  In this work the $e^{+}e^{-}\to l^{+}l^{-}$ scattering process is
investigated. The cross-section is calculated considering three different
effects: temperature, external magnetic field and chemical potential. The
effect due to an external field is inserted into the problem through a
redefinition of the fermionic field operator. Effects due to temperature and
chemical potential are introduced using the Thermo Field Dynamics formalism.
