DOI 10.1007/s100520000353
Precision luminosity for Z0 lineshape
measurements with a silicon-tungsten calorimeter
The OPAL Collaboration
G.Abbiendi
- et al.
2Dipartimento di Fisica dell' Università di Bologna and INFN,
40126 Bologna, Italy
Received: 28 September 1999 / Published online: 14 April 2000 - © Springer-Verlag 2000
Abstract
The measurement of small-angle Bhabha scattering
is used to
determine the luminosity at the OPAL interaction point
for the LEP I data recorded between 1993
and 1995.
The measurement is based on
the OPAL Silicon-Tungsten Luminometer
which is composed of two calorimeters
encircling the LEP beam pipe, on opposite sides of the
interaction point.
The luminometer detects electrons
from small-angle Bhabha scattering at angles between 25 and 58 mrad.
At LEP center-of-mass energies around the
,
about half
of all Bhabha electrons entering the detector fall within a 79 nb
fiducial acceptance region.
The electromagnetic showers generated in the stack of
1 radiation length tungsten absorber plates are
sampled by 608 silicon detectors
with 38,912 radial pads of 2.5 mm width.
The fine segmentation of the detector,
combined with the precise knowledge of its physical dimensions,
allows the
trajectories of incoming 45 GeV electrons or photons to be determined
with a
total systematic error of less than 7 microns. We have quantified all
significant sources of systematic experimental error in the luminosity
determination by direct physical measurement.
All measured properites of the luminosity event sample are found
to be in agreement with current theoretical expectations.
The total systematic measurement uncertainty is
,
significantly below the theoretical error of
currently assigned to the QED calculation of the Bhabha
acceptance, and contributes negligibly to the
total uncertainty in the OPAL measurement
of
,
a quantity of basic physical interest which depends
crucially on the luminosity measurement.
Copyright Società Italiana di Fisica, Springer-Verlag 2000
