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 2010: 20M hadronic Pb+Pb events, 300M p+p MB events 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 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) Radiation sees length ~tf at once 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 h/p ratio Agrees with world data Two methods: conversions and PHOS Good agreement h/p ratio Agrees with world data

Nuclear modification factor Charged hadron pT spectra Shape of spectra in Pb+Pb differ from p+p Large suppression RAA rises with pT  relative energy loss decreases

Comparing to theory All calculations show increase with pT 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 Bass et al, PRC79, 024901 Medium density tuned to RHIC data, scaled with multiplicity All calculations show increase with pT Well-known radiative formalisms ASW, WHDG predict too much suppression (HT better?) Ingredients: pQCD production Medium density profile tuned 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?

Mass-dependence indicates boost (common flow field) Elliptic flow v2 Reaction plane Density, pressure gradients convert spatial anisotropy into momentum space Mass-dependence indicates boost (common flow field) Agrees well with Hydrodynamical calculations  Viscosity small

High-pT v2 In-plane, out-of plan RAA High-pT v2  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 Leading particle 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 Di-hadron correlations: Simple and clean way to access di-jet fragmentation Background clearly identifiable No direct access to undelying kinematics (jet energy) Compare AA to pp 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 Di-hadrons at low pT measure bulk correlations 2 < pT,trig < 4 GeV 1 < pT,assoc < 2 GeV 0-2% central Alver and Roland, PRC81, 054905 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 Measurement: Charm RAA ≥ light hadrons light Expected energy loss Expect: heavy quarks lose less energy due to dead-cone effect 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 between charm 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 p+p charged jets well described by PYTHIA EMCal jet trigger commissioned in p+p

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

Extra slides

(from hard scattering) Jet Quenching 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? High-energy parton (from hard scattering) Hadrons

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

p+p reference

Multiplicity dependence of RAA RAA scales with dNch/dh ?

RAA compared to RHIC results Larger suppression at LHC and pT-dependence

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