Photon-hadron Correlation In Pp Collision At7TeV With ALICE

The microscopic structure of matter has historically been concerned by many scientists. So far, the Standard Model(SM) is a widely accepted theory of particle physics, which describes the elementary structure of macroscopic materials and the fundamental interactions among them. The basic components of this theory are: six different flavour of quarks, six different leptons, their anti-particles, and four gauge bosons (r, g and W±/Z0) involving three kinds of fundamental interactions:the elec-tromagnetic, strong and weak force; According to the SM, all macroscopic matters are composed of these elementary quarks and leptons with the interactions medi-ated by the gauge bosons. Quantum Chromodynamics (QCD) is the theory which describes the strong interaction between quarks and gluons in SM. QCD predict-s that in the condition of extremely high temperature or/and high baryon density, normal hadronic matter could go through a phase transition into a new deconfined quark-gluon plasma (QGP) phase. In nature, QGP has ever existed in the first few microseconds after the Big-Bang with an extremely high temperature and still exists in the cores of neutron stars where the baryon density is extremely high.According to QCD, it’s very likely that the deconfined QGP would be formed in the high energy heavy-ion collision, where a huge amount of energy would deposit in a small space. In the last thirties years, a series of accelerators have been built to study the properties of QGP, including the AGS (Alternating Gradient Synchrotron), SPS (Super Proton Synchrotron) and RHIC (Relativistic Heavy-Ion Collide). These accel-erators have contributed greatly to improve our understanding of QGP, and a scries of experimental phenomenons associated to the formation of QGP have been observed, such as collective flow, jet quenching, strangeness enhancement and thermal pho-ton. Especially, the jet quenching effect which causes the high transverse momentum yield suppression and back-to-back jet correlation disappear, arc important signals of QGP formation in the collisions. ALICE (A Large Ion Collider Experiment), one of the four major experiments at LHC (Large Hadron Collider) which has started from2009, is dedicated to the study of the heavy-ion collision. This new experimental facility is used to accelerate lead nuclei to the high energy of2.76TeV per nuclcon. It’s expected that a local hot and dense matter with an extremely high temperature and energy density would be produced at the early stage of the collision.The main purpose of the thesis is to study the parton fragmentation function with the measurement of the direct photon-hadron correlations, in proton-proton collisions at (SNN)=7TeV. This measurement provides a baseline for the medium modified parton fragmentation in heavy-ion collision where the hot and dense nuclear medium is likely to be formed. Due to the fact that the inclusive photons consist of direct and decay photons, the statistical subtraction method is used to extract the direct photon-hadron correlation distribution. The parton fragmentation function can be well approximated by the imbalance parameter xE of the direct photon-hadron correlation. Since there is not enough statistics of pp collision data at (SNN)=7TeV up to now, it is impossible to extract physical meaningful results with the statistical subtraction method. But the work in this thesis establishes the use of this method in the data analysis of ALICE experiment, which can be used effectively for measuring parton fragmentation and studying the medium effect for future analysis with enough statistics.In the first chapter, the basic knowledge of heavy-ion collision is introduced such as the Standard Model, the space-time evolution of heavy-ion collision and experimental phenomenons associated to QGP formation; The second chapter introduces briefly the layout of ALICE detector and the ALICE offline data analysis framework Ali-Root. In the third chapter, the background knowledge of the direct photon-charged hadron correlation and two methods are introduced: Statistical Subtraction and Iso-lation method. In the forth chapter, the neutral photon/π0/η reconstruction and identification, and the detailed steps and some results of the Statistical Subtraction method are introduced.