Beam Polarization at the ILC: the Physics Impact and the Accelerator Solutions

In this contribution accelerator solutions for polarized beams and their impact on physics measurements are discussed. Focus are physics requirements for precision polarimetry near the interaction point and their realization with polarized sources. Based on the ILC baseline programme as described in the Reference Design Report (RDR), recent developments are discussed and evaluated taking into account physics runs at beam energies between 100 GeV and 250 GeV, as well as calibration runs on the Z-pole and options as the 1TeV upgrade and GigaZ. The studies, talks and discussions presented at this conference demonstrated that beam polarization and its measurement are crucial for the physics success of any future linear collider. To achieve the required precision it is absolutely decisive to employ multiple devices for testing and controlling the systematic uncertainties of each polarimeter. The polarimetry methods for the ILC are complementary: with the upstream polarimeter the measurements are performed in a clean environment, they are fast and allow to monitor time-dependent variations of polarization. The polarimeter downstream the IP will measure the disrupted beam resulting in high background and much lower statistics, but it allows access to the depolarization at the IP. Cross checks between the polarimeter results give redundancy and inter-calibration which is essential for high precision measurements. Current plans and issues for polarimeters and also energy spectrometers in the Beam Delivery System of the ILC are summarized in reference (28). The ILC baseline design allows already from the beginning the operation with polarized electrons and polarized positrons provided the spin rotation and the fast helicity reversal for positrons will be implemented. A reversal of the positron helicity significantly slower than that of electrons is not recommended to not compromise the precision and hence the success of the ILC. Recently to use calibration data at the Z resonance for physics has been discussed. It looks promising but further studies are needed to evaluate and to optimize these measurements. Finally it should be remarked: many studies on different physics processes and scenarios at a future linear collider are done for high luminosities and high energy assuming small and well-known uncertainties. Polarization, especially positron polarization, is often considered as not that important. But in order to interpret data and to reduce ambiguities in the measurements, the polarization of electrons and positrons and their very precise knowledge are essential. The ILC design must offer this from the beginning to be prepared for the physics questions after years of LHC operation.