deletions | additions       diff --git a/Introduction.tex b/Introduction.tex  index fa5b8e8..bf490de 100644  --- a/Introduction.tex  +++ b/Introduction.tex 
...
The design study of the Future Circular Colliders (FCC) in a 100-km ring in the Geneva area has started at CERN at the beginning of 2014, as an option for a post-LHC particle accelerators. The study has an emphasis on proton-proton and electron-positron high-energy frontier machines~\cite{FCCWebSite}. In the current plans, the first step of the FCC physics programme would make use of the ${\rm e^+e^-}$ collider, called FCC-ee, with centre-of-mass energies ranging from below the Z pole to the ${\rm t\bar t}$ threshold and beyond. A first look at the physics case of the FCC-ee can be found in Ref.~\cite{Bicer_2014}.  In this first look, the focus regarding top-quark physics was on precision measurements of the top-quark mass, width and Yukawa coupling through a scan of the ${\rm t\bar t}$ production threshold, with $\sqrt{s}$ comprised between~340 and~350\,GeV. The expected precision on the top-quark mass is in turn used in a global electroweak fit to set constraints on weakly-coupled new physics up to a scale of 100\,TeV. Although not alluded to in the first look, new physics might also show up via significant deviations of the top-quark electroweak couplings with respect to their standard-model predictions. Theories in which the top quark and the Higgs boson are composite lead to such deviations. While these theories also lead to modifications of the Higgs boson couplings and of the Z decay width, it may well be that the The  inclusion of a direct measurement of the ttZ coupling in the global electroweak fit is therefore likely to  further constrain these theories. It is widely believed (as, e.g., in Ref.~\cite{Baer_2013}) that both a centre-of-mass energy well beyond the top-quark pair production threshold and a large longitudinal polarization of the incoming electron and positron beams are crucially needed to disentangle independently access  the tt$\gamma$ and the ttZ couplings. couplings for both chirality states of the top quark.  In Ref.~\cite{Baer_2013}, it is shown that the measurements of the total event rate, the forward-backward asymmetry, and the helicity asymmetry of the top quark with 500\,${\rm fb}^{-1}$ at $\sqrt{s}=500$\,GeV and with beam polarizations of ${\cal P} = \pm 0.8$, ${\cal P}^\prime = \mp 0.3$, allow for this distinction. In this short letter, however, the sensitivity to the top-quark electroweak couplings is estimated with an optimal-observable analysis of the lepton angular and energy distributions from ${\rm t\bar t}$ production in the $\ell \nu {\rm q \bar q b \bar b}$ final states (with $\ell = {\rm e}$ or $\mu$) at the FCC-ee, without incoming longitudinal beam polarization and with a centre-of-mass energy not significantly above the ${\rm t\bar t}$ production threshold. Such a sensitivity can be understood from the fact that the moderate top-quark polarization arising from its coupling to the Z is maximally transferred to the final state particles via the weak top-quark decay ${\rm t \to W b}$ with a 100\% branching fraction: the lack of initial polarization is compensated by the presence of substantial final state polarization, and by a larger integrated luminosity. A similar observation had already been made many years ago at LEP, with the measurement of $\sin^2\theta_W$ from the $\tau$ polarization in ${\rm Z} \to \tau^+\tau^-$ events.