Energy Loss Measurements at LEP 2

The accurate measurement of the W Boson mass at LEP requires to determine the beam energy to the highest possible precision. Present schemes rely on accurate energy determination in the range of 40 to 60 GeV using resonant depolarization and on precise extrapolations to high energy. Several methods based on measurements of the energy loss due to synchroton radiation have been studied. Different approaches such as the study of the damping time of transverse oscillations, the radio frequency sawtooth and the dependence of the synchroton tune on the total accelerating voltage are described and results are discussed.