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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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Yaxian.Mao@QM2011, 24/05/2011, Annecy
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
Yaxian.Mao@QM2011, 24/05/2011, Annecy
xE kinematics
o Charged/neutral trigger:
o Isolated trigger:
o Direct photon trigger :
trigger
associate
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• di-jet balance (no
kT)
•
Yaxian.Mao@QM2011, 24/05/2011, Annecy
• Leading particle trigger selection
• Azimuthal correlation between trigger and associate particles
Leading Particle Correlation
• Away side associate particle xE distribution
Near side
Away side
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Yaxian.Mao@QM2011, 24/05/2011, Annecy
xE distribution (S+B)
Background estimate for away
side xE:
• Assumption: isotropic in Δϕ
• Estimate from
• Normalize to the away side
hemisphere
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Near side
Away side
Yaxian.Mao@QM2011, 24/05/2011, Annecy
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. )
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PHENIX, PRD 74 (2006) 072002
Yaxian.Mao@QM2011, 24/05/2011, Annecy
pT dependence xE slope
• data exhibits uniform xE slope over a wide pTt range
DELPHI, EPJC, 13 (2000) 573
OPAL, ZPC, 69 (1996) 543
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• away side xE distribution different from real jet FF ( )• how to improve ? → next slide…
J. D. Bjorken, PRD8 (1973) 4098
Yaxian.Mao@QM2011, 24/05/2011, Annecy
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
Yaxian.Mao@QM2011, 24/05/2011, Annecy
ΔΦ correlation
Isolated charged
Charged hadron trigger:
Associate charged hadrons:
Isolation
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Yaxian.Mao@QM2011, 24/05/2011, Annecy
xE distribution: w/ (w/o) isolation
• Isolated trigger has steeper slope than non-isolated trigger
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Yaxian.Mao@QM2011, 24/05/2011, Annecy
xE slope for isolated trigger
• Exponential fit in the range → xE slope higher than the slope of non-isolated
trigger11
Yaxian.Mao@QM2011, 24/05/2011, Annecy
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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Yaxian.Mao@QM2011, 24/05/2011, Annecy
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
Yaxian.Mao@QM2011, 24/05/2011, Annecy
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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Yaxian.Mao@QM2011, 24/05/2011, Annecy
Inverse xE slope
• cluster trigger (mainly π0) and charged trigger has similar slope on xE distribution
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Yaxian.Mao@QM2011, 24/05/2011, Annecy
Thanks very much!
Yaxian.Mao@QM2011, 24/05/2011, Annecy
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
Yaxian.Mao@QM2011, 24/05/2011, Annecy
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
Yaxian.Mao@QM2011, 24/05/2011, Annecy
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
Yaxian.Mao@QM2011, 24/05/2011, Annecy
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
Yaxian.Mao@QM2011, 24/05/2011, Annecy
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
Yaxian.Mao@QM2011, 24/05/2011, Annecy
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