Interference radiative phenomena in the production of heavy unstable particles

We discuss the characteristic interference phenomena, including real as well as virtual radiative corrections, to the inclusive cross sections for production of heavy unstable systems such as $t\overline{t}$ or $W^{+}{W}^{-}$ pairs. We demonstrate explicitly the (fortunate) cancellation between emissions associated with the production of unstable particles and with their decays, and between emissions accompanying the decay stages. As a result the total interference effect is shown to be of $O\left(\frac{{\alpha }_i\Gamma }{M}\right)$ or better (where ${\alpha }_i=\alpha or {\alpha }_s$ as appropriate).