predictions of the Glashow weak interactions, can be tested The
(LEP)
Positron Collider to
1993, nearly
tector. The
Weinberg model, the theory of electrowith very high precision at the Large Electron CERN. During the period of data taking from 1990
at
Salam
two million Z-boson events have been recorded with the L3 de¬
sample
contains about 64000 Z-boson
decays
into
pairs (e+e~). This particular decay channel has been used
lings
of the electroweak neutral current to electrons.
cross
sections aee
as
well
electron-positron to study the coup¬
With the L3 detector
forward/backward asymmetries AeFB
as
have been
measured at different
energies of the LEP machine in the vicinity of the Z resonance. electromagnetic calorimeter BGO crystals are used to de¬ termine the energies of electrons, positrons and photons with high precision. Momentum and angular directions of charged particles are measured with the vertex chamber TEC. This drift chamber has been carefully calibrated using muon tracks of known momenta to optimize the resolution for the measure¬ In the
ment of transverse momenta.
At pj
achieved for 83 % of the tracks.
topology
—
Using
40 GeV
a
resolution of 68 % has been
the additional
knowledge of the event decays the charges charge confusion is de¬
of 'radiationless' e+e~ final states from Z-boson
high momentum tracks are assigned. The termined using fi+n~ events and is 3.1±0.2% in 1992 and 4.5±0.4% for an equivalent period in 1993. By restricting the acceptance to the high resolution regions of TEC it is possible to reduce the charge confusion to 2%. The measurement of the cross section for e+e~ —> (Z, 7) —* e+e~ is based on the selection of events with two electromagnetic clusters with high energy in the BGO calorimeter. The large statistics in the data taking periods 1992 of the two
and 1993 allow of the 0.0072
cross
precise determinations of the cross sections. The measurement section aee in 1992 on the Z-peak gives 1.053 ± 0.007 (stat.) ±
(syst.)
statistical
nb. The four datasets of 1993 at different beam
errors
of about 1.1% for each
contributions from the 1993
luminosity
period.
The
systematic
energies errors
have
contain
determination of 0.6% in 1992 and 0.3% in
respectively.
forward/backward asymmetry AeFB the signs of the e+e~-pair must be determined. After correcting the chargecharges confusion of TEC and inefficiencies of the BGO calorimeter the asymmetries for different energies near the Z-peak are obtained. The statistical errors are larger than the systematic errors and therefore the asymmetry AFB will re¬ main an interesting observable for future data taking periods. The measurements of cross sections and asymmetries are used to determine The values obtained the effective vector and axial vector couplings ]jy und ^ For the measurement of the
for each
.
by
the L3-collaboration in the years 1990-1993
Messpunkt
Wirkungsquerschnitte im Jahre 1992
am
mit
guter
Maximum der
Z-Resonanz liefert im betrachteten Intervall fur den Polarwinkel
von
44° bis
aee Wirkungsquerschnitt (stat.) (syst.) nb. Fur die vier Messperioden des Jahres 1993 bei verschiedenen Strahlenergien ergeben sich statistische Fehler von 1.1% pro Messpunkt. Der
136° einen
von
systematische
Fehler enthalt einen
Luminositat in der
=
Beitrag
Messperiode 1992,
1.053 ± 0.007
von
0.6%
aus
der
± 0.0072
Bestimmung
der
bzw. 0.3% fur 1993.
Fur die
Messung der Vorwarts-Ruckwarts Asymmetrie AFB mussen die beiLadungsvorzeichen der e+e~-Paare bestimmt werden. Nach Korrektur der Ladungskonfusion und der Ineffizienz des BGO-Kalorimeters werden die Asymmetrien fur die verschiedenen Strahlenergien im Bereich der Z-Resonanz den
bestimmt. matische
Die statistischen Fehler dieser Werte sind grosser als der syste¬
Fehler, sodass Messgrosse darstellt.
die
Asymmetrie AFB
6
auch weiterhin eine interessante
Wirkungsquerschnitten und Asymmetrien werden die Axialvektor-Kopplungskonstanten
