Centrality-dependence of direct photon production from Au+Au Collisions

1. Centrality-dependence of direct photon production from Au+Au Collisions F.M. Liu Central China Normal University, China T. HiranoUniversity of Tokyo, Japan K. Werner University of Nantes, France Y. Zhu Central China Normal University, China QM2009 Knoxville, March 30 - April 4, 2009

2. Outline • Motivations • Calculation approach • Results • Conclusion

3. Motivations • The properties of the hot dense matter created in heavy ion collision are of great interest, especially the critical behaviors. • As penetrating probes, direct photons can provide the inner information of the hot dense matter. • Theoretically, photon production can easily be studied both macroscopically and microscopically, which may be helpful for the study of hadron production. • What can we learn from direct photon observables? How is this signal related to hadronic signal?

4. Calculation approach • The space-time evolution of the created hot dense matter distributions of thermal partons and hadrons • The propagation of jets in plasma  distribution of hard partons • All sources of direct photons A precise calculation requires careful treatments on

5. Parameters are constrained with PHOBOS data Tested with hadrons’ yields, spectra, v2 and particles correlation For more details, read T. Hirano the evolution of the matter 3D ideal hydrodynamic equation Initial condition: Glauber model, EoS: QGP phase: 3 flavor free Q & G gas HG phase: hadronic gas PCE Described with

6. Distribution of hard partons MRST 2001 LO pDIS and EKS98 nuclear modification are employed Jet phase space distribution at τ=0: at τ>0:

7. Parton Energy Loss in a Plasma • Energy loss of parton i=q, g, • Energy loss per unit distance, i,e, with BDMPS D: free parameter • Every factor depends on the location of jet in plasma , i.e., fQGP: fraction of QGP at a given point

8. Fix D with pi0 suppression A common D=1.5 for various Centralities!

9. Sources of direct photons • Leading Order contr. from primordial NN scatterings • Thermal contribution Interactions of thermal partons are inside the rate! Coupling depends on temperature

10. Sources of direct photons • Jet photon conversion • Fragmentation contribution: Sources not included: Medium induced Bremsstrahlung Radiation from pre-equilibrium phase

11. Results • pt-spectra • Elliptic flow (at low pt)

12. pt-spectra The measured pt spectrum is reproduced with 4 sources. Jet quenching plays a role but not so evident here.

13. Direct photons are not suppressed? Reason: Due to the dominance of leading order contribution. Consequence: The high pt elliptic flow might be too small to be visible.

14. jet quenching effect at high pt • Jet quenching treatment is very important in fragmentation contribution • and jet photon-conversion contribution. • If one can separate the different sources via particle correlations, • then high pt suppression and v2 caused by jet quenching and by • geometry may be observed!

15. V2 of thermal photons • In the local rest frame, photons are emitted from the thermal bath isotropically. • Thermal photons’ v2 is caused by the Lorentz boost and accumulated with the space-time integration. • Both the strength and the asymmetry of the transverse flow are important. dominant source at low pt.

16. Time evolution of the transverse flow Energy-weighted Space-averaged Transverse flow gets stronger with time. The asymmetry increases with eccentricity.

17. thermal photons v2 time evolution Elliptic flow of thermal photons increases with time. Fraction emitted at earlier time Increases with pt. Elliptic flow of thermal photons decreases at high pt due to the abundant emission at early time.

18. V2(pt) at different centrality Elliptic flow of thermal photons does decrease at high pt.

19. Centrality dependence of pt-int. v2 Maximum pt-int. v2 appears at 40-50% centrality, due to the interplay between the strength and asymmetry of the transverse flow. This centrality dependence is similar to the measured hadronic v2. The measurement of elliptic flow of thermal photons( direct photons) is really needed to test models!

20. QGP phase and HG phase V2 from hadronic phase is much bigger than from QGP phase. V2 can carry different information than pt spectrum.

21. Dependence of EoS? Various input of EoS Elliptic flow is more sensitive to EoS than pt spectrum!

22. Conclusion • Ideal hydro model can reproduce the measured pt spectra of direct photons at different centrality with the four sources we considered. • Jet quenching plays a role in direct photon production. • pt-int. v2 reaches maximum at 40-50% centrality, due to the interplay between the strength and asymmetry of the transverse flow. Does this interplay play a role in hadronic elliptic flow? How? • Thermal photon V2 is more sensitive to EoS than pt spectra! • Measurement is needed to test the model.

23. Thank you!

24. RAA suppression from initial effect The dominant contribution at high pt is the LO contribution from NN collisions: Isosping mixture and nuclear shadowing: The isospin mixture and nuclear shadowing reduce Raa at high pt. This is the initial effect, not related to QGP formation.

25. Fix D with pi0 suppression • From pp collisions: • From AA collisions, parton energy loss is considered via modified fragmentation function Factorization scale and renormalization scale to be X.N.Wang’s formula

26. Why jets lose energy

27. Pt spectrum from pp collisions PRL 98, 012002 (2007) A good test for contributions from leading order + fragmentation without Eloss in AA collisions.

28. Thermal fraction