1. 1 Dihadron Tomography of High Energy AA Collisions in NLO pQCD Hanzhong Zhang Department of Physics, Shandong University Institute of Particle Physics, Central China Normal University Jinan, Jan. 9, 2008 Collaborators: Enke Wang Joseph F. Owens Xin-Nian Wang 1) Phys. Rev. Lett. 98(2007)212301 2) J. Phys. G. 34(2007)S801 3) To be submitted.

2. 2 Outline I.Introduction II.Modified fragmentation function model III.Numerical analysis on single hadron and dihadron production IV.Conclusions

3. 3 I. Introduction 1. What is “Dihadron tomography” ? 1) Medical x-ray tomography: see inside a “bone” by x-ray. 2) Jet tomography: see inside “QGP” by a parton jet, not only by single jet, but also by dijet. 3) Hadron/Dihadron tomography: we can’t “catch” a parton jet/dijet, but can “catch” a hadron/dihadron.

4. 4 2. How to know a tomography of QGP ? ---- Jet Quenching ! Jet quenching: Induced by multiple scattering in QGP medium, a parton jet will radiate gluon and lose its energy. hadrons q q leading particle leading particle N-N collision hadrons q q Leading particle suppressed leading particle suppressed A-A collision

5. 5 Jet quenching in 2→2 processes LO analysis of jet quenching in AA : 2→2 processes (tree level) A factor K=1.5-2 was put by hand to account for higher order corrections 3. Why NLO study? pQCD Parton Model

6. 6 Jet quenching in 2→3 processes 2→3 processes (tree level) NLO (Next to Leading Order ) corrections: One-loop corrections Jeff. Owens, PRD65(2002)034011; B.W. Harris and J. Owens, PRD65(2002)094032. K is absent NLO: More stronger quenching; More clearer QGP picture.

7. 7 4. Why dihadron tomography study? Single hadron suppression factor is found to be fragile to probe the dense matter. K. J. Eskola, et al, NPA747 (2005) 511-529 One of the motives of this work: How about dihadron? fragile or robust?

8. 8 Jet Quenching effect in AA is incorporated via a model of modified fragmentation functions: II. Modified fragmentation functions model (X. -N. Wang, PRC70(2004)031901) where Two contributions from jets in vacuum and medium! Jet energy loss

9. 9 the averaged scattering number, It determines the thickness of the outer corona where a parton jet survives in the overlapped region. the gluon density distribution,

10. 10 In 1-demension expanding medium, the total energy loss is written as a path integration: The energy loss per unit lenth with detailed balance: (Enke Wang and Xin-Nian Wang, PRL87(2001)142301) An energy loss parameter proportional to the initial gluon density

11. 11 In BDMPS calculation for the radiative parton energy loss, is equivalent to Baier, Dokshitzer, Mueller, Peigne, Schiff, NPB484(1997)265 where is a jet transport or energy loss parameter, reflects the ability of the medium to “quench” jets. By Wang2 and BDMPS formulas, estimate the average jet transport parameter by J. C. Solana and X. -N. Wang, hep-ph/0705.1352

12. 12 III. Numerical analysis on single hadron and dihadron production 1. Single hadron tomography 2. Dihadron tomography 3. Estimate jet transport parameter 4. Comparison between different shadowing 5. LHC predictions

13. 13 1. Single hadron tomography single inclusive or production

14. 14 The invariant p_T spectra of single hadron With, p_T spectra in AA is not also sensitive to the choice of Set, since p_T spectra in pp is not at all sensitive to the choice of

15. 15 Nuclear modification factor is 10% larger than is not sensitive to the initial gluon density

16. 16 Centrality dependence is not sensitive to, and

17. 17 Similar to the study by K. J. Eskola, H. Honkanen, C. A. Salgado, U. A. Wiedemann, NPA747 (2005) 511-529 is a fragile probe of dense matter. 1.68 loses its effectiveness as a good probe of dense matter Why the single hadron tomography is fragile to probe the dense matter? The bigger is, the flatter is.

18. 18 y x Single hadron Color strength = single hadron yield from partons in the square parton jet emission surface completely suppressed Single hadron is dominated by vertical surface emission corona thickness

19. 19 Is there a robust probe of the dense matter produced in AA collisions? Let’s see dihadron production! Trigger one hadron of a dihadron, check the other hadron --- the associated hadron 2. Dihadron tomography

20. 20 Fit dAu data by pp result to fix scales, Invariant mass No jet quenching in d+Au, The dihadron spectra in Au+Au collisions

21. 21 The dihadron suppression factor in Au+Au collisions

22. 22 If no jet quen- ching,

23. 23 Comparison between single hadron and dihadron tomography in Au+Au collisions dihadron single hadron

24. 24 1.68 Dihadron is a robust probe of dense matter. The curve is steeper than when

25. 25 comparison between single hadron and dihadron suppression factor for for single for dihadron

26. 26 Why does the dihadron behave more robust than single hadron to probe the dense matter? Single hadron is dominated by vertical surface emission dihadron ?

27. 27 partonic di-jet N S tangential y x triggered hadron associated hadron Color strength = dihadron yield from partons in the square Dihadron is from tangential surface emission + punch-through jets punch-through jets 25% left

28. 28 3. Estimate jet transport parameter

29. 29 4. Comparison between different shadowing p+Au@RHIC 200GeV

30. 30 Au+Au@RHIC 200GeV Single hadron and dihadron are all not sensitive to different shadowing at RHIC dihadron single hadron

31. 31 Trigger: 20GeV at LHC LHC 5. LHC predictions is estimated as 4.5-5. 5 GeV/fm at LHC Single hadron fragileDihadron robust

32. 32 There are much more punch-through jets in higher energy AA collisions, increases while decreases with collision energy. RHICLHC

33. 33 Different shadowing in p+Pb@LHC 5500GeV

34. 34 Different shadowing in Pb+Pb@LHC 5500GeV Single hadron not sensitive to different shadowing. Dihadron sensitive to different shadowing because of much more punch-through jets. dihadron single hadron

35. 35 2) Punch-through jets contributing to hadron spectra at LHC

36. 36 Why is only Dihadron Iaa at LHC sensitive to different shadowing parameterizations, HIJ, EKS, nDS, nPDF? 1) Punch-through jets are created from central system region; 2) Initial partons participating in strong interaction in central region should be associated with stronger shadowing effects than those initial partons in the outer layer of the system; 3) So punch-through jets manifest a strong shadowing effect. There are much more punch-through jet contributing to dihadron spectra at LHC than at RHIC. So does dihadron than single hadron. H. Zhang, J.F. Owens, E. Wang and X.-N. Wang, hep-ph/0000008

37. 37 Because of the stronger quenching effects, the single hadron is dominated by vertical surface emission; the dihadron is from tangential surface emission + punch- through jets. The dihadron is more sensitive to the initial gluon density than the single hadron. When becomes insensitive in higher energy A+A collision, is a sensitive probe of dense matter. -fit to both single and dihadron spectra can be achieved with a narrow range of the energy loss parameter at RHIC energy, it provide convincing evidence for the jet quenching description. 1) 2) 3) IV. Conclusions 4) Dihadron Iaa at LHC is found to be able to distinguish different shadowing parameterizations.

38. 38 Thank for your attention! 谢谢！

39. 39 Hard sphere model

40. 40 nuclear modification factor the formula of spectra in AA

41. 41 (Shi-Yuan Li and Xin-Nian Wang, PLB527(2002)85) (Enke Wang and Xin-Nian Wang, PRL87(2001)142301) (B. B. Back et al. [PHOBOS collaboration], PRC70(2004)021902)

42. 42 Nuclear shadowing effects only in small p T region So in large p T, medium effects only come from Jet Quenching !!!

43. 43 p-p data at 200GeV are used to fix scales, The invariant p_T spectra of single hadron

44. 44 Invariant mass: How to fix scales: If no medium effects, (X. –N. Wang, PLB 595(2004)165

45. 45 The dihadron azimuthal distributions

46. 46 The ratio between the yield/trigger in AA and in pp: If no jet quen- ching, PRL95(2005)152301 0.3

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