Towards precision polarimetry at the ILC: Concepts, simulation and testbeam results

At the International Linear Collider (ILC) the polarisation of the electron and positron beams at the e⁺e⁻ interaction point needs to be determined with a precision of δP/P = 0.25% or even better in case of electroweak precision measurements. This can only be achieved by combining long term polarisation measurements based on e⁺e⁻ collisions with fast measurements of Compton polarimeters placed up- and downstream of the e⁺e⁻ IP. In order to control spin transport effects and depolarisation during the collisions, the Compton polarimeters are aiming for yet unrivaled instrumental precision at the permille level. This can be achieved by dedicated magnet configurations, which need to be designed carefully to take into account the very different beam and background conditions up- and downstream of the IP and to match in an optimal way the Cherenkov detectors forseen for the Compton scattered beam electrons. While the design of the magnetic chicane can be studied currently in simulations only, testbeam studies with the former SLD polarimeter Cherenkov detector have been performed to better understand the sources of limitations of this excellent device.