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\subsection{Beam energy measurement}  Explain briefly the unique and superb beam energy measurement with resonant depolarization at circular colliders when transverse polarization is available. Compare with linear collider ? Mention the possibility of non-colliding bunches, and of continuous energy measurement. State a plausible beam energy precision at the Z pole and WW threshold.  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{doi:10.1016/0370-2693(92)90457-F}, depolarization~\cite{ doi:10.1016/0370-2693(92)90457-F },  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 beam energy to be known with a precision of 2 MeV. The intrinsic precision of the method, 0.1 MeV or better, was not attained at LEP1 because the polarization could not be maintained in collisions. The few measurements performed in specific runs with one beam had therefore to be extrapolated to “predict” the beam energy during collisions, with different conditions. This extrapolation was the dominant contributor to the 2 MeV uncertainty. At TLEP, instead, it is possible to inject a few non-colliding bunches out of the 4400 (Z pole) or 600 (WW threshold) colliding bunches without significant loss of luminosity, and apply resonant spin depolarization on those. The beam energy will therefore be measured continuously, in the exact same conditions as for the colliding bunches, with a statistical accuracy of 100 keV or (much) better. A precision of 0.1 MeV or better is therefore at hand for the Z width measurement, and the beam energy knowledge is not a concern for the W mass measurement at the WW threshold.