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

OutlineOutline 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 pT

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

• PYTHIA default selection selection – hard subprocesses

– low-pT 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

hep-ex/0609031

* PHENIX data can be explained well by PACIAE and PYTHIA

(a) Direct photon pT invariant cross section

(b) pT invariant cross section

* 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

In Au + Au at =200 GeV

(a) Direct photon pT distribution

* PHENIX data, from

PRL,94(05)232301

•Direct photons from

PACIAE simulation

Prompt dominates

* PACIAE results are consistent well with PHENIX data

“HM”: prompt + hadronic “HM & QGM”: prompt + thermal + hadronic

* 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

(b)0 and decay photon pT distributions

Prediction in p + p at =14TeV

(a) Direct photon pT distribution

* PACIAE and * PACIAE and

PYTHIA are self-PYTHIA are self-

consistent.consistent.

* More statistic for * More statistic for

PYTHIAPYTHIA

(b) pT distribution

Prediction in Pb + Pb at =5.5 TeV

(a) Direct photon pT distribution

“HM”: prompt + hadronic

“HM & QGM”: prompt + thermal + hadronic

(b)0 pT distributions

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

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