M. Acciarri et al., Measurements of cross sections and forward-backward asymmetries at the Z resonance and determination of electroweak parameters, EUR PHY J C, 16(1), 2000, pp. 1-40
We report on measurements of hadronic and leptonic cross sections and lepto
nic forward backward asymmetries performed with the L3 detector in the year
s 1993-95. A total luminosity of 103 pb(-1) nas collected at centre-of-mass
energies root s approximate to m(Z) and root s approximate to m(Z) +/- 1.8
GeV which corresponds to 2.5 million hadronic and 245 thousand leptonic ev
ents selected. These data lead to a significantly improved determination of
Z parameters. From the total cross sections, combined with our measurement
s in 1990-92, we obtain the final results:
m(Z) = 91 189.8 +/- 3.1 MeV, Gamma(Z) = 2502.4 +/- 4.2 MeV,
Gamma(had) = 1751.1 +/- 3.8 MeV, Gamma(l) = 84.14 +/- 0.17 MeV.
An invisible width of Gamma(inv) = 499.1 +/- 2.9 MeV is derived which in th
e Standard Model yields for the number of light neutrino species N-nu = 2.9
78 +/- 0.014. Adding our results on the leptonic forward-backward asymmetri
es and the tau polarisation, the effective vector and axial-vector coupling
constants of the neutral weak current to charged leptons are determined to
be (g) over bar(V)(l) = -0.0397+/-0.0017 and (g) over bar A(l) = -0.50153/-0.00053. Including our measurements of the Z --> b (b) over bar forward-b
ackward and quark charge asymmetries a value for the effective electroweak
mixing angle of sin(2)<(theta)over bar>(W) = 0.23093 +/- 0.00066 is derived
. All these measurements are in good agreement with the Standard Model of e
lectroweak interactions. Using all our measurements of electroweak observab
les an upper limit on the mass of the Standard Model Higgs boson of m(H) <
133 GeV is set at 95% confidence level.