High-pT results from ALICE

Marco van Leeuwen,Utrecht University,for the ALICE collaboration

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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 ~ 100-300 MeV

Sensitive to medium density, transport properties

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ALICE

Central tracker:|| < 0.9High resolution• TPC• ITS

Particle identification•HMPID •TRD•TOF

EM Calorimeters• EMCal• PHOS

Forward muon arm-4 < < -2.5

2010: 20M hadronic Pb+Pb events, 300M p+p MB events

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0 spectra0 spectraTwo methods: conversions and PHOS

Good agreement

Agrees with world data(not shown)

/ ratio

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Medium-induced radition

If < f, multiple scatterings add coherently

2ˆ~ LqE Smed

2

2

Tf k

Zapp, QM09

Lc = f,max

propagating

parton

radiatedgluon

Landau-Pomeranchuk-Migdal effectFormation time important

Radiation sees length ~f at once

Energy loss depends on density: 1

2

ˆq

q

and nature of scattering centers(scattering cross section)

Transport coefficient

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Nuclear modification factor

Charged hadron pT spectra

Nuclear modificationfactor

Shape of spectra in Pb+Pb differfrom p+p

Large suppressionRAA rises with pT relative energy loss decreases

ppTcoll

PbPbTAA dpdNN

dpdNR

/

/

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Comparing to theoryAdd model refs

Many theory calculations available

Ingredients:- pQCD production- Medium density profile

tuned to RHIC data, scaled- Energy loss model

Large spread of predictionsSome may be ruled out, need to explore systematics of theory

All calculations show increase with pT

More to come!

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Identified hadron RAA (strangeness)

Kaon, pion RAAsimilar

: RAA~1 at pT~3 GeVSmaller suppression,/K enhanced at low pT

pT ~8 GeV:All hadrons similar

Confirms partonic origin of suppression?

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Elliptic flow v2

Mass-dependence indicates boost(common flow field)

Agrees well with Hydrodynamical calculations

Density, pressure gradients convert spatial anisotropy into

momentum space

React

ion p

lane

)(2cos21 2

vNddN

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High-pT v2

In-plane, out-of plan RAA

Larger suppression out-of-plane:Path length dependent energy loss

High-pT v2

v2 is non-zero at high pT

Clear path length dependence of energy lossTheory calculations ongoing

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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

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Di-hadron correlationsA

LIC

E, a

rXiv:1

11

0.0

12

1

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: IAA

Near side: yield increases

Away side: yield decreases

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Di-hadron suppressionA

LIC

E, a

rXiv:1

11

0.0

12

1

Near side Away side

Near side: enhancementEnergy loss changes underlying kinematics+ radiated gluon fragments

Away side: suppressionEnergy loss reduces fragment pT

Surface bias effect: longer mean path length

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Comparing di-hadrons and single hadrons

Energy loss calculationsdepend on:-Initial production spectrum-Medium density profile/evolution-Energy loss model

Need simultaneous comparison to several measurements to constrain all aspects

Here: RAA and IAA

Of this set: YaJEM-D gives best description

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Di-hadrons at lower pT

Alver and Roland, PRC81, 054905

2 < pT,trig < 4 GeV1 < pT,assoc < 2 GeV0-2% central

Di-hadron structure at low pT: three peaks

Higher harmonics from initial state fluctuations (v3)

visible in final state

Di-hadrons at low pT measure bulk correlations

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Charm nuclear modification

Three decay channels studied:

KD 0

KD

KDD 0*

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 RAA ≥ light hadrons

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Heavy flavour, towards beauty

Horow

itz and Gyulassy, arX

iv:1107.2136

Expected difference betweencharm and light quarks not large

Heavy flavour electrons

Significant contribution from BAgrees with FONLL in p+p

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Jets in pp

EMCal (100º in azimuth)Installed in winter 2011/2012

p+p charged jets well described by PYTHIA

EMCal jet trigger commissionedin p+p

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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

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Jets in heavy ion collisions

Reconstructed jet spectrumDominated by background fluctuations for pT < 60-80 GeV (central events)

Unfolding of fluctuations needed: in progress…

jetbkgmeasTjetT App ,,

Subtract uncorrelated background:

Fluctuations remain after subtraction

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2011 Pb+Pb run

• Expect 500-1000 Hz hadronic• Integrated lumi 10-20x 2010

– EMCal jet trigger– Forward muons (J/, 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

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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

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Jet Quenching

1) 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