Luminosity measurement in the L3 detector at LEP

Abstract One of the limiting factors in the determination of the electroweak parameters from cross section measurements of e + e − annihilation close to the Z pole is the precision of the luminosity measurement. The luminosity monitor of the L3 detector at LEP and the analysis of its data are described. Using a combination of a BGO calorimeter and a 3-layer silicon tracker, the absolute luminosity has been measured with an experimental precision of 0.08% in 1993 and 0.05% in 1994. The measurement relies on a detailed understanding of small-angle elastic e + e − (Bhabha) scattering from the experimental and theoretical point of view, as well as an excellent knowledge of the detector geometry.