Review of beam energy measurements at VEPP-4M collider

Another approach allows to calculate beam energy via the maximum energy of backscattering laser photons. The Compton BackScattering (CBS) monitor allows continuous on-line monitoring of the beam energy with accuracy about 150 keV, which is critical during the τ lepton mass measurement. The statistical error for a 1 h period is about 100 keV, the present systematic error is 50-70 keV.