Movable spin rotators as designed for HERA \cite{cite:HERA-beams} would therefore allow a program of longitudinally polarized beams at the Z peak. (The spin rotator design foreseen for LEP requires tilting the experiments and is unpractical for TLEP.) For the same level of polarization in collisions as that observed at LEP, and assuming that a fraction of the bunches can be selectively depolarized, a simultaneous measurement \cite{Blondel:1987wr} of the beam polarization and of the left-right asymmetry \(A_{\rm LR}\) can be envisioned at TLEP.

As mentioned above, the maximum level of polarization is limited by the increase of the beam energy spread when the beam energy increases. The establishment of longitudinal polarization at higher energies therefore requires a cancellation of depolarizing effects, by reducing the spin-tune spread associated with the energy spread. Siberian snake solutions \cite{cite:Wienans-TLEP4} invoking combinations of spin rotators situated around the experiments and polarization wigglers are being discussed. They take advantage of the fact that the TLEP arcs have very low fields, which can be overruled by polarization wigglers suitably disposed around the ring. An example is displayed in Fig. \ref{fig:longitudinal}. These schemes need to be worked out and simulated before the feasibility of longitudinal polarization in high-energy collisions can be asserted.