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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-pT 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 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
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
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
p0 spectra in p+p p0 spectra Two methods: conversions and PHOS Good agreement h/p ratio Agrees with world data
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
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!
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?
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
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
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)
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
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
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
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
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
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
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
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
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)
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
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
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?
p0 RAA p0→gg with conversions Good agreement between charged p and p0
Multiplicity dependence of RAA RAA scales with dNch/dh ?
RAA compared to RHIC results Larger suppression at LHCand pT-dependence
Heavy flavour electrons Heavy flavour electrons (p+p) RAA for electrons, muons Significant contribution from B Agrees with FONLL in p+p