Direct photon production in RHIC and LHC energies

1. Direct photon production in RHIC and LHC energies Xiao-Mei Li, Shou-Ping Li, Shou-Yang Hu, Ben-Hao Sa China Institute of Atomic Energy Dai-Mei Zhou, Zhi-Guang Tan Institute of Particle Physics, Huazhong Normal University

2. Outline 1. INTRODUCTION 2. PACIAE MODEL 3. CALCULATION DETAILS 4. RESULTS 5. CONCLUSIONS

3. 1. INTRODUCTION

4. • Motivation (a) A good electromagnetic probe for earlier dynamics and QGP formation (b) Play special role, in judging whether high pT suppression is initial or final state effect (c) There is lack of microscopic description for direct photon production data

5. • Direct photon cataloged into (a)Prompt direct photon produced in hard processes (b)Thermal direct photon produced in parton evolution stage: or in string (or parton) fragmentation (not included in PYTHIA) (c)Hadronic direct photon produced in hadron evolution stage

6. • Decay photon mainly from decays • Both direct and decay photons can be described in a hadron and parton cascade model： PACIAE PA: hadron and parton cascade CIAE: China Institute of Atomic Energy

7. 2. PACIAE MODEL

8. PACIAE based on PYTHIA, composed of four parts (a) Parton initial state (b) Parton evolution (scattering) (c) Hadronization (d) Hadron evolution (rescattering)

9. (a) Parton initial state •In PACIAE model, nucleus-nucleus collision is decomposed into nucleon-nucleon collisions. Nucleons in nucleus are distributed randomly according to Wood-Saxon distribution. • Nucleon-nucleon collision is described by PYTHIA with string fragmentation switched-off. • Therefore, consequence of nucleus-nucleus collision is a configuration of ( ) , diquark (anti- diquark) and g. Diquark (anti-diquark) is forced to split into ( ) randomly.

10. (b) Parton evolution (scattering) • Up to now, only 2 2 processes are considered， LO pQCD cross section (PL, B70(1977)234)is used. • There are nine partonic 2 2 processes globally.

11. • If LO pQCD differential cross section is denoted as then for , can be expressed as That has to be regularized as by introducing color screening mass

12. • Total cross section of subprocesses then is • Using above cross sections, parton scattering can be simulated by MC

13. (c) Hadronization • Partons begin to hadronize when their interactions have ceased (freeze-out) • Hadronized by: - Fragmentation model *Field-Feynman model (IF) *Lund string fragmentation model - Coalescence model

14. (d) Hadron evolution (rescattering) • Only rescatterings among are considered • For simplicity, is assumed as 0.85 at high energy • Usual two-body collision model is used

15. 3. CALCULATION DETAILS

16. (a)Two kinds of simulations PACIAE can run in case of • Hadron cascade only, corresponding results will be indicated by“HM” • Hadronand parton cascade completely, corresponding results will beindicated by “HM & QGM”

17. (b) Two selections for hard (soft) subprocesses • PYTHIA default selection – hard subprocesses – low-pT processes • PYTHIA prompt direct photon selection PYTHIA sampling for prompt direct photon

18. (c) PYTHIA default selectionevent is generated for decay photon (or p0 ) PYTHIA prompt direct photon selection event is generated for direct photon and is corrected by cross section (d) Model parameters are fixed in all the calculations

19. 4. RESULTS

20. in p + p at There is prompt direct photon only, as thermal direct photon in string (or parton) fragmentation is not included

21. (a) Direct photon pT invariant cross section *PHENIX data, from hep-ex/0609031 * PHENIX data can be explained well by PACIAE and PYTHIA

22. (b) p0pT invariant cross section * p0PHENIXdata, from PRL, 91(03)241803 * The trends in PACIAE and PYTHIA are consistent to PHENIX data, but a little bit lower than PHENIX data

23. In Au + Au at =200 GeV

24. (a) Direct photon pT distribution * PHENIX data, from PRL,94(05)232301 • Direct photons from PACIAE simulation “HM”: prompt + hadronic “HM & QGM”: prompt + thermal + hadronic Prompt dominates * PACIAE results are consistent well with PHENIX data

25. (b) p0 and decay photon pT distributions * p0PHENIX data, from PRL,91(03)072301 * “HM” seems OK “HM & QGM” is too hard in pT>2GeV/c The softening of QGP spectrum is not enough and we have to improve model to have more parton-parton and hadron-hadron collisions * The trends in decay photon are similar to p0

26. Prediction in p + p at =14TeV

27. (a) Direct photon pT distribution * PACIAE and PYTHIA are self- consistent. * More statistic for PYTHIA

28. (b) p0pT distribution

29. Prediction in Pb + Pb at =5.5 TeV

30. (a) Direct photon pT distribution “HM”: prompt + hadronic “HM & QGM”: prompt + thermal + hadronic

31. (b) p0pT distributions

32. 5. CONCLUSIONS

33. • PACIAE (PYTHIA) seems work for p+p at RHIC energy • Au+Au, the pT spectrum in HM & QGM has to be softenedand be studied further •Give the predictions of direct photon production for LHC energy and need to be proved by LHC experiments

34. Support Frames Production

35. Honeycomb Boxes Production 2. Design in the computer 1. Make honeycomb panels 3. Manufactured by program control 4. Production

36. Readout Strips Production

37. Laboratory construction at CIAE

38. Thanks for your attention!