Two-Particle Correlations with Neutral Pion and Direct Photon Triggers in pp and Pb+Pb Collisions with ALICE at the LHC

Two-particle correlations 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 $p_{\rm T}$ region and called the trigger particle, which is presumably from jet fragmentations. The so called associated particles from lower $p_{\rm T}$ 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 $p_{\rm T}$ and enhancement with double-peak at the low $p_{\rm T}$ on the away side, and ``ridge'' structure in pseudo-rapidity direction at the low $p_{\rm T}$ 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 $\gamma$-jet events produced from the QCD Compton scattering process, $q + g \rightarrow q + \gamma$ and $q + \bar{q} \rightarrow g + \gamma$ 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 $\pi^{0}$-hadron correlations and direct photon-hadron correlations, where the $\pi^{0}$ and photons are detected by the electromagnetic calorimeters (\mblue{EMCal}) and the charged hadrons are reconstructed by the central barrel detector system. In the $\pi^{0}$-hadron correlations, the azimuthal angle distribution of the correlations and the per-trigger yield modification factor, $I_{\rm AA} = \frac{Y^{\rm PbPb}}{Y^{\rm pp}}$, on the near side and away side are measured in pp and Pb+Pb collisions at $\sqrt{s_{NN}}$ = 2.76 TeV. In central Pb+Pb collisions, an away side suppression from in-medium energy loss is observed ($I_{\rm AA} \approx 0.6$), which is from the effects of partons energy loss. Moreover, there is an enhancement above unity of ($I_{\rm AA} \approx 1.2$) on the near side which has not been observed with any significance at lower collision energies. The significant near side enhancement of $I_{\rm AA}$ in the $p_{\rm T}$ region observed shows that the near side parton is also subject to medium effects. $I_{\rm AA}$ 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 $p_{\rm T}$ 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 azimuthal angle distribution of the correlations and the parton fragmentation function in pp at $\sqrt{s}$ = 7 TeV. 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 at $8.0 < p_{\rm T}^{iso, \gamma} < 25.0~{\rm GeV}/c$. 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 $\sqrt{s}$ = 7 TeV, 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: Chapter 1 presents 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 Chapter 2. Chapter 3 gives 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 Chapter 4, 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 Chapter 5 to 7, the selection criteria of the analysis data, analysis details of neutral pion-hadron correlations and direct photon-hadron correlations are discussed. Chapter 5 summarizes the selection criteria of data, clusters and tracks. The $\pi^{0}$ identification at EMCal and its trigger correlations are presented in Chapter 6. Chapter 7 deals with the analysis of direct photon-hadron correlations extracted from the pp collisions at $\sqrt{s}$= 7 TeV 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 Chapter 8