Calibration of the Vepp-4m Collider Beam Energy by Infrared Laser

For J/ψ, ψ′, and τ masses measurement with the KEDR detector, the VEPP-4M collider is planned to operate at 1200–2000 MeV beam energies. Electron beam energy calibration with backscattered laser photons, performed at BESSY I storage ring [1], has shown the accuracy compatible with that obtained by resonant spin depolarization technique. We propose the non-head-on configuration of electron-laser beams interaction: electron beam intersects the CO2 laser cavity axis at close to π/2 angle. The resulting spectrum of backscattered Compton photons has two edges, their sum is used to determine the electron beam energy and depends weakly from the intersection angle. The estimated accuracy for beam energy calibration is about 10−4.