Fermion mass effects in $e^+ e^- \rightarrow 4f$ and $e^+ e^- \rightarrow 4f\gamma$ with cuts

Abstract.

The fermion mass effects in $e^+ e^- \rightarrow 4f$ and in the corresponding bremsstrahlung reactions in the presence of realistic cuts are studied. It is shown that, for some four–fermion final states, the mass effects become sizable to the extent that they may affect the accuracy of theoretical predictions which is required to be better than 1%.