2020 Impact factor 4.590
Particles and Fields
Eur. Phys. J. C 16, 1-40
DOI 10.1007/s100520000390

Measurements of cross sections and forward-backward asymmetries at the Z resonance and determination
of electroweak parameters

The L3 Collaboration

M.Acciarri26 - et al.

26 INFN-Sezione di Milano, 20133 Milan, Italy

Received: 4 February 2000 / Published online: 18 May 2000 - © Springer-Verlag 2000
Dedicated to the memory of Prof. Dr. Klaus Schultze

We report on measurements of hadronic and leptonic cross sections and leptonic forward-backward asymmetries performed with the L3 detector in the years 1993-95. A total luminosity of $103\ \ensuremath{\mathrm{pb^{-1}}} $ was collected at centre-of-mass energies $\ensuremath{\sqrt{s}}\approx \ensuremath{m_{\mathrm{Z}}} $ and $\ensuremath{\sqrt{s}}\approx \ensuremath{m_{\mathrm{Z}}}\pm 1.8\ \ensuremath{\mathrm{Ge\kern -0.1em V}} $which corresponds to 2.5 million hadronic and 245 thousand leptonic events selected. These data lead to a significantly improved determination of \ensuremath{\mathrm {Z}} parameters. From the total cross sections, combined with our measurements in 1990-92, we obtain the final results:

An invisible width of $\ensuremath{\Gamma_{\mathrm{inv}}} = \ensuremath{499.1}\pm \ensuremath{2.9}\ \ensuremath{\mathrm{Me\kern -0.1em V}} $is derived which in the Standard Model yields for the number of light neutrino species $N_{\mathrm{\nu}} = \ensuremath{2.978}\pm \ensuremath{0.014} $. Adding our results on the leptonic forward-backward asymmetries and the tau polarisation, the effective vector and axial-vector coupling constants of the neutral weak current to charged leptons are determined to be $\ensuremath{\ensuremath{\bar{g}_{\mathrm{V}}}^{\mathrm{\ell}}} = \ensuremath{-0.0397}\pm \ensuremath{0.0017} $ and $\ensuremath{\ensuremath{\bar{g}_{\mathrm{A}}}^{\mathrm{\ell}}} = \ensuremath{-0.50153}\pm \ensuremath{ 0.00053} $. Including our measurements of the $\ensuremath{\mathrm {Z}}\rightarrow\ensuremath{\mathrm{b\bar{b}}} $ forward-backward and quark charge asymmetries a value for the effective electroweak mixing angle of $\ensuremath{\sin^2\!\overline{\theta}_{\mathrm{W}}} = \ensuremath{0.23093}\pm \ensuremath{0.00066} $ is derived. All these measurements are in good agreement with the Standard Model of electroweak interactions. Using all our measurements of electroweak observables an upper limit on the mass of the Standard Model Higgs boson of $\ensuremath{m_{\mathrm{H}}} < \ensuremath{133}\ \ensuremath{\mathrm{Ge\kern -0.1em V}} $ is set at 95% confidence level.

Copyright Società Italiana di Fisica, Springer-Verlag 2000