Self-polarization for energy calibration at FCC-ee Z and WW energies
In plain words
Measuring a collider's beam energy to a part per million uses spins that line up by themselves through synchrotron radiation; at higher energies this alignment is weak and easily destroyed.
Precise statement
Resonant depolarization gives the beam energy to below 1e-6 relative precision if a transverse polarization of about 5-10 percent builds up by the Sokolov-Ternov effect. For a $\sim 9e6\, \mathrm{cm}$ circumference ring at $E = 45.6\, \mathrm{GeV}$ and $E \sim 80\, \mathrm{GeV}$, determine the equilibrium polarization and build-up time with polarization wigglers, including depolarization from energy spread $\sigma_E/E \sim 4e-4 \text{ to } 1e-3$ and lattice errors, and whether calibration at the WW threshold is feasible.
What would settle it
Spin-diffusion calculations with realistic errors and wiggler settings, validated against LEP polarization data.