Two-particle correlations in p+p and Pb+Pb collisions at SPS energies

In this thesis two-particle correlations in pseudorapidity and azimuthal angle in p+p collisions at beam momenta: 20, 31, 40, 80, and 158 GeV/c are presented. Data were recorded in the NA61/SHINE experiment at the CERN Super Proton Synchrotron (SPS). The results are compared to the EPOS and the UrQMD models as well as to the results from various experiments at the Relativistic Heavy-Ion Collider (RHIC) and the Large Hadron Collider (LHC). Dedicated comparison analysis was done also on NA49 data of Pb+Pb collisions. The inclusive results in p+p show correlation structures connected with resonance decays, Bose-Einstein statistics, momentum conservation, and strings fragmentation. No structures connected with hard processes were observed even at 158 GeV/c beam momentum. The EPOS model reproduces data fine except of Bose-Einstein enhancement; the UrQMD model shows many disagreements with data. The results provide an insight into forgotten realm of soft physics where jet peaks do not cast shadows onto two-particle correlations landscape. In the first chapter a general look at the domain of heavy-ion physics is presented. The Standard Model is briefly discussed as well as Quark-Gluon Plasma with its signatures and location in the phase diagram of strongly interacting matter. The second chapter brings an overview of particle correlations. Firstly, two-particle correlation function $C(\Delta\eta,\Delta\phi)$ is defined. Then, historical and recent analyses on two-particle correlations are presented. Many results from experiments at RHIC and LHC are shown together with discussion on correlation structures appearing in the plots. In the third chapter, the NA61/SHINE experiment is introduced. Firstly, the detector setup is described as well as current and future upgrades. Then, the software part of the experiment is mentioned with its methods of calibration, reconstruction and data simulation. The fourth chapter contains the main part of the analysis of proton-proton interactions. After description of event and track selections, the main results from the inclusive analysis are presented together with detector effects correction method. Calculation of statistical and estimation of systematic uncertainties is depicted. Then, data results are compared to the EPOS and the UrQMD models. Next, two sub-analyses of correlations (with no transverse momentum restrictions and semi-inclusive analysis in multiplicity bins) are presented. The chapter ends with a comparison with results from similar analyses in experiments at RHIC and LHC. In the fifth chapter two-particle correlations in Pb+Pb interactions at 20$A$ and 158$A$ GeV/c are presented. They are compared to the results from p+p collisions. The analysis of $C(\Delta\eta,\Delta\phi)$ with changing two-track distance is performed. The thesis is closed with a summary in the sixth chapter.