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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}, 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.  A technique unique to $\epem$ rings, called resonant spin depolarization [15], will 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 be-   cause 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 LEP3, TLEP,  instead, it will be is  possible to inject a few non-colliding bunches in addition to out of  the 200 4400 (Z pole) or 600 (WW threshold)  colliding bunches, bunches without 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. bunches, with a statistical accuracy of 100 keV or (much) better.  A beam energy uncertainty precision  of 0.1 MeV or better,   i.e.   , with a 10   6   relative precision, better  is therefore at hand, crucial to measure hand for  the Z width with measurement, and the beam energy knowledge is not a concern for the W mass measurement at  the required accuracy. WW threshold.