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 ~ 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 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

4. 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

5. 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

6. 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

7. Comparing to theory All calculations show increase with pT
HT: X-N Wang et al, arXiv: (PRC)
HT: Majumder, Shen, arXiv:
TR: T. Renk et al, arXiv: (PRC)
WHDG: Horowitz and Gyulassy, arXiv:
Bass et al, PRC79,
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!

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. 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

10. 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

11. 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)

12. Di-hadron correlations
associated 
trigger
After background subtraction
ALICE, arXiv:
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

13. Di-hadron suppression
Near side
Away side
ALICE, arXiv:
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 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,
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
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

17. Heavy flavour, towards beauty
RAA: Heavy flavour electrons, D
Horowitz and Gyulassy, arXiv:
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

18. 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

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 < 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. (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

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 LHC and 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