Energy calibration at LEP using Nuclear Magnetic Resonance probes

The accurate Standard Model investigations carried out at LEP require knowledge of the beam energies of the order of a few 10-5. The resonant depolarisation method, used for absolute calibration in de dicated experiments, cannot be used to monitor continuously the beam energy during the physics runs. Moreover appreciable polarisation of the beams has not been measured above energies of 55 GeV. A me thod for continuous energy monitoring based on Nuclear Magnetic Resonance (NMR) probes mounted in tunnel magnets has been in use at LEP since 1995. The average field of the dipole magnets is sampled v ia 24 NMR probes mounted in the gap of the C-shaped yokes on top of the vacuum chamber. The probes are distributed over the 27 km of the accelerator. The probes are used for the continuous monitoring of the field during LEP operation and to determine the absolute field value.