Monte Carlo Study Of Neutral Meson Decay Photon Elliptic Flow In Pb+Pb Collisions At S_NN^1/2=5.5TeV

Through study of many scientists since ancient times, people found matter con-sists of atom, atom consists of electron and nucleus, nucleus consists of proton and neutron. With development of particle accelerator, people found proton and neutron consist of quark and gluon.In normal state, quarks are confined in mesons or baryons. According to The Quantum Chromo-Dynamics (QCD). quarks and gluons of hadron would be liberated from color confinement in the finite volume at an extreme temper-ature and density, transformed to a new state consisted of free quarks and gluons.This new state is called "Quark Gluon Plasma (QGP)". Heavy ion collisions are meant to detect QGP.A key observable to study the thermodynamic properties of the QGP is the 'elliptic flow', i.e. the azimuthal anisotropy in the momenta distribution of the particles produced in the collision, which can be connected to the Equation of State of the system.The direct photons do not interact strongly with the hot and dense medium because of its large mean free path, so they can carry out direct thermody-namical and kinematical information on the state of production source.Direct photons are a important probe to study the early stage of heavy-ion collisions. In the heavy-ion collision experiments at the Relativistic Heavy-Ion Collider (RHIC), a strong suppression has been observed in hadron yields at high transverse momentum (pT) in central Au+Au collisions compared with p+p collisions scaled by the number of binary nucleon-nucleon collisions. The observed high-pT suppression is attributed to a characteristic of energy loss of hard-scattered partons passing through the high density matter. On the contrary, non suppres- sion of high-pT direct photon yields has been observed in Au+Au collisions. ALICE is a dedicated heavy ion detector for the reconstruction of lead-lead collisions at the Large Hadron Collider.The main purpose of the ALICE experiment is to study the properties of the QGP at collision energies never achieved before.Although the measurements of direct photon as a penetrating probe provides di-rect and more reliable information of the early stage of collisions, photons are emitted at all the stage of collisions such as hard-scattering on the initial stage, interaction of quarks and gluons on the thermalized phase, and the hadron gas on the late stage.It is complexity to achieve direct photon by experiment.In non-central heavy-ion colli-sions, a geometrical shape of the collision zone becomes almond like shape.The direct photon yield is measured experimentally by the subtraction of hadronic decay photon from the inclusive photon distribution.A main source of the hadronic decay photon contamination isπ0 emission。besidesπ0, the influence ofη,ρ0,ω,η1 decay are also considered.Since components besidesπ0are not measured directly, the photons from hadrons are estimated by Monte-Carlo simulation. The transverse momentum distribution function ofπ0 is obtained by a fit of the generated spectra ofπ0,η,ρ0,ω,η1 produced by Hijing, simulating (?) collision. The simulated dN/dpT distribution are defined by a formula,similar to the Levy distribution.According to mT scaling,η,ρ0,ω,η1 pt distribution can be got according to to pt distribution. Then flow information were added to the event, making elliptic flow ofπ0,η,ω,ρ0,η1 be consistent with result of RHIC. u2 which depending on pT increases linearly before arriving a saturation value. It suggests that flow is created at early stage of collision, and is stable at expanding stage. Then Event Plane method is used to analyse u2 which depends on pT The pT distribution of decay photon u2 is consistent with its mother hadron u2.