\subsection{Measurements with MegaTop}

With an integrated luminosity of  the order of $130~\infb$ per year and per experiment, TLEP will be a top factory as well, with over one million $\ttbar$ pairs produced in five years (hence the ``MegaTop'' appellation) at $\sqrt{s} \sim 345$~GeV. The precise measurement of the cross section at the $\ttbar$ production threshold is sensitive to the top-quark pole mass, $m_{\rm top}$, the total top-quark decay width, $\Gamma_{\rm top}$, as well as to the Yukawa coupling of the top quark to the Higgs boson, $\lambda_{\rm top}$, through the virtual exchange of a Higgs boson between the two top quarks. 

The production cross section at threshold~\cite{g_Manohar_Stewart_Teubner_2001}, corrected for QCD effects up to the next-to-next-to-leading order, is displayed in Fig.~\ref{fig:ttbar} for $m_{\rm top} = 174$~GeV, with and without the effects of initial-state radiation (present at all $\epem$ colliders) and of beamstrahlung (only affecting linear colliders). As mentioned in Section~\ref{sec:beamstrahlung}, the absence of beamstrahlung at TLEP slightly increases the steepness, hence the sensitivity to the top-quark mass, and absolute value of the cross-section profile at the $\ttbar$ threshold. The corresponding numbers of events expected at TLEP are given in Table~\ref{tab:TopCross}.