1. 28 April 2008MYX@Nanjing 0 Yaxian Mao, Daicui Zhou, Yves Schutz In ALICE Physics Workgroup: High p T and photons ( for ALICE collaboration -- Wuhan)

2. 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 (  E ≈ 20 GeV)  The jet cannot be reconstructed in the AA environment 28 April 2008MYX@Nanjing 1 Fragmentation  Jet Prompt  00

3. q q g g γ Photon sources  Direct photons (the signal)  Prompt pQCD photons (E  > 10 GeV)  g Compton scattering  qq annihilation  Fragmentation  Photons produced by the medium (E γ < 10 GeV)  Bremsstrahlung  Jet conversion  Thermal 28 April 2008MYX@Nanjing 2 q qg γ q q g γ q g q γ q qg γ

4. Photon sources  Decay photons (the background)  Hadrons, mainly π 0  But suppressed by the medium 28 April 2008MYX@Nanjing 3 EE Rate Hadron Decay QGP Thermal Jet conversion Bremsstrahlung pQCD prompt 10GeV

5. High p T 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 28 April 2008MYX@Nanjing 4

6. From RHIC to LHC  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 2008MYX@Nanjing 5

7. From RHIC to LHC  Cross section of hard probes increased by large factors  10 5 for very high p T jets  Differential measurements become possible  Jet fragmentation function  Photon tagging 28 April 2008MYX@Nanjing 6

8. 28 April 2008MYX@Nanjing 7  =100º  |  | < 0.12  E/E = 3%/  E PHOS TPC Charged hadrons: Photons:  =360º  |  | < 0.9  p/p < 5% at E < 100GeV  hh Measurement with ALICE@LHC

9. Strategy for a feasibility study  Identify prompt photons with ALICE PHOS detector (PID + Isolation Cut)  Construct  -charged hadrons correlation from 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 2008MYX@Nanjing 8

10. Monte Carlo Data production   +jet in final state   – jet @ √s = 14 TeV  Prompt  is the signal under study: 6×10 5 events (5 GeV < E  < 100 GeV)  2 jets in final state  jet – jet @ √s = 14TeV  These events constitute the background: high-p T  0 [O(  S )] and fragmentation [O(  2 S )]: 24×10 5 events (5 GeV < E jet < 200 GeV)  ALICE offline framework AliRoot  Generator: PYTHIA 6.214; PDF: CTEQ4L  Luminosity: L int = 10 pb -1  Acceptance: two PHOS modules  [-0.13, 0.13];  = [259, 301] 28 April 2008MYX@Nanjing 9

11. Generated prompt photon from -jet events  Cross section of generated photons from  -jet events in pp@14TeV  Dashed lines are the simulation bins 28 April 2008MYX@Nanjing 10  -jet in pp@14TeV

12. 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 28 April 2008MYX@Nanjing 11 jet-jet in pp@14TeV (π 0 → γ+γ)

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

14. Photon PID Efficiency  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 2008MYX@Nanjing 13 true 70% false 20%

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

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

18. Fragmentation function  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 = p T /E T jet  Requirement: reconstruction of jet energies 28 April 2008MYX@Nanjing 17 R =  (  -  0 ) 2 + (  -  0 ) 2 =1 (  0,  0 ) R IP Jet

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

20. -hadron correlation  Momentum imbalance variable  z  -h = -p T h · p T  / |p T  | 2  In leading-order kinematics (  s )  z  -h  p T h / p T   According momentum conservation,  p T  = k  = E parton  Therefore,  (exp.) z  -h  z (th.) 28 April 2008MYX@Nanjing 19

21. Kinematics conditions on CF  Photon and hadron momenta cuts must be very asymmetric: p T  cut >> p T h cut  Photon must be produced directly from the partonic process and not from a jet fragmentation: isolated and p T  > 20GeV/c  Photon – hadrons are back to back:  /2 <  < 3  /2 28 April 2008MYX@Nanjing 20

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 28 April 2008MYX@Nanjing 21

23. Summary  Tagging jets with direct prompt photons is the unique approach to identify low energy jets (E jet < 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 … 28 April 2008MYX@Nanjing 22

24. Thanks for your attention! 28 April 2008MYX@Nanjing 23

25. Back up

26. Hard Probes: an essential tool  Hard probes involve high momentum (p T ) or high mass transfer:  p T, M >>  QCD : pertubative regime of QCD thus calculable  1/(p T, M) <<  QGP formation : produced in early phase of the collision  p T, M >> T medium : they decouple from the medium  Observe how the medium (AA) modifies the hard probes as compared to its vacuum (pp) properties 28 April 2008MYX@Nanjing 25

27. Di-hadron correlation No back-to-back high p T correlation 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 2008MYX@Nanjing 26 Jet

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

29. Performance Momentum resolution in central barrel better than 4% Energy resolution in PHOS better than 1.5% for E > 10 GeV 28 April 2008MYX@Nanjing 28 PHOSCentral Barrel, pp

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