Guy Wilkinson

Abstract:

Measurements of the cross section and forward-backward asymmetry for the reaction e+e- → μ+μ- using the DELPHI detector at LEP are presented. The data come from a scan around the Z0 peak at seven centre of mass energies, giving a sample of 3858 events in the polar angle region 22° < θ < 158°. From a fit to the cross section for 43° < θ < 137°, a polar angle region for which the absolute efficiency has been determined, the square root of the product of the Z0 → e+e- and Z0 → μ+μ- partial widths is determined to be (ΓeΓμ) 1 2 = 85.0 ± 0.9(stat.) ± 0.8(syst.) MeV. From this measurement of the partial width, the value of the effective weak mixing angle is determined to be sin2(θw) = 0.2267 ± 0.0037. The ratio of the hadronic to muon pair partial widths is found to be Γh/Γμ = 19.89 ± 0.40(stat.) ± 0.19(syst.). The forward-backward asymmetry at the resonance peak energy ECMS = 91.22 GeV is found to be AFB = 0.028 ± 0.020(stat.) ± 0.005(syst.). From a combined fit to the cross section and forward-backward asymmetry data, the products of the electron and muon vector and axial-vector coupling constants are determined to be VeVμ = 0.0024 ± 0.0015(stat.) ± 0.0004(syst.) and AeAμ = 0.253 ± 0.003(stat.) ± 0.003 (syst.). The results are in good agreement with the expectations of the minimal standard model. © 1991.

Abstract:

A search for light Higgs bosons was performed using the data sample collected in 1990 by the DELPHI detector at LEP, at centre of mass energies between 88.2 and 94.2 GeV. Using the process e+e-→H0+Z0*, Z0*→ff, it is possible to exclude the existence of the standard model Higgs particle with a mass between 0 and 210 MeV/c2 at the 99% confidence level. Extending this analysis to the minimal supersymmetric standard model restricts the lightest neutral Higgs boson to masses above 28 GeV/c2 irrespective of the value of the mixing angle. © 1991 Springer-Verlag.

Abstract:

In four-jet events from e+e- →Z0 →multihadrons one can separate the three principal contributions from the triple-gluon vertex, double gluon-bremsstrahlung and the secondary quark-antiquark production, using the shape of the two-dimensional angular distributions in the generalized Nachtmann-Reiter angle θNR* and the opening angle of the secondary jets. Thus one can identify directly the contribution from the triple-gluon vertex without comparison with a specific non-QCD model. Applying this new method to events taken with the DELPHI-detector we get for the ratio of the colour factor Nc to the fermionic Casimir operator CF: Nc CF= 2.55 ± 0.55 (stat.) ± 0.4 (fragm. + models) ± 0.2 (error in bias) in agreement with the value 2.25 expected in QCD from Nc=3 and CF= 4 3. © 1991.