Determination of the LEP center-of-mass energy from Zgamma events
  Radiative returns to the Z resonance (Zgamma events) provide a method to determine the centre-of-mass energy of the high energy e+e- colliders and already at LEP2. The method is applied for the data collected with the ALEPH detector in 1997 at 181-184 GeV, using e+e- -> gammaZ -> hadrons events. 
ALEPH Collaboration