Direct photon production in pp and AA collisions 合肥, Dec 5 - 7, 2009 刘复明 华中师范大学粒子物理研究所 FML, T.Hirano, K.Werner, Y. Zhu, Phys.Rev.C79:014905,2009. FML, T.Hirano, K.Werner, Y. Zhu, Phys.Rev.C80:034905,2009.

2 Direct photons and hadrons PRL94,232301(2005)

3 Outline Sources of direct photons Results Conclusion

4 Sources of direct photons Direct photons: photons produced in all processes except decay. In pp collisions, there are two main sources: 2. Fragmentation contribution: High order contribution 1. Leading Order contribution

5 Sources of direct photons In AA collisions, the two sources are kept, but Initial nuclear effect Jets lose energy in plasma 1.Leading Order contribution 2. Fragmentation contribution:

6 Sources of direct photons In AA collisions, additional sources appear due to plasma formation: 3. Thermal contribution 4. Jet-photon conversion More additional sources : Medium induced Bremsstrahlung Radiation from pre-equilibrium phase

7 the evolution of plasma Initial condition: thermalized QCD matter at rest at Evolution: 3D ideal hydrodynamic equation described with 3+1D ideal hydrodynamics EoS: 1st order phase transition at QGP phase: 3 flavor free Q & G gas HG phase: hadronic gas PCE Freeze-out:

8 Initial conditio based on Glauber model Parameterized rapidity distribution in pp collisions Energy density or entropy distribution in the space:

9 initial condition based on EPOS Strings are constructed randomly in NN collisions. String segments overlap in the space and form the core and corona region.

10 Energy Density at Initial Stage Uniform distribution is assumed along longitudinal direction in 2+1D hydrodynamics Parameters constrained with PHOBOS data Both are tested with hadrons’ yields, spectra, v2 and particles correlation

11 Fix jet energy loss with pi0 suppression f QGP : QGP fraction : E-loss per unit distance in BDMPS formulism A common D=1.5 for various Centralities! D

12 Results: Pt spectra of direct photons Direct photon production from AuAu collisions at top RHIC energy is well explained in a large pt range at all centralities.

13 Results: R AA of direct photons Due to the LO source, the effect of jet quench is much less evident in direct photon production than in hadronic production.

14 pt dependence of thermal photon v2 Elliptic flow of thermal photons decreases at high pt due to the abundant emission at early time.

15 Centrality dependence of pt-int. v2 The strength and asymmetry of transverse flow  Maximum appears at 40-50% centrality. pt-integrated v2 is mainly determined by thermal contribution.

16 Rapidity dependence The rapidity distribution can not identify different initial conditions; But the elliptic flow of thermal photons “remembers” the initial conditions.

17 Direct photons from PP  pQCD works so well! Two sources: 1. LO order contribution 2. Hard parton fragmentation

18 RHIC LHC Multiple (hard) scattering becomes very important. For some triggered events, very high energy density may be reached from string overlapping. P+P at 7,10, 14, 5.5 TeV From end of 2009 to end of 2010 ALICE PHOS Pt range 100Mev ~100GeV

19 Plasma evolution in pp (EPOS)

20 Conclusion 1.The measured pt spectra of direct photons at different centralities are well produced with the four sources we considered. 2.We predict thermal photons’ v2 reaches maximum at 40-50% centrality, due to the interplay between the strength and asymmetry of the transverse flow. 3.The rapidity distribution of direct photons’ v2 is able to identify the initial longitudinal energy/entropy distribution. 4.Direct photon may be a useful tool to test if a plasma can be formed in (not finished)

21 Thank you!

22 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.

23 Jet production at tau=0 MRST 2001 LO pDIS and EKS98 nuclear modification are employed Jet phase space distribution at τ=0: at τ>0:

24 Why jets lose energy

25 Thermal photons from eta_s source Parameterized initial condition EPOS initial condition

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