Six fermion processes at future e^+ e^- colliders: signal and irreducible background for top, WWZ and Higgs physics in charged current final states

We compute several total and differential cross sections relevant to top, WWZ and Higgs physics at future $e^+e^-$ colliders taking into account the full set of Feynman diagrams for six fermion final states. We examine in particular charged current processes, in which final particles cannot be formed by three Z's decay. We include in our calculations initial state radiation and beamstrahlung effects, and the most important QCD corrections in an approximate (naive) form. We also compare this complete approach with production $\times$ decay approximation.