Semileptonic six fermion processes at future $e^+ e^-$ colliders: signal and irreducible background for top and WWZ physics

We compute several total and differential cross sections relevant to top and WWZ physics at future $e^+e^-$ colliders taking into account the full set of Feynman diagrams for six fermion final states. We also include in our calculations initial state radiation and beamstrahlung effects, and the most important QCD corrections in an approximate (naive) form. We compare such a complete approach with "production x decay" approximation and we suggest that in many physical studies the former is needed.