Polarized beams for CEPC: physics requirements, design strategy, and enabling technologies

Polarized lepton beams can substantially extend the physics capability of the Circular Electron Positron Collider (CEPC). Transversely polarized pilot bunches enable resonant-depolarization beam-energy calibration at the Z and W energies, while longitudinally polarized colliding beams would provide an additional spin observable for precision electroweak measurements and searches for physics beyond the Standard Model. This article reviews the polarization requirements for CEPC and presents an updated source-to-collider strategy for generating, preserving, manipulating, and measuring beam polarization. The proposed scheme combines a polarized electron source, a dual-purpose positron damping and polarizing ring, spin-preserving transport through the injector chain, booster-lattice optimization to avoid strong spin resonances, solenoid-based spin rotators in the collider ring, and Compton polarimetry. Recent studies indicate that polarization transmission above 70% through the injector chain is achievable for applications at the Z and W energies, whereas polarization at the Higgs and tt̅ energies remains challenging because of stronger spin-resonance effects. The associated R&D program is summarized, and the main open issues for the CEPC Engineering Design Phase are identified.