The determination of the mass and the width of the Z boson at CERN's LEP accelerator, an e + e storage ring with a circumference of approximately 27 kilometres, imposes heavy demands on the knowledge of the LEP counter-rotating electron and positron beam energies. The precision required is of the order of 1 MeV or 20 ppm frequency. Due to its size the LEP collider is inuenced by various macroscopic and regional factors such as the position of the moon or seasonal changes of the rainfall in the area, as reported earlier. A new and not less surprising eect on the LEP energy was observed in 1995: railroad trains in the Geneva region perturb the dipole eld. A parasitic ow of electricity, originating from the trains, travels along the LEP ground cable and the vacuum chamber, interacting with the dipole eld. An account of the phenomenon with its explanation substantiated by dedicated measurements is presented.to be submitted to Nuclear Instruments and Methods
Radiative spin-polarization has been used extensively at LEP to accurately measure the beam energy around the Z resonance. As the LEP physics has moved on to the W boson the calibration based on polarization must be extended towards higher beam energies. This is difficult as the depolarizing effects of spin resonances grow rapidly with beam energy. At LEP it has been possible for the first time to measure transverse beam polarization at 60.6 GeV. To allow a build-up of polarization the tunes and the energy were chosen accurately. A low phase advance optics was used and careful orbit correction was carried out using dynamic beam based alignment data. Harmonic spin matching was applied both in a deterministic and a novel semi-empirical way.
The LEP spectrometer performed calibrations of the beam energy in the 2000 LEP run, in order to provide a kinematical constraint for the W boson mass measurement. The beam was deflected in the spectrometer by a steel core dipole, and the bending angle was measured by Beam-Position Monitors on either side of the magnet. The energy determination relies on measuring the change in bending angle when ramping the beam from a reference point at 50 GeV to an energy within the LEP W physics regime, typically 93 GeV. The ratio of integrated bending fields at these settings (approximately 1.18 Tm/0.64 Tm) must be known with a precision of a few 10 −5 .The paper reports on the field mapping measurements which were conducted to determine the bending integral under a range of excitation currents and coil temperatures. These were made in the laboratory before and after spectrometer operation, using a test-bench equipped with a moving arm, carrying an NMR probe and Hall probes, and in the LEP tunnel itself, with a mapping trolley inside the vacuum chamber. The mapping data are related to local readings supplied by fixed NMR probes in the dipole, and a predictive model developed which shows good consistency for all datasets within the estimated uncertainty, which is 14 × 10 −5 for the moving arm, and 3 × 10 −5 for the mapping trolley. Measurements are also presented of the field gradient inside the dipole, and of the environmental magnetic fields in the LEP tunnel. When applied to the spectrometer energy calibrations, the bending field model calculates the ratio of integrated fields with an estimated uncertainty of 1.5 × 10 −5 .To be submitted to Nucl. Inst. and Meth. A.
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