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

High- p T particle correlations in pp collisions at LHC/ALICE. Yaxian Mao (for the ALICE collaboration)   IOPP-Wuhan,China & LPSC-Grenoble, France. 1. Objective.

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

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  1. High-pT particle correlations in pp collisions at LHC/ALICE • Yaxian Mao • (for the ALICE collaboration) •   IOPP-Wuhan,China & LPSC-Grenoble, France 1

  2. Objective • Understand the jet properties in pp to exploit jets as probes of the QCD medium in AA. • Methods: • Full Jet reconstruction • Two-particle correlations: • Leading Particle (LP) approximates jet direction 2

  3. xE kinematics associate • Charged/neutral trigger: • Isolated trigger: • Direct photon trigger : trigger • di-jet balance (no kT) 3

  4. Leading Particle Correlation • Leading particle trigger selection • Azimuthal correlation between trigger and associate particles Near side Away side • Away side associate particle xE distribution 4

  5. xE distribution (S+B) Near side Away side • Background estimate for away side xE: • Assumption: isotropic in Δϕ • Estimate from • Normalize to the away side hemisphere 5

  6. 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. ) h PHENIX, PRD 74 (2006) 072002 6

  7. pT dependence xE slope DELPHI, EPJC, 13 (2000) 573 OPAL, ZPC, 69 (1996) 543 J. D. Bjorken, PRD8 (1973) 4098 • data exhibits uniform xE slope over a wide pTt range • away side xE distribution different from real jet FF ( ) • how to improve ? → next slide… 7

  8. IP   R trigger Isolation Thursday Poster: N. Arbor Enrich the trigger sample with <zt> → 1 • estimate hadronic (charged only) activity around the trigger • R (= 0.4) of the cone • ε(= 10%) fraction of trigger pT • trigger (pTt) is isolated if ∑cone pTh± < ε* pTt Isolation < zt > ~ 0.5 < ztisolated> ~ 0.8 8

  9. ΔΦ correlation Isolated charged Charged hadron trigger: Associate charged hadrons: Isolation 9

  10. xEdistribution: w/ (w/o) isolation • Isolated trigger has steeper slope thannon-isolated trigger 10

  11. xE slope for isolated trigger • Exponential fit in the range • →xE slope higher than the slope of non-isolated trigger 11

  12. Conclusion and Outlook Two 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 • a description closer to the fragmentation function • the potential to distinguish quark and gluon jet fragmentation • Isolated direct photons (dominated by quark jets from “Compton scattering”) will allow to achieve 〈zt〉= 1  preview in the next few slides … 12

  13. 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

  14. ΔΦ correlation Cluster trigger pp@√s=7 TeV cluster trigger: pTt ∈ [4, 5],[5,6], [6,7],[7,8] GeV/c associate charged hadrons: pTa > 1 GeV/c • Azimuthal correlation independent on the calorimeters 14

  15. Inverse xE slope • cluster trigger (mainly π0) and charged trigger has similar slope on xE distribution 15

  16. Thanks very much!

  17. 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) 4/10

  18. 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) 10/10

  19. Dihadron ⟨zt⟩≠1; xE ≠ FF PHENIX, Phys. Rev D 74, 072002 (2006) CCOR, Physica Scripta 19, 116 (1979) 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. xE ~ za/<zt> <zt>=0.85 measured ⇒Dqπ(z)~e-6za e-5.3xE

  20. Photon Tagged Jets: xE ~ FF X.N. Wang, Z. Huang PRC55 (1997) 3047-3061 γ energy ≈ jet energy Also can measure IAA, ratio of the direct photon-hadron yields

  21. Outline • Objective • 2→2 kinematics (LO) • Non-isolated trigger hadron correlation • Isolated trigger-hadron correlation • neutral cluster-hadron correlation • Conclusion

  22. ΔΦ correlation Isolated cluster neutral cluster trigger: Associate charged hadrons: Isolation 22

  23. 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

  24. Or Bjorken “parent-child relation” and the “trigger bias” Final state parton invar. Dist. Trigger bias Small z fragments strongly suppressed Mean z Parent-child rel. Jet cross section has The “same” shape as Particle cross section J. D. Bjorken, PRD8 (1973) 4098

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