Determination of the LEP centre-of-mass energy from Zγ events

Abstract Radiative returns to the Z resonance ( Zγ events) are used to determine the LEP2 centre-of-mass energy from the data collected with the ALEPHdetector in 1997. The average centre-of-mass energy is measured to be: E CM =182.50±0.19 ( stat. )±0.08 ( syst. ) GeV in good agreement with the precise determination by the LEP energy working group of 182.652±0.050 GeV. If applied to the measurement of the W mass, its precision translates into a systematic erroron M W which is smaller than the statistical error achieved from the corresponding dataset.