High- p T particle correlations in pp collisions at LHC/ALICE

Yaxian.Mao@QM2011, 24/05/2011, Annecy

High-pT particle correlations in pp

collisions at LHC/ALICEYaxian Mao

(for the ALICE collaboration)

IOPP-Wuhan,China & LPSC-Grenoble, France

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Objective• Understand the jet properties in pp to exploit jets

as probes of the QCD medium in AA.

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• Methods:

o Full Jet reconstruction

o Two-particle correlations:

o Leading Particle (LP) approximates jet direction


xE kinematics

o Charged/neutral trigger:

o Isolated trigger:

o Direct photon trigger :

trigger

associate

3

• di-jet balance (no

kT)

•


• Leading particle trigger selection

• Azimuthal correlation between trigger and associate particles

Leading Particle Correlation

• Away side associate particle xE distribution

Near side

Away side

4


xE distribution (S+B)

Background estimate for away

side xE:

• Assumption: isotropic in Δϕ

• Estimate from

• Normalize to the away side

hemisphere

5

Near side

Away side


h

xE distribution (bkg. subtracted)

• Low xE (<0.4) → power law (away/near side jet momentum imbalance) • High xE (>0.4) → exponential ( )

→ Extract xE exponential inverse slope in the range [0.4, 0.8]

(n is slope of inclusive dist. )

6

PHENIX, PRD 74 (2006) 072002


pT dependence xE slope

• data exhibits uniform xE slope over a wide pTt range

DELPHI, EPJC, 13 (2000) 573

OPAL, ZPC, 69 (1996) 543

7

• away side xE distribution different from real jet FF ( )• how to improve ? → next slide…

J. D. Bjorken, PRD8 (1973) 4098


IsolationEnrich the trigger sample with <zt> → 1

• estimate hadronic (charged only) activity around the trigger o R (= 0.4) of the cone o ε(= 10%) fraction of trigger pT

• trigger (pTt) is isolated if ∑cone pT

h± < ε* pTt

Rtrigger

IP

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< zt > ~ 0.5 < ztisolated> ~ 0.8

Isolation

Thursday Poster: N. Arbor


ΔΦ correlation

Isolated charged

Charged hadron trigger:

Associate charged hadrons:

Isolation

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xE distribution: w/ (w/o) isolation

• Isolated trigger has steeper slope than non-isolated trigger

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xE slope for isolated trigger

• Exponential fit in the range → xE slope higher than the slope of non-isolated

trigger11


Conclusion and OutlookTwo particle correlation is a suitable tool to study the jet

properties:

- Charged triggers lead to an xE slope corresponding to

the 〈 zt〉 ~ 0.5 region

- Isolation introduces a trigger bias towards 〈 zt〉 ~ 0.8 providing

o a description closer to the fragmentation function

o the potential to distinguish quark and gluon jet fragmentation

- Isolated direct photons (dominated by quark jets from quark jets from

“Compton scattering”“Compton scattering”) will allow to achieve 〈 zt〉 = 1

preview in the next few slides … preview in the next few slides …

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Neutral cluster triggers

• Photons are detected as clusters of cells in the calorimeters

• No photon PID applied yet

• Cluster dominated by π0 contribution

PYTHIA

PTcluster PTtrue 13


ΔΦ correlationCluster trigger

• Azimuthal correlation independent on the calorimeters

pp@√s=7 TeV

cluster trigger: pTt ∈ [4, 5],[5,6], [6,7],[7,8] GeV/c

associate charged hadrons:

pTa > 1 GeV/c

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Inverse xE slope

• cluster trigger (mainly π0) and charged trigger has similar slope on xE distribution

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Thanks very much!


The ALICE detectors (2010)

• Photons are detected in the EM calorimeters

- PHOS ( ⎜η⎜< 0.12, ΔΦ = 60°)

- EMCAL ( ⎜η⎜< 0.7, ΔΦ = 40°)

• Charged particles are detected by the central tacking system

- ITS+TPC ( ⎜η⎜< 0.9, ΔΦ = 2π) 17

4/10


The ALICE detectors (2011)

• Photons are detected in the EM calorimeters

- PHOS ( ⎜η⎜< 0.12, ΔΦ = 60°)

- EMCAL ( ⎜η⎜< 0.7, ΔΦ = 100°)

• Charged particles are detected by the central tacking system

- ITS+TPC ( ⎜η⎜< 0.9, ΔΦ = 2π) 18

10/10


Dihadron ⟨zt⟩≠1; xE ≠ FF

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PHENIX, Phys. Rev D 74, 072002 (2006)

Measured dN/dxE compared with dN/dxE calculated using FF of quark Dq ∝ (−8.2 z ) (solid line) and gluon Dg ∝ (−11.4 z ) (dashed line) from LEP.

CCOR, Physica Scripta 19, 116 (1979)

xE ~ za/<zt><zt>=0.85 measured⇒Dq

π(z)~e-6zae-5.3x

E


Photon Tagged Jets: xE ~ FF

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γ energy ≈ jet energy

X.N. Wang, Z. Huang PRC55 (1997) 3047-3061

Also can measure IAA, ratio of the direct photon-hadron yields


Outline

• Objective

• 2→2 kinematics (LO)

• Non-isolated trigger hadron correlation

• Isolated trigger-hadron correlation

• neutral cluster-hadron correlation

• Conclusion

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ΔΦ correlation

Isolated cluster neutral cluster

trigger:

Associate charged hadrons:

Isolation

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Trigger associated momentum distributions

• trigger = gdir, assoc = hadron : folding only over assoc FF

• trigger=hadron, assoc=hadron : folding over trigger and assoc. FF

S’ = distribution of of assoc. partons when the trigger pTt is fixed


Or Bjorken “parent-child relation” and the “trigger

bias”Final state parton invar. Dist.

Mean z

Trigger biasSmall z fragments

strongly suppressed

Parent-child rel.Jet cross section hasThe “same” shape

asParticle cross section

J. D. Bjorken, PRD8 (1973) 4098

High- p T particle correlations in pp collisions at LHC/ALICE

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