| Since a long time ago, the ultimate constituents of matter have always puzzled the mankind and been researching. It began with the theory of atomism (indestruc-tible atom) speculated by Democritus, a philosopher of ancient Greece, followed by the modern atomic theory proposed by John Dalton with the law of multiple proportions in1803. In the early20th century, the electron was discovered by J. J. Thomson through the measurement of mass to charge ratio in his explorations on the properties of cathode rays, and Ernest Rutherford theorized that atoms have their charge concentrated in a very small nucleus through his discovery and interpretation of Rutherford scattering with the gold foil experiment. With more experiments built for researching, more particles, such as proton, neutron and quark, are discovered in succession. All the discoveries let us clearly know that the atom consists of the nucleus and electrons, the nucleus is made up of protons and neutrons which are composed of fundamental particles, quarks and gluons.Currently, it is widely known that the ultimate constitutes of the matter are three generation quarks, their anti-quarks and three leptons. These fundamental particles have strong, weak and electromagnetic interactions by mediating gluons, W±and Z0bosons, and photons. A famous theory, Standard Model(SM), estab-lished to describe the electromagnetic, weak, and strong nuclear interactions and the fundamental particles, is success in explaining a wide variety of experimental results. The strong force of quarks and gluons is described by a theory of Quantum Chromodynamics (QCD). Three significant features, confinement, asymptotic free-dom and chiral symmetry restoration, reveal the main characteristics of QCD. The strong interaction increases or the coupling constant as describing the strong inter-action strength becomes larger with the momentum transfer decreasing. Therefore, the quarks and gluons are confined in the hadrons in the normal world with low mo-mentum transfers, known as confinement. The second feature is called asymptotic freedom. According to the asymptotic freedom, the coupling constant αs becomes smaller and the interaction is perturbative with the momentum transfer increas-ing (equivalently at short distances). The third characteristic associated with QCD is chiral symmetry restoration. The chiral symmetry exists as an exact symmetry only when the mass parameter of a quark is strictly zero. At low energy region, Lattice QCD is a well-established non-perturbative approach to solving the quan-tum chromodynamics theory of quarks and gluons. According to the Lattice QCD prediction, a new matter, Quark-Gluon Plasma (QGP), which consists of decon- fined quarks and gluons, is excepted to be created at extremely high temperature and/or high baryons density. More results demonstrate that the QGP matter may be created in the universe after a microsecond of the Big Bang due to the formed extremely high temperature or in the interior of the neutron stars with high baryons density. Therefore, it is great significant to search for the characteristic signatures of the QGP and research its properties for understanding the evolution and forma-tion of the early stages of the universe. However, it is impossible to extract directly the signatures of the early stages of the universe due to its long time evolution and explore the interior of the neutron stars. So how to create the QGP matter under the normal laboratory conditions is a great challenge.Ultra-relativistic heavy-ion collision experiment is considered as an available ap-proach to producing the QGP phase. In the heavy-ion collisions, two Lorentz con-tracted nuclei approach to each other with velocities nearly equal to the velocity of light and have colliding. In the colliding instant, both contracted nuclei pass through each other in the region of geometrical overlap. Many processes of parton-parton hard scatterings occur in the overlap region, which result in depositing a large amount of energy in a limit volume. The energy density is so high that a new matter state consisting of defined quarks and gluons is created. Since1960’s, a series of heavy ion accelerators, such as Alternating Gradient Synchrotron (AGS), Super Proton Synchrotron (SPS) and Relativistic Heavy Ion Collider (RHIC), have been built to search for the QGP signatures and research its properties.In European Organization for Nuclear Research (CERN), the current biggest accelerator, Large Hadron Collider (LHC), was designed at1998and run successfully at the end of2009. As one of four experiments, A Large Ion Collider Experiment (ALICE), whose aim is to study the physics of the strongly interacting matter at extreme energy densities. The centre-of-mass energy of (?)=2.76TeV of Pb+Pb collisions running in2010is about14higher than the highest energy of RHIC. Hence, it is excepted that the QGP created at LHC has longer lifetime and larger volume than at RHIC. This provides much better conditions for searching for the QGP and studying its properties. In the heavy-ion collisions, the formed QGP only exists in a short time and then fragment into a great variety of final hadrons. In this case, we can only study the QGP phase by different measurements from the final particles. Up to now, some main measurements which are considered as the signatures of the formation of the QGP are strangeness enhancement, suppression of J/Ψ production, direct photons and thermal di-leptons, jet quenching, collective flow, and so on.Two-particles correlation is considered as a powerful probe for understanding the properties of the strongly interacting hot and dense medium. In such an analysis, a particle is chosen from higher pT region and called the trigger particle, which is presumably from jet fragmentations. The so called associated particles from lower PT region are always from the other fragmentation of the jet, or another production, such as collective flow. At RHIC and LHC, the measurements of the azimuthal angle distribution from two-particle correlations in A+A collisions show a strong suppression even disappeared at the high pT and enhancement with double-peak at the low pT on the away side, and "ridge" structure in pseudo-rapidity direction at the low pT on the near side compared to pp collisions. All the measurements can be explained as the effects of the hot and dense medium, and imply the Quark-Gluon Plasma is indeed formed in the heavy-ion collisions. When the direct photon is selected as the trigger particle, the correlations probably tag the γ-jet events produced from the QCD Compton scattering process, q+g→q+γ and q+q→g+γ annihilation process. In these processes, the photons momenta in the center-of-mass frame are approximately balanced by that of the recoil partons. The photons do not occur energy loss when going through the medium due to only electromagnetic interactions happen between photons and other particles because of the large mean free path of photons. The fragments of the recoil partons have rich information, such as the parton fragmentation function with the medium effects, due to the interactions of the recoil partons and medium.In this thesis, the medium effects and the parton fragmentation function are measured by π0-hadron correlations and direct photon-hadron correlations, where the π0and photons are detected by the electromagnetic calorimeters (EMCal) and the charged hadrons are reconstructed by the central barrel detector system. In the π0-hadron correlations, the azimuthal angle distribution of the correlations and the per-trigger yield modification factor,IAA=YPbPb/Ypp, on the near side and away side are measured in pp and Pb+Pb collisions at (?)=2.76TeV. In central Pb+Pb collisions, an away side suppression from in-medium energy loss is observed (IAA≈0.6), which is from the effects of partons energy loss. Moreover, there is an enhancement above unity of (IAA≈1.2) on the near side which has not been observed with any significance at lower collision energies. The significant near side enhancement of IAA in the pT region observed shows that the near side parton is also subject to medium effects.IAA is sensitive to (ⅰ) a change of the fragmentation function,(ⅱ) a possible change of the quark/gluon jet ratio in the final state due to the different coupling to the medium, and (ⅲ) a bias on the parton pT spectrum after energy loss due to the trigger particle selection. In the direct photon-hadron correlations, both isolation technique and statistical subtraction method are used to extract, the direct photons and measure the azimmthal angle distribution of the correlations and the parton fragmentation function in pp at (?)=7TeV. The isolation technique used for the analysis is based on the physics that there is no particle or only a few particles around the leading order direct photons. The parton fragmentation function is measured and compared to the theory calculations at8.0<pTiso,γ<25.0GeV/c. The statistical subtraction method is based on the fact that all photons consist of direct photons and decay photons from hadrons decay. Since there is no enough statistics of pp collisions at (?)=7TeV, it is impossible to extract significant results with the statistical subtraction method. But the work in this thesis develops the method in ALICE data analysis, which can be used quickly for measuring the parton fragmentation function and studying the medium effects in the next running.This thesis is organized as follows:Chapter1presents the Standard Model of the particle physics including the description of the Higgs boson and the Quantum Chromodynamics, the Lattice QCD predication and the QCD phase diagram. The space-time evolution of the heavy-ion collisions and some significant measurements for searching for the QGP phase from SPS, RHIC and LHC are summarized in Chapter2. Chapter3gives an overview of the ALICE experiment and a description of the ALICE online and offline systems. The analysis framework for measuring the correlations is also presented shortly in this chapter. In Chapter4, the analysis method of two-particle correlations is introduced as well as the measurements of the correlations with the triggers as charged hadrons, neutral pions and direct photons from RHIC and LHC. From Chapter5to7, the selection criteria of the analysis data, analysis details of neutral pion-hadron correlations and direct photon-hadron correlations are discussed. Chapter5summarizes the selection criteria of data, clusters and tracks. The π0identification at EMCal and its trigger correlations are presented in Chapter6. Chapter7deals with the analysis of direct photon-hadron correlations extracted from the pp collisions at (?)=7TeV with the methods of the isolation and the statistical subtraction. At last, the discussion and outlook to the work in this thesis are addressed in Chapter8. |
