Leptons, e.g. muons, are ideal tools to explore the hot and dense matter created at heavy-ion collider experiments, because they have minimal final state interactions and as a result are able to preserve information of the hot medium. This thesis focuses on two leptonic physics results of the STAR experiment at Brookhaven National Lab – dimuons and muonic atom production. The first measurement of dimuon production at low invariant mass is presented, using data collected from Au + Au collisions at psNN = 200 GeV. An excess of the dimuon yield over known hadronic contributions in the mass region 0.2 - 0.55 GeV/c2 is found. This excess might be sensitive to modified ⇢ meson spectrum function in hot medium, which has been proposed to be related to chiral symmetry restoration. This thesis also presents the first search results of muonic atoms in heavy-ion collisions. Femtoscopic correlations indicate hadrons and muons are produced at the same space-time point, providing a signature of atom ionization at the detector beam pipe. Invariant mass signals are observed for Kμ and pμ atoms and their antimatter counterparts. The measured yields and a calculation from a coalescence model are found not in agreement, suggesting significant other sources in the calculation may be needed.