Guy Wilkinson

Abstract:

The total and differential cross-sections for the reaction e+e- → γγ(γ) are measured at centre of mass energies around 91 GeV using an integrated luminosity of 4.7 pb-1. The aggreement with QED prediction is good. Consequently there is no evidence for non-standard channels which would have the same experimental signature. The lower limits on the QED cuttoff parameters are Λ+ > 113 GeV and Λ- > 95 GeV. An upper limit on the effective coupling between a possible excited electron and the gamma is derived. At 95% confidence level the branching ratios for Z0 decay into π0γ, ηψ and γγγ are below 1.5 × 10-4, 2.8 × 10-4 and 1.4 × 10-4 respectively. © 1991.

Abstract:

The multiplicity distributions of charged particles in restricted rapidity intervals in Z⁰ hadronic decays measured by the DELPHI detector are presented. The data reveal a shoulder structure, best visible for intervals of intermediate size, i.e. for rapidity limits around ±1.5. The whole set of distributions including the shoulder structure is reproduced by the Lund Parton Shower model. The structure is found to be due to important contributions from 3-and 4-jet events with a hard gluon jet. A different model, based on the concept of independently produced groups of particles, "clans", fluctuating both in number per event and particle content per clan, has also been used to analyse the present data. The results show that for each interval of rapidity the average number of clans per event is approximately the same as at lower energies. © 1991 Springer-Verlag.

Abstract:

This paper presents an analysis of the multiplicity distributions of charged particles produced in Z⁰ hadronic decays in the DELPHI detector. It is based on a sample of 25364 events. The average multiplicity is ch>=20.71±0.04(stat)±0.77(syst) and the dispersion D=6.28±0.03(stat)±0.43(syst). The data are compared with the results at lower energies and with the predictions of phenomenological models. The Lund parton shower model describes the data reasonably well. The multiplicity distributions show approximate KNO-scaling. They also show positive forward-backward correlations that are strongest in the central region of rapidity and for particles of opposite charge. © 1991 Springer-Verlag.

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.