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 Let’s come to the first part. This work is contributed by those people.

Outline 1. INTRODUCTION 2. PACIAE MODEL 3. CALCULATION DETAILS 4. RESULTS 5. CONCLUSIONS the first part consists of five small parts

1. INTRODUCTION Fist let’s come to the introduction

• Motivation dynamics and QGP formation (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 Why do we study direct photon, first because direct photon Second Third: (i.e. phenix data )that is why we use PACIAE to study direct photon production phenix 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 In the relativistic heavy-ion collision, there are two kinds of photons: direct and decay. For direct photon, it can be cataloged into.., which is produced... Where such as quark anti-quark annihilation, quark gluon compton scattering However which is not

• Both direct and decay photons can be 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 Then come to decay, as we know most of decay photons are from eta－prim PACIAE: hadron and parton cascade and CIAE

2. PACIAE MODEL Come to

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

• In PACIAE model, nucleus-nucleus collision is (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. The first part of PACIAE is string fragmentatio（switched-off）是第一步的还是第二步的 Thermal photon 分不分 bias 和default 关掉string fragmentation是为了生成QGP而截断强子化

(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. Perturbative among them,

• If LO pQCD differential cross section is denoted as then for , can be expressed as That has to be regularized as as the formula one Can be described formula 3 by introducing color screening mass mu 2 by introducing color screening mass

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

• Partons begin to hadronize when their (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 Now let’s come to the third part: We assume Can be 不产生直接光子，PYTHIA的string fragmentation没考虑产生光子

• Only rescatterings among (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 Forth part o PACIAE model: In PACIAE model, For simplicity, assume the ratio of inelastic cross section and total cross section is 0.85 at high energy is used here

3. CALCULATION DETAILS Let’s come to the calculation details

PACIAE can run in case of • Hadron cascade only, corresponding (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” We can perform three kinds of simulations with PACIAE.//The simulations with PACIAE can be performed in three methods One is Corresponding result is … since in this case the final state is Hadronic Matter, The other is Since here the final state is hadron matter and is evolved from the QGM，Quark-gluon matter.

(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 We have two selections for hard (soft) subprocesses. First one is hard subprocesses and low pt processes are used Such as Low pt =Soft

(c) PYTHIA default selection event is generated for decay photon (or p0 ) PYTHIA prompt direct photon selection event is generated for direct photon and is corrected by cross section 产生thermal也是根据前面的default和bias选择吗？（不产生in pythia）？ Direct photon ＝ The cross section of total inclusive photon production –the cross section of inclusive decay photon (which from the pai 0 Pt spectrum of another experiment） 直接光子： p+p分布归一后乘以直接光子截面 Au+Au分布归一化到the ratio of direct photon cross section over (to) total cross section (d) Model parameters are fixed in all the calculations

4. RESULTS

There is prompt direct photon only, as in p + p at There is prompt direct photon only, as thermal direct photon in string (or parton) fragmentation is not included Square root of s first In p + p collision， prompt direct photon amounts nearly one per event

(a) Direct photon pT invariant cross section * PHENIX data, from hep-ex/0609031 * PHENIX data can be explained well by PACIAE and PYTHIA This pic shows The full circle with error bar is from PHENIX data The open circle is … Curve， the results of pp直接光子的总截面0.00777(PACIAE)乘以归一化了的分布 p+p, PACIAE和PYTHIA区别:self-consistent

(b) p0 pT invariant cross section * p0 PHENIX data, from PRL, 91(03)241803 * The trends in PACIAE and PYTHIA are consistent to PHENIX data, but a little bit lower than PHENIX data This pic shows Upper panel The full circle with error bar is PHENIX data taken from this paper Open circle is the results from PACIAE model Tri-angle is the results from PYTHIA　model Correspond each other pp的非弹截面(~pai_0截面）×pai_0归一化分布 (default算出来的， 某一pT的粒子数/总粒子数）

In Au + Au at =200 GeV Now let’s come to the gold-gold collision at 200 GeV First let’s see one – something like 246 Why is it so small? Because for example, … Thermal direct photon from string fragmentation is not included The cross section ..So … can not compete with … The number of decay photon is large, 3828 and pai 0 is 1815, decay photon should be double of pai 0. Prompt 有没有选过了，还是真实值， Double of pai 0 Very few Quite a lot Number small and large

(a) Direct photon pT distribution * PHENIX data, from PRL,94(05)232301 * Direct photons for “HM”: prompt + hadronic “HM & QGM”: prompt + thermal + hadronic Introduction pic: The error here means the exp. data just give us an upper limit, the lower limit could be zero. For HM, include For QGM, include Compare the three spectrums, we could understand prompt photon domination. The results from PACIAE model are quite different from expr. 归一到直接光子总数（直接光子截面除以几何截面作为纠偏因子），结果非常好 0.00777×246/[0.9×pai×（Ra+Rb)2] Prompt dominates * PACIAE results are consistent well with PHENIX data

(b) p0 and decay photon pT distributions * p0 PHENIX 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 Let’s see the of Au+Au at 200 GeV Upper plot shows Open triangle. Full triangle is from PACIAE HM simulation spectrum To soften 归一化到pai_0总数，pai_0要不要也乘那个因子:不要， 因为是default算出来的 The softening of QGP spectrum is not enough and have to improve model …

Prediction in p + p at =14TeV Let me give the prediction for LHC energy. first

(a) Direct photon pT distribution * PACIAE and PYTHIA are self- consistent. * More statistic for PYTHIA This pic shows The full circle with error bar is from PHENIX data The open circle is … Curve， the results of pp直接光子的总截面0.00777(PACIAE)乘以归一化了的分布 p+p, PACIAE和PYTHIA区别:self-consistent

(b) p0 pT distribution This pic shows Upper panel The full circle with error bar is PHENIX data taken from this paper Open circle is the results from PACIAE model Tri-angle is the results from PYTHIA　model Correspond each other pp的非弹截面(~pai_0截面）×pai_0归一化分布 (default算出来的， 某一pT的粒子数/总粒子数）

Prediction in Pb + Pb at =5.5 TeV Now let’s come to the gold-gold collision at 200 GeV First let’s see one – something like 246 Why is it so small? Because for example, … Thermal direct photon from string fragmentation is not included The cross section ..So … can not compete with … The number of decay photon is large, 3828 and pai 0 is 1815, decay photon should be double of pai 0. Prompt 有没有选过了，还是真实值， Double of pai 0 Very few Quite a lot Number small and large

(a) Direct photon pT distribution “HM”: prompt + hadronic “HM & QGM”: prompt + thermal + hadronic Introduction pic: The error here means the exp. data just give us an upper limit, the lower limit could be zero. For HM, include For QGM, include Compare the three spectrums, we could understand prompt photon domination. The results from PACIAE model are quite different from expr. 归一到直接光子总数（直接光子截面除以几何截面作为纠偏因子），结果非常好 0.00777×246/[0.9×pai×（Ra+Rb)2]

(b) p0 pT distributions Let’s see the of Au+Au at 200 GeV Upper plot shows Open triangle. Full triangle is from PACIAE HM simulation spectrum To soften 归一化到pai_0总数，pai_0要不要也乘那个因子:不要， 因为是default算出来的 The softening of QGP spectrum is not enough and have to improve model …

5. CONCLUSIONS

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

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

Merci beaucoup!