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