1 / 31

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 ?.

metea
Download Presentation

Gamma- Hadron Correlation MeasureMENT WITH ALICE at LHC

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  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

More Related