Jet quenching and direct photon production F.M. Liu 刘复明 Central China Normal University, China T. Hirano 平野哲文 University of Tokyo, Japan K.Werner University of Nantes, France Y. Zhu 朱燕 Central China Normal University, China Mainly based on arXiv: hep-ph/ v2 ATHIC2008 Tsukuba Oct 13-15

2 Outline Motivations Calculation approach Results Conclusion

3 Motivations Heavy ion collisions at various centralities 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.

4 direct photons, jets and plasma PRL94,232301(2005), PRL96,202301(2006) 1.Jet queching gives different effects to direct photons? 2. Direct photons (thermal, jet-photon conversion) 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.

5 Calculation approach A precise calculation requires careful treatments on The space-time evolution of the created hot dense matter The propagation of jets in plasma ( interaction between jet + plasma) All sources of direct photons

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

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

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

9 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

10 Raa(pi0, %) at high pt gives D=1.5 A common D for various Centralities!

11 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

12 Sources of direct photons Jet photon conversion Fragmentation contribution: similar to pi0 production Ignored contributions: Medium induced radiation (mainly at low pt ) radiation from pre-equilibrium phase (short time)

13 Results

14 Centrality dependent pt-spectra(1) PHENIX data: PRL 98, (2007) & a rXiv: Our predictions coincide with the precise measurement!

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

16 Pt spectrum from pp collisions The PHENIX fit of pp spectrum is used for Raa of thermal photons. PRL 98, (2007) A good test for contributions from leading order + fragmentation without Eloss in AA collisions.

17 Raa: energy loss Data is reproduced within theory uncertainty. E loss makes about 40% decrease of total photon production Centrality independent ? central and peripheral results differ by less than 5% with Eloss by about 20% at intermediate pt w/o Eloss PRL94,232302(2005);J.Phys.G34, S ,2007

18 Centrality-dependent suppression E loss does play a important role in fragmentation contribution and jet photon-conversion contribution. This is centrality-dependent, similar to the suppression to pi0 production.

19 Competition btw different sources Thermal and LO dominate low and high pt region respectively. Raa is not sensitive to E loss, because of the centrality dependence of them. When collisions move to perpherial, JPC becomes less important while fragmentation becomes more important.

20 Information from Thermal photons Energy density at plasma center High Temp. from fitting pt spectrum  A higher Temp. plasma More yields (shines) of thermal photons  A bigger-size (longer-life) plasma. Raa due to thermal source

21 V2 of thermal photons Contrary to hadronic v2 (ideal hydro predicted increase monotonically), the elliptic flow of thermal photons decrease at high pt! ( Information for the earlier evolution of the plasma?)

22 Time evolution At initial time there is no transverse flow, so v2 vanishes. A big fraction of energetic thermal photons are emitted at early time: More than 50\% at pt=3GeV/c and more than 70\% at pt=4GeV/c within the first 0.3fm/c, though the whole evolution time is about 20fm/c.

23 Discussion and Conclusion Parton energy loss does make 40% decrease of Raa(γ) Raa(γ) is independent of centrality (within 5% accuracy) because of 1) the dominance of leading order contribution 2) strong suppression to JPC and frag. contributions due to E-loss Thermal photons can provide information of the temperature and size of the plasma via the slope of pt spectrum and the yields. The elliptic flow of thermal photons is predicted to first increase and then decrease with pt, contrary to hadronic v2, which does not carry the early information of the QGP.

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

25 Thank you!

26 Thermal fraction