A Study of Number-Ratio Fluctuations in Gold-Gold Interactions at sqrt(s_NN)=200 GeV

Sammanfattning: The properties of the basic building blocks of nuclear matter, the quarks, and their interactions are not known in detail, largely because observations are complicated by the confinement of quarks in composite particles. New insights can be gained from the study of nuclear matter in a deconfined phase, like the form of matter that the Universe consited of early in its evolution according to the Big Bang theory. Collisions between heavy atomic nuclei at high energies can be used to probe the phases of nuclear matter. When high enough energy densities are reached in these collisions, a transition to the deconfined quark-gluon plasma phase and back may take place, with expected impacts on the fluctuations in the medium. The amplitude of these fluctuations will depend on the order of the phase transition. Thus, studies of fluctuations provide information on the properties of nuclear matter under different conditions. This thesis describes a detailed study of kaon-to-pion and proton-to-pion particle-number fluctuations in collisions of high-energy Gold nuclei. Experimental data from collisions at a centre-of-mass energy of 200 GeV per nucleon-nucleon pair are analysed within the collaboration of the PHENIX experiment at Brookhaven National Laboratory, New York (USA). The results show that there is little room for dynamical fluctuations, resulting from a sharp phase transition, in these collisions. This study demonstrates how important it is to analyse fluctuations in charge combinations of different particle species separately, when attempting to disentangle the various contributions to the observed fluctuations.