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

  4. Medium-induced radiation Radiation sees length ~tf at once Landau-Pomeranchuk-Migdal effect Formation time important Energy loss radiated gluon CR: color factor (q, g) : medium density L: path length m: quark mass (dead cone eff) propagating parton Path-length dependence Ln n=1: elastic n=2: radiative (LPM regime) n=3: AdS/CFT (strongly coupled) Energy loss depends on density: and nature of scattering centers (scattering cross section) Transport coefficient

  5. p0 spectra in p+p p0 spectra Two methods: conversions and PHOS Good agreement h/p ratio Agrees with world data

  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 HT: X-N Wang et al, arXiv:1102.5614 (PRC) HT: Majumder, Shen, arXiv:1103.0809 TR: T. Renk et al, arXiv:1103.5308 (PRC) WHDG: Horowitz and Gyulassy, arXiv:1104.4958 All calculations show increase with pT Well-known radiative formalisms ASW, WHDG predict too much suppression(HT better?) Bass et al, PRC79, 024901 Medium density tuned to RHIC data, scaled with multiplicity • Ingredients: • pQCD production • Medium density profiletuned to RHIC data, scaled • Energy loss model Need time to sort out theory uncertainties:More to come!

  8. Identified hadron RAA (strangeness) L: RAA~1 at pT~3 GeV/c Smaller suppression, L/K enhanced at low pT Kaon, pion RAA similar pT ~8 GeV/c: All hadrons similar partonic energy loss + pp-like fragmentation?

  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  Viscosity small

  10. High-pT v2 High-pT v2 In-plane, out-of plan RAA in-plane  v2 is non-zero at high pT multi-particle methods suppress non-flow Larger suppression out-of-plane Clear path length dependence of energy loss Theory calculations ongoing

  11. Di-hadron correlations I: Underlying event in p+p Azimuthal distribution wrt leading track Leadingparticle Transverse region Multiplicity in transverse region More underlying event in data than in MC generators Being used to tune MC generators(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 Need simultaneous comparison to several measurements to constrain geometry and E-loss Here: RAA and IAA Three models: ASW: radiative energy loss YaJEM: medium-induced virtuality YaJEM-D: YaJEM with L-dependent virtuality cut-off (induces L2) None of these works well without tuning

  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 RAA: Heavy flavour electrons, D Horowitz and Gyulassy, arXiv:1107.2136 Expected difference betweencharm and light quarks not large Significant contribution from B expected at pT > 4 GeV Hints at large E-loss for B Next: separate out B

  18. Jets in pp EMCal (100º in azimuth) Installed in winter 2010/2011 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/c in central events Measure background fluctuations ‘in situ’: Random cones, embedding give similar results not gaussian: tail from jets sgauss = 10 GeV/c 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/c (central events)

  21. 2011 Pb+Pb run • Expect >1kHz 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. Extra slides

  24. 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?

  25. p0 RAA p0→gg with conversions Good agreement between charged p and p0

  26. p+p reference

  27. Multiplicity dependence of RAA RAA scales with dNch/dh ?

  28. RAA compared to RHIC results Larger suppression at LHCand pT-dependence

  29. Heavy flavour electrons Heavy flavour electrons (p+p) RAA for electrons, muons Significant contribution from B Agrees with FONLL in p+p

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