Precision luminosity for Z(0) lineshape measurements with a silicon-tungsten calorimeter

The measurement of small-angle Bhabha scattering is used to determine the l uminosity at the OPAL interaction point for the LEP I data recorded between 1993 and 1995. The measurement is based on the OPAL Silicon-Tungsten Lumin ometer which is composed of two calorimeters encircling the LEP beam pipe, on opposite sides of the interaction point. The luminometer detects Electro ns from small-angle Bhabha scattering at angles betn een 25 and 58 mrad. At LEP center-of-mass energies around the Z(0), about half of all Bhabha elec trons entering the detector fall within a 79 nb fiducial acceptance region. The electromagnetic showers generated in the stack of 1 radiation length t ungsten absorber plates are sampled by 608 silicon detectors with 38,912 ra dial pads of 2.5 mm width. The fine segmentation of the detector, combined with the precise knowledge of its physical dimensions, allows the trajector ies of incoming 45 GeV electrons or photons to be determined with a total s ystematic error of less than 7 microns. We have quantified all significant sources of systematic experimental error in the luminosity determination Lv direct physical measurement. All measured properites of the luminosity eve nt sample are found to bk in agreement with current theoretical expectation s. The total systematic measurement uncertainty is 3.4x10(-4), significantl y below the theoretical error of 5.4x10(-4) currently assigned to the QED c alculation of the Bhabha acceptance, and contributes negligibly to the tota l uncertainty in the OPAL measurement of Gamma(inv)/Gamma(l+l-), a quantity of basic physical interest which depends crucially on the luminosity measu rement.