CERN: Another leap at LEP

When the 1994 physics run at CERN's LEP electron-positron collider finished at the end of November, integrated luminosity reached 64.5 inverse picobarns (October 1994, page 1), more than one and a half times as much as collected in 1993, itself 40% more than the 1992 score. So far the four experiments have seen well over 12 million Z particles. During most of the year, eightbunch 'pretzel' running (October 1992, page 17) was standard. After five years, the LEP team have few remaining tricks up their sleeves, and the main emphasis in 1994 was to go for optimal efficiency (maximum vertical beam-beam tune shift). This paid off and by late summer, the luminosity aggregate had already surpassed 60 inverse picobarns. In preparation for the higher energy LEP2 scheme, scheduled to begin operations in 1996, additional beam separators and superconducting radiofrequency accelerating cavities were installed in late summer. Towards the end of the year another harbinger of LEP2 was the abandoning of eight-bunch pretzel running in favour of 'trains' of bunches, foreseen as the standard scheme for LEP2. During 1994, LEP operations surpassed the magic figure of an inverse picobarn of integrated luminosity in 24 hours, while peak luminosity attained 2.2 x 10³¹ cm^-2 s^-1 well above the design figure of 1.3 x 10³¹. (In our November issue, we wrongly cited this design luminosity as 1.7 x 10³¹.) Polarization gives a precision fix on beam energy (September 1993, page 5) to within 1.5 MeV at the Z resonance. LEP has developed into such a precision machine that it is making its mark on geodesy. The effect of earth tides on the beams, once hailed as a spectacular breakthrough in accuracy (January 1993, page 4), is now considered unfortunate background, with people grumbling about their inability to do anything about it. The behaviour of LEP beams is now limited by another factor nobody can do anything about - rainfall! LEP can now detect the height of the water table in the nearby Jura mountains and the level of water in Lake Geneva