of QCD matter. Heavy-ion collisions produce ‘quasi-thermal’ QCD matter Dominated by soft partons p ~ T ~ 100-300 MeV. Hard-scatterings produce ‘quasi-free’ partons

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High-p T results from ALICE

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  1. High-pT results from ALICE Marco van Leeuwen, Utrecht University, for the ALICE collaboration

  2. Hard probes of QCD matter Heavy-ion collisions produce‘quasi-thermal’ QCD matter Dominated by soft partons p ~ T ~ 100-300 MeV Hard-scatterings produce ‘quasi-free’ partons  Initial-state production known from pQCD  Probe medium through energy loss Use the strength of pQCD to explore QCD matter Sensitive to medium density, transport properties

  3. ALICE • EM Calorimeters • EMCal • PHOS • Central tracker: • |h| < 0.9 • High resolution • TPC • ITS • Particle identification • HMPID • TRD • TOF Forward muon arm -4 < h < -2.5 2010: 20M hadronic Pb+Pb events, 300M p+p MB events

  4. p0 spectra p0 spectra Two methods: conversions and PHOS Good agreement h/p ratio Agrees with world data (not shown)

  5. Medium-induced radition Zapp, QM09 Lc = tf,max Radiation sees length ~tf at once Landau-Pomeranchuk-Migdal effect Formation time important radiated gluon propagating parton If l < tf, multiple scatterings add coherently Energy loss depends on density: and nature of scattering centers (scattering cross section) Transport coefficient

  6. Nuclear modification factor Nuclear modificationfactor Charged hadron pT spectra Shape of spectra in Pb+Pb differfrom p+p Large suppression RAA rises with pT relative energy loss decreases

  7. Comparing to theory Add model refs Many theory calculations available • Ingredients: • pQCD production • Medium density profiletuned to RHIC data, scaled • Energy loss model Large spread of predictions Some may be ruled out, need to explore systematics of theory More to come! All calculations show increase with pT

  8. Identified hadron RAA (strangeness) L: RAA~1 at pT~3 GeV Smaller suppression, L/K enhanced at low pT Kaon, pion RAA similar pT ~8 GeV: All hadrons similar Confirms partonic origin of suppression?

  9. Elliptic flow v2 Reaction plane Density, pressure gradients convert spatial anisotropy intomomentum space Mass-dependence indicates boost(common flow field) Agrees well with Hydrodynamical calculations

  10. High-pT v2 In-plane, out-of plan RAA High-pT v2 Larger suppression out-of-plane: Path length dependent energy loss  v2 is non-zero at high pT Clear path length dependence of energy loss Theory calculations ongoing

  11. Di-hadron correlations I: Underlying event in p+p Multiplicity in transverse region Azimuthal distribution wrt leading track More underlying event in data than in MC generators Being used to tune MC gens (Pythia, Herwig, etc)

  12. Di-hadron correlations associated  trigger After background subtraction ALICE, arXiv:1110.0121 Background Compare AA to pp • Di-hadron correlations: • Simple and clean way to access di-jetfragmentation • Background clearly identifiable • No direct access to undelying kinematics • (jet energy) Near side: yield increases Away side: yield decreases Energy loss+fragmentation Quantify/summarise: IAA

  13. Di-hadron suppression Near side Away side ALICE, arXiv:1110.0121 Near side: enhancement Energy loss changes underlying kinematics + radiated gluon fragments Away side: suppression Energy loss reduces fragment pT Surface bias effect: longer mean path length

  14. Comparing di-hadrons and single hadrons • Energy loss calculationsdepend on: • -Initial production spectrum • Medium density profile/evolution • Energy loss model Need simultaneous comparison to several measurements to constrain all aspects Here: RAA and IAA Of this set: YaJEM-D gives best description

  15. Di-hadrons at lower pT Alver and Roland, PRC81, 054905 2 < pT,trig < 4 GeV 1 < pT,assoc < 2 GeV 0-2% central Higher harmonics from initial state fluctuations (v3) visible in final state Di-hadron structure at low pT: three peaks Di-hadrons at low pT measure bulk correlations

  16. Charm nuclear modification Expected energy loss light Expect: heavy quarks lose less energy due to dead-cone effect Measurement: Charm RAA≥ light hadrons Three decay channels studied: Most pronounced for bottom Use PID to identify daughters where possible

  17. Heavy flavour, towards beauty Heavy flavour electrons Horowitz and Gyulassy, arXiv:1107.2136 Expected difference betweencharm and light quarks not large Significant contribution from B Agrees with FONLL in p+p

  18. Jets in pp EMCal (100º in azimuth) Installed in winter 2011/2012 EMCal jet trigger commissioned in p+p p+p charged jets well described by PYTHIA

  19. Jets in heavy ion collisions Large uncorrelated background density in heavy ion collisions r ~ 170 GeV in central events Measure background fluctuations ‘in situ’: Random cones, embedding give similar results not gaussian: tail from jets sgauss = 10 GeV for central events

  20. Jets in heavy ion collisions Subtract uncorrelated background: Fluctuations remain after subtraction Unfolding of fluctuations needed: in progress… Reconstructed jet spectrum Dominated by background fluctuations for pT < 60-80 GeV (central events)

  21. 2011 Pb+Pb run • Expect 500-1000 Hz hadronic • Integrated lumi 10-20x 2010 • EMCal jet trigger • Forward muons (J/y, heavy flavour decays) • Online centrality trigger • Large increase of central events • RAA light, charm etc • Large sample of mid-central collisions • Flow at high pT, charm flow 2012: p+Pb running – First tests promising

  22. Conclusion • First round of parton energy loss results available: • Single hadron, di-hadron suppression • RAA similar for all measured hadrons at pT > 8 GeV • Dependence on reaction plane angle • Heavy quarks (charm only for now) • Need careful comparisons with theory, RHIC to constrain theory • Jet reconstruction being worked on • Need stats, control background fluctuations • 2011 run will bring factor ~10 increase for main results

  23. Jet Quenching High-energy parton (from hard scattering) Hadrons • How is does the medium modify parton fragmentation? • Energy-loss: reduced energy of leading hadron – enhancement of yield at low pT? • Broadening of shower? • Path-length dependence • Quark-gluon differences • Final stage of fragmentation outside medium? 2) What does this tell us about the medium ? • Density • Nature of scattering centers? (elastic vs radiative; mass of scatt. centers) • Time-evolution?

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