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Centrality-dependent pt spectra of Direct photons at RHIC

Centrality-dependent pt spectra of Direct photons at RHIC. F.M. Liu 刘复明 Central China Normal University, China T. Hirano University of Tokyo, Japan K.Werner University of Nantes, France Y. Zhu 朱燕 Central China Normal University, China. liufm@iopp.ccnu.edu.cn. Outline.

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Centrality-dependent pt spectra of Direct photons at RHIC

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  1. Centrality-dependent pt spectra of Direct photons at RHIC F.M. Liu 刘复明Central China Normal University, China T. HiranoUniversity of Tokyo, Japan K.Werner University of Nantes, France Y. Zhu 朱燕Central China Normal University, China liufm@iopp.ccnu.edu.cn

  2. Outline • Motivations • Calculation approach • Results • Discussion and conclusion Hefei

  3. Motivations • Heavy ion collisions at various centralitis offer us various bulks of hot dense matter. • The interaction between jets ( hard partons) and the bulk has received notable interest, i.e. jet quenching is one of the most exciting observables at RHIC. • The interaction of partons inside the bulk and the properties of the bulk are of great interest, which may offer us some insight to quark confinement. Hefei

  4. direct photons, jets and plasma PRL94,232301(2005), PRL96,202301(2006) • Jet queching gives different effects to direct photons? 2. Thermal photons and JPC photons are penetrating probes for the interaction of partons inside the bulk and the interaction between jet and bulk. We can make cross check of the properties of the medium. Hefei

  5. Calculation approach • A realistic description of the space-time evolution of plasma • A good jet topology • A reasonable treatment of jet energy loss ( interaction between jet + plasma) • A careful consideration of all sources of direct photons A precise calculation requires Hefei

  6. Space-time evolution of Plasma Described with ideal hydrodynamics in full 3D space Constrained with PHOBOS data Tested with hadrons’ yields, spectra, v2 and particles correlation For more details, read T. Hirano Hefei

  7. Jet (hard parton) Topology MRST 2001 LO pDIS and EKS98 nuclear modification are employed Jet phase space distribution at τ=0: at τ>0: Hefei

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

  9. Fix parton energy loss parameter D • 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 Hefei

  10. Raa(pi0, %) at high pt gives D=0.5 A common D for various centralities should be required! Hefei

  11. Sources of direct photons • Leading Order contr. from primordial NN scatterings (does not depend on the plasma) • Thermal contribution Interactions of thermal partons are inside the rate! Hefei

  12. Sources of direct photons • Jet photon conversion • Fragmentation contribution: similar to pi0 production, modified fragmentation function is used. Ignored contributions: Induced radiation (higher order) radiation from pre-equilibrium phase (short time) Hefei

  13. Results Hefei

  14. Centrality dependent pt-spectra(1) PHENIX data: PRL 98, 012002 (2007) & arXiv:0801.4020 Precise data and predictions coincide with each other! Hefei

  15. Centrality dependent pt-spectra(2) PRL94,232302(2005) Hefei

  16. Pt spectrum from pp collisions PRL 98, 012002 (2007) Side proof for our results of LO + Frag_w/o_E loss in AA. The PHENIX fit of pp spectrum is used by us to calculated the nuclear modification factor of direct photons, Raa. Hefei

  17. Raa: scale, centrality & energy loss PRL94,232302(2005);J.Phys.G34, S1015-1018,2007 • Data is reproduced within theory uncertainty. • Almost independent of centrality for pt > 6 GeV/c, why? • Visible, but not sensitive to energy loss, why? Hefei

  18. Jet quenching & bulk volume • Parton energy loss does play a role in fragmentation contribution and JPC. • This is centrality-dependent, similar to the suppression to pi0 production. Hefei

  19. Competition btw different sources Raa is not sensitive to E loss at high pt, due to the dominance of LO. Raa is almost independent to centrality at high pt, due to the compensation btw the two sources, JPC & frag. when collisions move from central to peripheral. Hefei

  20. Information from Thermal photons Raa due to thermal source Energy density at plasma center • The slope of pt spectrum at low pt region (dominant by thermal photons) • can reflect the temperature (energy density, …) of the bulk. • The yields of photons (mainly low pt ) is roughly proportional to Ncoll., which • reflect the bulk volume. Hefei

  21. Discussion and Conclusion • Parton energy loss does make a visible but not so significant effect in Raa(γ) compared to Raa (π). D=0.5 is to understand. • The modification of Raa(γ) by jet quenching is at the same degree of that by theory scales, which is quite different from the case of Raa (π) . • Raa(γ) is weakly dependent on centrality for pt> 6GeV/c, due to 1) the dominance of leading order contribution 2) the compensation btw JPC and fragmentation contribution. • As a penetrate probe, thermal photons can reflect the properties of bulk, i.e., temperature and volume via the slope of pt spectrum and the yields. The interaction between thermal partons gives a cross check to the information offered by parton energy loss. Hefei

  22. Thank you! One more word: v2 of direct photons is expected to offer much more Information. Our results and experiment measurements are coming soon! Hefei

  23. Why RAA decreases at high pt? purely leading order calculation: Isosping mixture and nuclear shadowing make an evident decrease. Hefei

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