The Future Circular electron-positron Collider, FCC-ee, is designed for unprecedented precision for particle physics experiments from the Z-pole to above the top-pair-threshold. This demands a precise knowledge of the centre-of-mass energy (ECM) and collision boosts at all four interaction points and all operation energies.
From the Z-pole to the W-pair-region, determining the average beam energies is foreseen using resonant depolarization, with a precision of a few keV, using transversely polarized non-colliding pilot bunches. While wigglers are foreseen to improve the polarization time, misalignment and field errors can limit the achievable polarization level, and might alter the relationship between the resonant depolarization frequency and the beam energies. Strong synchrotron radiation losses range from 40 MeV per turn at the Z-pole, to 10 GeV per turn at the highest beam energy of 182.5 GeV can lead to different ECM and boosts for each interaction point. Beamstrahlung generates further energy losses. Other sources of ECM bias stem from collision offsets and must be controlled. A first evaluation has been published in 2019 with promising results. Further studies are ongoing in the framework of the Feasibility Study to be delivered in 2025 and a first set of further improvements on energy calibration, polarization and monochromatization are presented here.