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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

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9-12/7/2008Hefei2 Outline Motivations Calculation approach Results Discussion and conclusion

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9-12/7/2008Hefei3 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.

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9-12/7/2008Hefei4 direct photons, jets and plasma PRL94,232301(2005), PRL96,202301(2006) 1.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.

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9-12/7/2008Hefei5 Calculation approach A precise calculation requires 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

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9-12/7/2008Hefei6 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

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9-12/7/2008Hefei7 Jet (hard parton) Topology MRST 2001 LO pDIS and EKS98 nuclear modification are employed Jet phase space distribution at τ=0: at τ>0:

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9-12/7/2008Hefei8 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., f QGP : fraction of QGP at a given point

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9-12/7/2008Hefei9 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

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9-12/7/2008Hefei10 Raa(pi0, %) at high pt gives D=0.5 A common D for various centralities should be required!

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9-12/7/2008Hefei11 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!

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9-12/7/2008Hefei12 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)

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9-12/7/2008Hefei13 Results

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9-12/7/2008Hefei14 Centrality dependent pt-spectra(1) PHENIX data: PRL 98, 012002 (2007) & a rXiv:0801.4020 Precise data and predictions coincide with each other!

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9-12/7/2008Hefei15 Centrality dependent pt-spectra(2) PRL94,232302(2005)

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9-12/7/2008Hefei16 Pt spectrum from pp collisions The PHENIX fit of pp spectrum is used by us to calculated the nuclear modification factor of direct photons, Raa. PRL 98, 012002 (2007) Side proof for our results of LO + Frag_w/o_E loss in AA.

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9-12/7/2008Hefei17 Raa: scale, centrality & energy loss Data is reproduced within theory uncertainty. Almost independent of centrality for pt > 6 GeV/c, why? Visible, but not sensitive to energy loss, why? PRL94,232302(2005);J.Phys.G34, S1015-1018,2007

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9-12/7/2008Hefei18 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.

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9-12/7/2008Hefei19 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.

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9-12/7/2008Hefei20 Information from Thermal photons 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 N coll., which reflect the bulk volume. Raa due to thermal source

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9-12/7/2008Hefei21 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.

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9-12/7/2008Hefei22 Thank you! One more word: v2 of direct photons is expected to offer much more Information. Our results and experiment measurements are coming soon!

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9-12/7/2008Hefei23 purely leading order calculation: Isosping mixture and nuclear shadowing make an evident decrease. Why R AA decreases at high pt?

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