-hadron correlation measurement to probe fragmentation function with in ALICE @ LHC

1. -hadron correlation measurementto probe fragmentation function within ALICE@LHC Presented by: Renzhuo Wan Institute of Particle Physics Central China Normal University, Wuhan, China and Yaxian Mao, Gustavo Conesa, Yves Schutz,Chuncheng Xu, Zhongbao Yin and Daicui Zhou For the ALICE Collaboration and CCNU group The international (7th national) workshop for QCD phase transition and heavy ion collisions July 10~13, 2008, Hefei, Anhui

2. Outline • Motivation • ALICE experiment • γ-hadroncorrelations measurement • Summary

3. Motivation The medium is almost transparent to photons Balancing the jet and the photon provides a measurement of the medium modification experienced by the jet Study of fragmentation function

4. Hard scattered partons interact with the color dense medium • Probe the energy loss • From pp 900GeV, 10TeV and 14TeV to PbPb 5.5TeV

5. LHC/ALICE experiment • Explore the properties of nuclear matter at extreme high energy density • Explore the dynamics of the transition from deconfined matter toward hadronic matter Photons: PHOS  =100º || < 0.12 E/E = 3%/E Charged hadrons: ITS, TPC, TRD, TOF  =360º || < 0.9 p/p < 5% at pT < 100GeV

6. Photon sources γ q q g γ γ q q g q g q q γ g q q g γ q g • Decay photons • Hadrons, mainly π0, ~90% q γ γ q g q g q • Direct photons －Prompt pQCD photons (Eg > 20 GeV) • g compton scattering • qqbar annihilation • Fragmentation － Photons produced by the medium (Eγ < 10 GeV) • Bremsstrahlung • Jet conversion • Thermal

7. Strategy • Identify prompt photons with ALICE PHOS detector (PID + Isolation Cut) • Construct-charged hadrons correlationfrom detected events (detector response) • Compare the imbalance distribution(CF) to thefragmentation 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 in AA

8. MC 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: 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]

9. Photon identification (PID) • We can discriminate , e and0from anything else, based on: • CPV : Charged particle identification • TOF : Identification of massive low pT particles • PHOS: Hadron rejection via shower shape analysis • Shower from: • single photon/e : e1/e2 = 1 • p0 (pp0 > 30 GeV/c) : e1/e2 > 1

10. Isolation Cut (1) IP TPC   R candidate PHOS • Prompt g are produced isolated • Cone size • pT threshold candidate isolated if: • no particle in cone with pT > pT thres • pT sum in cone, ΣpT < ΣpTthres • pp collisions; R = 0.3, ΣpTthres = 2.0 GeV/c • Identification probability 98 % • Misidentification 3 %

11. Isolation Cut (2) ×10 ×3 • S/B: • ~ 0.1 at pT =25 GeV/c (generated events) • ~ 0.3 at pT =25 GeV/c (reconstructed events + SSA) • > 10 at pT = 25 GeV/c (after IC selection)

12. Fragmentation Function R IP (0, 0) • 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 • Possible in pp • Difficult in AA

13. γ-hadron correlation pTh pTg • Momentum imbalance variable • z-h= -pTh · pT / |pT|2 • In leading-order kinematics (s) • z-h pTh / pT • According to momentum conservation, • pT = k = Eparton • So, • (exp.) z-h z (th.)

14. Kinematics condition 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

15. Azimuthal correlation for γ-jet

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

17. Underlying Event (UE) 99% For pt h > 2GeV/c, contribution from UE represents less than 15% • Hadrons spatial distribution from underlying events (UE) is isotropic: UE (|-hadron |<0.5 ) ≅ UE (0.5 <|-hadron |<1.5 ) • Calculate UE contribution on the same side as photon where there is no jet contribution

18. Comparison CF with FF pT cut Fragmentation & decay • Imbalance distribution is equivalent to fragmentation function for z = 0.12 – 0.8.

19. 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.

20. Summary The modification is best measured in the jet fragmentation function A valuable approach to access the medium properties Currently the only way to identify low energy jets (< 50 GeV) in AA Verified the feasibility of such a measurement with the ALICE experiment in pp at 14 TeV

21. Thank you !