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\subsection{Beam energy measurement}  As mentioned above, transverse polarization can be naturally established at TLEP at the Z pole and at the WW threshold. A technique unique to $\epem$ rings, called resonant spin depolarization~\cite{on_Blondel_Aβmann_Dehning_1992}, can therefore be used to measure the beam energy with high precision. This technique was developed and successfully used during the LEP1 programme, and allowed the average  beam energy to be known with a precision of 2 1  MeV. The intrinsic precision of the method, 0.1 MeV or better, was notbe  fully exploited at LEP1 because no attempt was made to maintain the polarization in collisions. Regular measurements were performed by separating the beams at the end of physics fills, but it was realized that the energy actually drifted with time because of e.g. tides and trains, so that an extrapolation had to be made to “predict” the beam energy during collisions. This extrapolation was the dominant contributor to the 2 MeV systematic uncertainty on the Z mass and width. At TLEP, instead, it will be possible to keep a few non-colliding bunches out of the 4400 (Z pole) or 600 (WW threshold) colliding ones without significant loss of luminosity, and apply resonant regular spin depolarization on those. This will allow continuous beam energy measurements, in the exact same conditions as for the colliding bunches, with an accuracy of 100 keV or so for eachh measurement. Given the statistics available, a precision of 0.1 MeV or better will therefore be at hand for the Z mass and width measurements. Similarly the beam energy uncertainty on the W mass should be less 0.1 MeV.