Gamma-jet tomography of high-energy nuclear collisions in NLO pQCD

1. Gamma-jet tomographyof high-energy nuclear collisions in NLO pQCD Han-Zhong Zhang Institute of Particle Physics, Huazhong Normal University, China Collaborators: Enke Wang, J. Owens and X.-N. Wang Weihai, 08-14 August, 2009

2. OUTLINE • Jet Quenching • Gamma-jet in NLO pQCD • Same (energy loss) formalism for calculating gam-hadr, single/dihadron • Gamma-jet tomography: volume vs surface emission • Conclusions H. Z. Zhang, J. Owens, E. Wang and X. –N. Wang, Phys. Rev. Lett, 103(2009)032302; 98(2007)212301

3. leading particle hadrons q q hadrons leading particle Leading particle suppressed hadrons q q hadrons N-N collision A-A collision leading particle suppressed A powerful tool for the study of Quark Matter Jet quenching: X.-N.Wang and M.Gyulassy, Phys.Rev.Lett.68,1480(1992) The hard jet loses a significant amount of its energy via radiating gluon induced by multiple scattering.

4. Three kinds of hard probes of Quark Matter • Single jet  Single hadron spectra • Dijet  Hadron-triggered away-side hadron spectra • Gamma-jet  Photon-triggered away-side hadron spectra Experimental studies of jet quenching, suppression observed Single hadron Dihadron Gamma-hadron STAR Pre.

5. Single and di-jet tomography H. Z. Zhang, J. Owens, E. Wang and X. –N. Wang, PRL98(2007)212301 Dihadron Single Hadron

6. Gamma-jet probe PQCD parton model: LO (tree level) NLO corrections: (e.g. 23)

7. Try to eliminate fragmentation photon An “isolation” cut (IC) is often applied on the electromagnetic signal to separate the direct photons from other sources. Gamma Most accompanying hadrons are within a cone of angle radius H. Baer, J. Ohnemus, and J. F. Owens, Phys. Rev. D. 42, 61(1990) Jet

8. Direct vs. fragmentation photons Data : PHENIX, PRL 98 (2007) 012002 Inclusive photons “the measured photon samples … are expected to be isolated from parton jet activity.” With IC, the fragm. only 10%. we will focus mainly on photons with isolation cuts

9. The effective fragmentation functions in medium in vacuum (X. -N. Wang , PRC70(2004)031901) The jet energy loss in a 1D expanding system: Energy loss parameter (a parameterization form of theory calculations)Enke Wang , X. -N. Wang , PRL87(2001)142301)

10. Track any jet inside the medium Jet energy loss is dependent on its transverse momentum and initial space origination, a function of the azimuthal angle, the passing distance, the medium particle density along the trajectory

11. determined in a same energy loss formalism in most central Au+Au single hadron dihadron Actually we will choose epsilon_0 1.68GeV/fm for gam-hadr

12. t(r) r/fm Hard sphere versus Woods-Saxon A hard sphere overlap geometry differs at most about 10% from a Wood-Saxon geometry.

13. Calculate gamma-hadron in the same formalism as for single/dihadron in HIC • Same model: NLO pQCD parton model • Same geometry: hard sphere • Same PDFs in vacuum: CTEQ6M • Same FFs in vacuum: KKP • Same energy loss parameterization • and =1.68GeV/fm

14. Gamma-triggered hadron spectra The per-trigger photon-hadron spectra D_AA is a sum of FF’s of the away-side jets (quark and gluon) weighted with the fractional gamma-jet cross sections. Sometimes we call D_AA as the photon-triggered fragmentation functions.

15. H.Z. Zhang, J.F. Owens, E. Wang and X.-N. Wang , Phys. Rev. Lett, 103(2009)032302 • Fit data very well in p+p for different values of p_T^trig. • Fit data well in Au+Au. Agreement is not nontrivial. It reinforces the success of the parton energy loss picture. • Hadron-triggered FF’s > Photon-triggered FF’s.

16. Mainly because the fraction of hadron-triggered gluon jets is larger than the fraction of photon-triggered gluon jets at same pt^trig, And the hadron yield of gluon jets is larger than that of quarks. 50% 10%

17. More stronger dependence of on Quantify suppressions of hadron- and gamma-triggered FF’s in A+A relative to p+p collisions due to JQ • In LO energy balance limits for gam-hadr • NLO radiative correction permits for G-hadr. The two effects cause NLO very different from LO . NLO needed • Compared with h-h , Gam-hadr has more dependence of on .

18. Large : more susceptible to Eloss --- Surface emission • Even a small amount energy loss can greatly suppress the large- gam-hadr yield. • Only those jets originating near and escaping through the surface will contribute without energy loss. • Large- is mainly determined by the thickness of the corona of the surface emission.

19. Small : encounter more Eloss --- Volume emission • For contributing to small- gam-hadr, high energy jet is “wealthy” enough to lose finite energy, and can therefore originate near the center region, --- volume emission. • Intermediate- gam-hadr are determined by the competition of the two emission mechanism.

20. Different centrality dependence of the suppression for different values of z_T • z_T < 0.6, volume emissions dominate, weaker dependence. • z_T > 1, surface emissions dominate, stronger dependence

21. Conclusions • Gamma-hadron correlation described well by NLO pQCD • Gamma-hadron suppression consistent with single hadron and dihadron suppression • Volume & surface emission in different region of kinematic (small and large z_T) • Gamma-jet study toward true tomography of dense matter Thank for your attention!

22. Gamma-jet by NLO pQCD parton model J. F. Owens, Rev. Mod. Phys. 59, 465(1987) LO pQCD NLO pQCD