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In case of an UA(1) symmetry restoration in a hot and dense hadronic matter,the mass of the produced hadrons and mesons can significantly change, and theirproduction cross-section can also be modified.  In this M.Sc. Thesis I search for the signature of an eta' enhancement in thePHENIX dilepton spectrum in 200 GeV Au+Au collisions, which has a significantenhancement in the low-mass region, e.g. in the (0.1-1.0) GeV invariantelectron-positron mass range. This is also the region of the eta' meson'sDalitz-decay (eta'->ee gamma), so the eta' enhancement might be responsible forat least a part of the excess. Other mesons' (other) properties can also bechanged in the hot, dense medium, but in this thesis I focus on the massmodification of the eta' meson. To explore the role of eta', I used EXODUSsimulations to generate different dilepton spectra, corresponding to differenteta' properties. The conclusion here was that the excess can not be describedwith just the eta' mass modification, but the agreement with data has beenimproved significantly.  Another idea which might brings us closer to understand the excess is toexamine the radial flow of the mesons, which was not included in the originalPHENIX analysis. Radial flow is important in the low-pT range, where itdescribes the particles' spectra well: just in the region where the dileptonspectrum has the most contributions from. Thus examining the effect of theradial flow seems to be inevitable, as it might be responsible for certainparts of the excess.  The results summarized here are work in progress, obtained with the frameworkof the PHENIX Collaboration at RHIC.

Experimental and Theoretical Studies: Analysis of Low-mass Dilepton Enhancement in 200 GeV Au+Au Collisions at RHIC (thesis)