High-p T results from ALICE Marco van Leeuwen, Utrecht University, for the ALICE collaboration

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

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

4  0 spectra Two methods: conversions and PHOS Good agreement Agrees with world data (not shown)  /  ratio

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

6 Nuclear modification factor Charged hadron p T spectra Nuclear modification factor Shape of spectra in Pb+Pb differ from p+p Large suppression R AA rises with p T  relative energy loss decreases

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

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

9 Elliptic flow v 2 Mass-dependence indicates boost (common flow field) Agrees well with Hydrodynamical calculations Density, pressure gradients convert spatial anisotropy into momentum space Reaction plane

10 High-p T v 2 In-plane, out-of plan R AA Larger suppression out-of-plane: Path length dependent energy loss High-p T v 2 v 2 is non-zero at high p T  Clear path length dependence of energy loss Theory calculations ongoing

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

12 Di-hadron correlations ALICE, arXiv: associated  trigger 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 After background subtraction Energy loss+fragmentation Quantify/summarise: I AA Near side: yield increases Away side: yield decreases

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

14 Comparing di-hadrons and single hadrons Energy loss calculations depend on: -Initial production spectrum -Medium density profile/evolution -Energy loss model Need simultaneous comparison to several measurements to constrain all aspects Here: R AA and I AA Of this set: YaJEM-D gives best description

15 Di-hadrons at lower p T Alver and Roland, PRC81, < p T,trig < 4 GeV 1 < p T,assoc < 2 GeV 0-2% central Di-hadron structure at low p T : three peaks Higher harmonics from initial state fluctuations (v 3 ) visible in final state Di-hadrons at low p T measure bulk correlations

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

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

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

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

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

Pb+Pb run Expect Hz hadronic Integrated lumi 10-20x 2010 –EMCal jet trigger –Forward muons (J/ , heavy flavour decays) Online centrality trigger –Large increase of central events R AA light, charm etc –Large sample of mid-central collisions Flow at high p T, charm flow 2012: p+Pb running – First tests promising

22 Conclusion First round of parton energy loss results available: –Single hadron, di-hadron suppression –R AA similar for all measured hadrons at p T > 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 1)How is does the medium modify parton fragmentation? Energy-loss: reduced energy of leading hadron – enhancement of yield at low p T ? 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