\section{Precise measurements of the EWSB parameters}
\label{sec:EWSB}

Electroweak loops have the remarkable property of being sensitive to the existence of weakly-coupled particles, even if they cannot be directly produced or observed in current experiments. For example, the measurements of the Z resonance line-shape parameters, undertaken  at LEP during a dedicated scan in 1993, led to a prediction of the top quark mass $m_{\rm top}$ of $172\pm 20$~GeV by the time of the Moriond conference in March 1994~\cite{cite:Pietrzyk}. The uncertainty on $m_{\rm top}$ was dominated by the range of assumptions for the Higgs boson mass, varied from 60 to 1000~GeV. When the top quark was discovered at the Tevatron in 1995, and its mass measured with precision of a few~GeV within one standard deviation of the prediction, the Electroweak fits of the LEP data became sensitive to the only remaining unknown quantity in the Standard Model, the Higgs boson mass $m_{\rm H}$, predicted to be $m_{\rm H} = 99^{+28}_{-23}$~GeV~\cite{_Lys_Murayama_Wohl_et_al__2012}. It is remarkable that the observation of the H(126) particle at the LHC falls, once again, within one standard deviation of this prediction. 

These two historical examples are specific of the Standard Model, with its particle content -- and nothing else. Now that the Higgs boson mass is measured with a precision of a fraction of a~GeV, and barring accidental or structural cancellations, these fits rule out the existence of any additional particle that would have contributed to the Electroweak loop corrections in a measurable way. As emphasized in Ref.~\cite{_Lys_Murayama_Wohl_et_al__2012}, the corrections to the W and Z masses do not necessarily decouple when the mass of new additional particles increase (contrary to the corrections to, e.g., $(g-2)_{\mu}$). For example, the top-quark loop correction scales like $(m^2_{\rm top} - m^2_{\rm b}) / m^2_{\rm W} $. The Electroweak loop corrections are also delicately sensitive to the details of the Electroweak Symmetry Breaking Mechanism. 

As summarized in Section~\ref{sec:exp}, the TLEP physics programme offers the potential of  considerable improvements in the precision of a large number of Electroweak observables. The outstandingly large luminosity, the precise energy definition, the absence of energy bias due to beamstrahlung, and an accurate energy calibration with resonant depolarization, are among the unique characteristics of TLEP towards an unparalleled precision for most of the measurements. 