THE INFLUENCE OF TRAIN LEAKAGE CURRENTS ON THE LEP DIPOLE FIELD

The determination of the mass and the width of the Z boson at CERN's L EP accelerator, an e(+)e(-) storage ring with a circumference of appro ximately 27 km, imposes heavy demands on the knowledge of the LEP coun ter-rotating electron and positron beam energies. The precision requir ed is of the order of 1 MeV or approximate to 20 ppm. Due to its size, the LEP collider is influenced by various macroscopic and regional fa ctors such as the position of the moon or seasonal changes of the rain fall in the area, as reported earlier. A new and not less surprising e ffect on the LEP energy was observed in 1995: railroad trains in the G eneva region perturb the dipole held. A parasitic flow of electricity, originating from the trains, travels along the LEP vacuum chamber, af fecting the LEP dipole held. An account of the phenomenon with its exp lanation substantiated by dedicated measurements is presented. (C) 199 8 Elsevier Science B.V. All rights reserved.