1. 1 A NLO Analysis on Fragility of Dihadron Tomography in High-Energy Nuclear Collisions Enke Wang Institute of Particle Physics, Central China Normal University Collaborator: Hanzhong Zhang (CCNU) Joseph. F. Owens (Florida) Xin-Nian Wang (LBNL) PRL 98 (2007) 212301 (nucl-th/0701045 )

2. 2 Outline Introduction Dihadron Production in NLO pQCD Nuclear Modification Factor of Hadron- trigged Fragmentation Function Summary

3. 3 Hard-scattering between partons in pp. Fragmentation of partons produce back-to-back jets of hadrons. Jets are clustered in angle and rich in high-pt particles. Jets produced in AA traverse and interact with the medium, lose energy and thus carry information of the medium. I. Introduction q q leading particle leading particle p-p collision q q Leading particle suppressed leading particle suppressed A-A collision Jet Quenching

4. 4 P T dependence of sensitive to energy dependence of dE/dx Nuclear Modification Factor: E. Wang, X.-N. Wang, Phys. Rev. C 64(2001) 034901 Shadowing Effect No Energy Loss Energy Loss Leading Particle Suppression No Medium Effect Medium Effect

5. 5 hadrons q q leading particle leading particle well calibrated: can be calculated by pQCD. q q hadrons Au+Au AA over binary-scaled pp p T trig >4 GeV/c, 2<p T assoc <4 GeV/c away-side particles suppressed at high pT leading particle suppressed Experimental Observation of Jet Quenching

6. 6 Monojet quenching? Surface Emission In surface emission R AA depend on the thickness of the outer corona which varies very slowly with the initial gluon density Motivation: R AA from single hadron spectra is insensitive to the initial gluon density What is the sensitive probe to initial gluon density ? outer corona

7. 7 II. Dihadron Production in NLO pQCD

8. 8 Average parton energy loss in medium at formation time: Energy loss parameter proportional to the initial gluon density Parameterization of Energy Loss Enke Wang and Xin-Nian Wang, PRL87(2001)142301

9. 9 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

10. 10 Jet quenching in 2→3 processes 2→3 processes (tree level) NLO (Next to Leading Order ) corrections: One-loop corrections J. Owens, PRD65 (2002) 034011; B.W. Harris and J. Owens, PRD65 (2002) 094032

11. 11 p-p data at 200GeV are used to fix scales, Single hadron production in p+p

12. 12 Jet Quenching effects lead to the modification of FF Modified Fragmentation Function in A+A KKP (X. -N. Wang, PRC70(2004)031901) averaged scattering number (opacity)

13. 13 Single hadron production in Au+Au Single hadron spectra and R AA at different (initial gluon density)

14. 14 Fit dAu data by using pp result to fix scales: Invariant mass: d+Au (no jet quenching): Dihadron Production in Au+Au Hadron-triggered fragmentation function

15. 15 If no jet quen- ching, Centrality Dependence of D AA

16. 16 III. Nuclear Modification Factor of Hadron-trigged Fragmentation Function

17. 17 y x Single hadron Color strength = single hadron yield from partons in the square parton jet emission surface completely suppressed Surface Emission of Single Hadron Production corona thickness

18. 18 partonic di-jet tangential y x triggered hadron associated hadron Color strength = dihadron yield from partons in the square punch-through jets 25% left Surface Emission + Punch-through jet in Dihadron Production

19. 19 Nuclear Modification Factor of Hadron-trigged Fragmentation Function

20. 20 If no jet quenching PRL95(2005)152301 Centrality Dependence of I AA

21. 21 Sensitivity of I AA to Initial Gluon Density reach minima, It provide convincing evidence for jet quenching description At LHC energy dihadron spectra have more contribution from dijet with higher initial energy, they are less suppressed than at RHIC energy, R AA become very flat (insensitive) with initial gluon density.

22. 22 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) VI. Summary

23. 23 Thank you for your attention!

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