  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 their
production cross-section can also be modified.
  In this M.Sc. Thesis I search for the signature of an eta' enhancement in the
PHENIX dilepton spectrum in 200 GeV Au+Au collisions, which has a significant
enhancement in the low-mass region, e.g. in the (0.1-1.0) GeV invariant
electron-positron mass range. This is also the region of the eta' meson's
Dalitz-decay (eta'->ee gamma), so the eta' enhancement might be responsible for
at least a part of the excess. Other mesons' (other) properties can also be
changed in the hot, dense medium, but in this thesis I focus on the mass
modification of the eta' meson. To explore the role of eta', I used EXODUS
simulations to generate different dilepton spectra, corresponding to different
eta' properties. The conclusion here was that the excess can not be described
with just the eta' mass modification, but the agreement with data has been
improved significantly.
  Another idea which might brings us closer to understand the excess is to
examine the radial flow of the mesons, which was not included in the original
PHENIX analysis. Radial flow is important in the low-pT range, where it
describes the particles' spectra well: just in the region where the dilepton
spectrum has the most contributions from. Thus examining the effect of the
radial flow seems to be inevitable, as it might be responsible for certain
parts of the excess.
  The results summarized here are work in progress, obtained with the framework
of the PHENIX Collaboration at RHIC.
