Direct photon production in p + p and Au + Au collisions at = 200GeV 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

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

1. INTRODUCTION

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

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

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

2. PACIAE MODEL

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

(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.

(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.

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

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

(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

(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

3. CALCULATION DETAILS

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

(b) Two selections for hard (soft) subprocesses selection PYTHIA default selection – hard subprocesses – low-p T processes PYTHIA prompt direct photon selection PYTHIA sampling for prompt direct photon

(c) PYTHIA default selection event is generated for decay photon (or  0 ) 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

4. RESULTS

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

*   PHENIX data, from PRL, 91(03) * PHENIX data can be explained well by PACIAE and PYTHIA * The trends in decay photon are similar to   (a)  0 p T invariant cross section and decay photon p T distribution

(b) Direct photon invariant cross section * PHENIX data, from hep-ex/ * PACIAE and PYTHIA are consistent with PHENIX data within error bar

In Au + Au at =200 GeV

*   PHENIX data, from PRL,91(03) * “HM” seems OK “HM & QGM” is too hard in p T >2GeV/c The softening of QGP spectrum is not enough and have to improve model to have more parton-parton and hadron-hadron collisions * The trends in decay photon are similar to   (a)  0 and decay photon p T distributions

(b) Direct photon p T distribution * PHENIX data, from PRL,94(05) * Direct photons for Prompt dominates * PACIAE results are consistent well with PHENIX data “HM”: prompt + hadronic “HM & QGM”: prompt + thermal + hadronic

5. CONCLUSIONS

PACIAE (PYTHIA) seems work for p+p Au+Au, the p T spectrum in HM & QGM has to be softened and be studied further

Direct photon excess is sure, question is how much it is Prompt direct photon dominance is sure, problem is how strong it is

Thanks for your attention and have a great time in China!