$e^+e^-\to t\bar t\to bW^+\bar b W^-$ (Fig.\[figinterfer\]a) and the single-resonant amplitudes describing the processes $e^+e^-\to t+\bar b W^-\to
bW^+\bar b W^- $ and $e^+e^-\to bW^+\bar t\to bW^+\bar b W^-$ (Figs.\[figinterfer\]b-i) in the $t\bar t$ threshold limit for $m_t\to\infty$.

![ Full theory Feynman diagrams describing the process $e^+e^-\to bW^+\bar b W^-$ with one or two intermediate top or antitop quark propagators. The circle in diagram (a) represents the QCD form factors for the $t\bar t$ vector and axial-vector currents. \[figinterfer\] ](figures/fig2.ps){width="14cm"}

Note that diagram (a) dominates in the non-relativistic limit due to two resonant top/antitop lines, while diagrams (b-i) are $v^2$-suppressed having only one resonant top/antitop line. Diagram (a) also contains a subleading $v^2$-suppressed contribution that has to be accounted for. Diagrams with no top/antitop line are suppressed by $v^4$ and do not need to be considered. This also means that pure background diagrams containing no intermediate top quark can be neglected at this order. We also note that to find literal agreement between full and effective theory matrix elements one has to replace the $i\epsilon$ terms in the resonant full theory top propagators by the Breit-Wigner term $i m_t\Gamma_t/2$. The circle shown in Fig.\[figinterfer\]a represents the QCD form factors for 