deletions | additions       diff --git a/Longitudinal Polarization Experiments.tex b/Longitudinal Polarization Experiments.tex  index 311183e..a65873a 100644  --- a/Longitudinal Polarization Experiments.tex  +++ b/Longitudinal Polarization Experiments.tex 
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  Measurements with longitudinal polarization of the $\epem$ system require maintaining beam polarization in collisions. Transverse beam polarization of 40\% in collisions was observed at LEP at Z pole energies (45 GeV per beam) with one collision point, a beam-beam tune shift of 0.04, and a single bunch luminosity of $10^{30}/cm^{2} /s$ ~\cite{cite:LEP-beam-beam-pol}. This would translate for TLEP, taking into account of the smaller value of $\beta_y^{*}$ and the larger number of bunches, in a luminosity of around $10^{35}/cm^{2} /s $ for the same total storage ring beam-beam tune shift. In addition to the polarization wigglers, movable spin rotators as designed for HERA~\cite{cite:HERA-beams} would allow a program of longitudinal polarized beams at the Z peak. The spin rotator design foreseen for LEP requires tilting the experiments and is unpractical for TLEP. Assuming the same level of polarization in collisions at TLEP than what was observed at LEP, and that a fraction of the bunches can be selectively depolarized to ensure longitudinal polarization of the $\epem$ system, a simultaneous measurement~\cite{Blondel:1987wr} of the beam polarization and of the left-right asymmetry $A_{LR}$ can be envisaged. For one year of data taking a precision on $A_{LR}$ of the order of $10^{-5}$ – or a precision on $\sin^2\theta_W^{eff}$ of the order of $10^{-6}$ would be achieved. Other beam polarization asymmetries for selected final states, $A_{FB}^{pol,f}$, will allow precise measurements of the electroweak couplings as well as being an interesting tool for flavour selection.   Obtaining longitudinal polarization at higher energies requires a cancellation of depolarization 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 and can be overruled by polarization wigglers suitably disposed around the ring. These schemes will need to be worked out and simulated before the feasibility of longitudinal polarization in high energy collisions can be asserted.