  In the high-temperature quark-gluon plasma and its subsequent hadronic matter
created in a high-energy nucleus-nucleus collision, the quark-antiquark
potential depends on the temperature. The temperature-dependent potential is
expected to be derived from the free energy obtained in lattice gauge theory
calculations. This requires one to study the relation between the
quark-antiquark potential and the quark-antiquark free energy. When the
system's temperature is above the critical temperature, the potential of a
heavy quark and a heavy antiquark almost equals the free energy, but the
potential of a light quark and a light antiquark, of a heavy quark and a light
antiquark and of a light quark and a heavy antiquark is substantially larger
than the free energy. When the system's temperature is below the critical
temperature, the quark-antiquark free energy can be taken as the
quark-antiquark potential. This allows one to apply the quark-antiquark free
energy to study hadron properties and hadron-hadron reactions in hadronic
matter.
