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Gamma- Hadron Correlation MeasureMENT WITH ALICE at LHC

Yaxian Mao, Daicui Zhou, Yves Schutz In ALICE Physics Workgroup: High p T and photons ( for ALICE collaboration -- Wuhan). Gamma- Hadron Correlation MeasureMENT WITH ALICE at LHC. Fragmentation g. p 0. Jet. Why -jet/hadron correlation ?.

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Gamma- Hadron Correlation MeasureMENT WITH ALICE at LHC

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  1. Yaxian Mao, Daicui Zhou, Yves Schutz In ALICE Physics Workgroup: High pT and photons ( for ALICE collaboration -- Wuhan) Gamma-Hadron Correlation MeasureMENT WITH ALICE at LHC MYX@Nanjing

  2. Fragmentation g p0 Jet Why -jet/hadron correlation? • The photon 4-momentum remains unchanged by the medium and sets the reference of the hard process • Balancing the jet and the photon provides a measurement of the medium modification experienced by the jet • Allows to measure jets in an energy domain (E < 50 GeV) where • The jet looses a large fraction of its energy (DE ≈ 20 GeV) • The jet cannot be reconstructed in the AA environment Promptg MYX@Nanjing

  3. Photon sources γ γ q q • Direct photons (the signal) • Prompt pQCD photons (Eg > 10 GeV) • g Compton scattering • qq annihilation • Fragmentation • Photons produced by the medium (Eγ < 10 GeV) • Bremsstrahlung • Jet conversion • Thermal g q g q q γ g q γ q g q q γ g g q MYX@Nanjing

  4. Photon sources Rate Hadron Decay • Decay photons (the background) • Hadrons, mainly π0 • But suppressed by the medium QGP Thermal Jet conversion Bremsstrahlung pQCD prompt 10GeV Eg MYX@Nanjing

  5. High pT hadrons suppression • Hard scattered partons interact with the color dense medium • The energy loss is imprinted in the fragmentation hadrons • The medium is transparent to photons MYX@Nanjing

  6. From RHIC to LHC MYX@Nanjing The medium is formed with energy densities larger by a factor 2-3: a different QGP ? The lifetime of the QGP is increased by a factor 2-3: more favorable for observation 28 April 2008 5

  7. From RHIC to LHC MYX@Nanjing • Cross section of hard probes increased by large factors • 105 for very high pT jets • Differential measurements become possible • Jet fragmentation function • Photon tagging 28 April 2008 6

  8. Measurement with ALICE@LHC h Charged hadrons: TPC D =360º || < 0.9 p/p < 5% at E < 100GeV Photons: PHOS g D =100º || < 0.12 E/E = 3%/E MYX@Nanjing

  9. Strategyfor a feasibility study MYX@Nanjing • Identify prompt photons with ALICE PHOS detector (PID + Isolation Cut) • Construct-charged hadrons correlationfrom detected events (detector response) • Compare the imbalance distribution(CF) and fragmentation function(FF) • Do the same study in -jet events(signal) and jet-jet events(background). • Estimate the contribution of hadrons from underlying events. • Start with pp, base line measurement for AA measurement 28 April 2008 8

  10. Monte Carlo Data production • +jet in final state  – jet @ √s = 14 TeV • Prompt  is the signal under study: 6×105 events (5 GeV < E  < 100 GeV) • 2 jets in final state  jet –jet @ √s = 14TeV • These events constitute the background: high-pTp0 [O(S)] and fragmentation [O(2S)]: 24×105events(5 GeV < E jet < 200 GeV) • ALICE offline framework AliRoot • Generator: PYTHIA 6.214; PDF: CTEQ4L • Luminosity: Lint = 10 pb -1 • Acceptance: two PHOS modules  [-0.13, 0.13];  = [259, 301] MYX@Nanjing

  11. -jet in pp@14TeV Generated prompt photon from -jet events • Cross section of generated photons from -jet events in pp@14TeV • Dashed lines are the simulation bins MYX@Nanjing

  12. jet-jet in pp@14TeV (π0→γ+γ) Generated decay photon from jet-jet events • Cross section of generated photons from jet-jet events in pp@14TeV • Dashed lines are the simulation bins MYX@Nanjing

  13. Photon identification (PID) MYX@Nanjing • We can discriminate , e and0from anything else : based on: • CPV : Charged particle identification • TOF : Identification of massive low pT particles • EMCA: Hadron rejection via shower topology (SSA) 28 April 2008 12

  14. Photon PID Efficiency true 70% MYX@Nanjing • discriminate  and 0 (SSA) 30 GeV < E < 100 GeV • High  identification efficiency, ~ 70%, • Misidentification efficiency decreasing from 70% to 20% • Not good enough 28 April 2008 13 false 20%

  15. Isolation cut (IC) IP TPC   R candidate PHOS MYX@Nanjing • Prompt g are likely to be produced isolated • Cone size • pT threshold candidate isolated if: • no particle in cone with pT> pTthres • pT sum in cone, ΣpT < ΣpTthres • pp collisions;R = 0.3, ΣpTthres = 2.0 GeV/c • Identification probability 98 % • Misidentification 3 % • Signal/Background >10 28 April 2008 14

  16. Isolation cut (IC) MYX@Nanjing • S/B: • ~ 0.07 at pT =20 GeV/c (generated events) • ~ 0.1 at pT =20 GeV/c (detected events) • > 10 at pT = 20 GeV/c (after IC selection) 28 April 2008 15

  17. Photon spectrum after one year data taking • Estimated counting statistics in one pp run for 2 PHOS modules • Systematic errors from misidentified 0 MYX@Nanjing

  18. R IP Jet Fragmentation function Jet • Construct jets within jet finder in PYTHIA; • Calculate fragmentation function of these jets: the distribution of charged hadrons as a function of the fraction of jet momentum z = pT/ETjet • Requirement: reconstruction of jet energies (0, 0) R = (-0)2 + (-0)2 =1 MYX@Nanjing

  19. Tagging jet with photon leading max min R EMCal TPC IP g PHOS MYX@Nanjing • Search identified prompt photon(PHOS)with largest pT(E g > 20 GeV) • Search leading particle: • -leading180º • Eleading> 0.1 E • Reconstruct the jet: • Particles around the leading inside a cone • A more simpler method is … 28 April 2008 18 18

  20. g-hadron correlation • Momentum imbalance variable • z-h= -pTh · pT / |pT|2 • In leading-order kinematics (s) • z-h pTh / pT • According momentum conservation, • pT = k = Eparton • Therefore, • (exp.) z-h z (th.) MYX@Nanjing

  21. Kinematics conditions on CF • Photon and hadron momenta cuts must be very asymmetric: pTcut >> pThcut • Photon must be produced directly from the partonic process and not from a jet fragmentation: isolated and pT> 20GeV/c • Photon – hadrons are back to back: /2 <  < 3/2 MYX@Nanjing

  22. Experimental imbalance distribution • Statistical errors correspond to one standard year of data taking with 2 PHOS modules. • Systematic errors is contributed by decay photon contamination and hadrons from underlying events. • Imbalance distribution is equivalent to fragmentation function for z = 0.1 – 0.8 MYX@Nanjing

  23. Summary • Tagging jets with direct prompt photons is the unique approach to identify low energy jets (Ejet < 50 GeV) in AA • The medium modification on jets is best measured in the jet fragmentation function • The fragmentation function can be measured in photon – charged hadrons correlations • The feasibility of such a measurement with the ALICE experiment has been evaluated in pp at 14 TeV • Next time , different kinematics cuts and AA … MYX@Nanjing

  24. Thanks for your attention! MYX@Nanjing

  25. Back up

  26. Hard Probes: an essential tool • Hard probes involve high momentum (pT) or high mass transfer: • pT, M >> QCD : pertubative regime of QCD thus calculable • 1/(pT, M) << t QGP formation: produced in early phase of the collision • pT, M >> T medium: they decouple from the medium • Observe how the medium (AA) modifies the hard probes as compared to its vacuum (pp) properties MYX@Nanjing

  27. Di-hadron correlation Jet Jet MYX@Nanjing • No back-to-back high pTcorrelation in central Au+Au collisions compared to dAu or pp collisions: the hard scattered parton looses energy via gluon radiation when traversing the color dense medium 28 April 2008 26

  28. Di-hadron correlation Jet Jet MYX@Nanjing • Back-to-back low pTcorrelation reflects the radiated energy through the fragmentation of low pT gluons 28 April 2008 27

  29. Performance Central Barrel, pp PHOS • Momentum resolution in central barrel better than 4% • Energy resolution in PHOS better than 1.5% for E > 10 GeV MYX@Nanjing

  30. Imbalance distributionfrom NLO (by F. Arleo) MYX@Nanjing Within higher kinematics condition, the medium effects can be measured by imbalance distribution (CF) instead of fragmentation function (FF). 28 April 2008 29

  31. Underlying events estimation • Based on: • Hadrons spatial distribution from underlying events (ue) is isotropic: ue (|-hadron |<0.5 ) ≅ ue (0.5 <|-hadron |<1.5 ) • Strategy: • Calculate ue contribution on the same side as photon where there is no jet contribution MYX@Nanjing

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