Guy Wilkinson

Abstract:

This paper describes a measurement of the Michel parameters, (Formula presented.), and the average (Formula presented.) helicity, (Formula presented.), in τ lepton decays together with the first measurement of the tensor coupling in the weak charged current. The (Formula presented.) pairs were produced at the LEP e⁺e⁻ collider at CERN from 1992 through 1995 in the DELPHI detector. Assuming lepton universalityin the decays of the τ the measured values of the parameters were: (Formula presented.). The strength of the tensor coupling was measured to be κ^Wτ = −0.029 ± 0.036 ± 0.018. The first error is statistical and the second error is systematic in all cases. The results are consistent with the V −A structure of the weak charged current in decays of the τ lepton.

Abstract:

Inclusive distributions of charged particles in hadronic W decays are experimentally investigated using the statistics collected by the DELPHI experiment at LEP during 1997, 1998 and 1999, at centre-of-mass energies from 183 to around 200 GeV. The possible effects of interconnection between the hadronic decays of two Ws are not observed. Measurements of the average multiplicity for charged and identified particles in qq̄ and WW events at centre-of-mass energies from 130 to 200 GeV and in W decays are presented. The results on the average multiplicity of identified particles and on the position ξ* of the maximum of the ξp = -log(2p/√s) distribution are compared with predictions of JETSET and MLLA calculations.

Abstract:

An updated analysis using about 1.5 million events recorded at √s = Mz with the DELPHI detector in 1994 is presented. Eighteen infrared and collinear safe event shape observables are measured as a function of the polar angle of the thrust axis. The data are compared to theoretical calculations in script O(α²s) including the event orientation. A combined fit of αs and of the renormalization scale xμ in script O(α²s) yields an excellent description of the high statistics data. The weighted average from 18 observables including quark mass effects and correlations is αs(M²z) = 0.1174±0.0026. The final result, derived from the jet cone energy fraction, the observable with the smallest theoretical and experimental uncertainty, is αs(M²z) = 0.1180 ± 0.0006(exp.) ± 0.0013(hadr.) ± 0.0008(scale) ± 0.0007(mass). Further studies include an αs determination using theoretical predictions in the next-to-leading log approximation (NLLA), matched NLLA and script O(α²s) predictions as well as theoretically motivated optimized scale setting methods. The influence of higher order contributions was also investigated by using the method of Padé approximants. Average αs values derived from the different approaches are in good agreement.

Abstract:

During 1993 and 1995 LEP was run at 3 energies near the Z⁰ peak in order to give improved measurements of the mass and width of the resonance. During 1994, LEP operated only at the Z⁰ peak. In total DELPHI accumulated data corresponding to an integrated luminosity of approximately 116 pb^-1. Analyses of the hadronic cross-sections and of the cross-sections and forward-backward asymmetries in the leptonic channels used the most precise evaluations of the LEP energies. In the dimuon channel, events with a photon radiated from the initial state have been used to probe the cross-sections and asymmetries down to PETRA energies. Model independent fits to all DELPHI lineshape and asymmetry data from 1990 to 1995 have been carried out giving values of the resonance parameters: MZ = 91.1863 ± 0.0028 GeV ΓZ = 2.4876 ± 0.0041 GeV σ0 = 41.578 ± 0.069 nb R1 = 20.730 ± 0.060 AFB⁰ = 0.0187 ± 0.0019. These values are significantly more precise than those previously published. The results are interpreted in terms of the Standard Model.

Abstract:

The probability P(c → D*+) that a charm quark fragments into a D*+ meson and the c → l+ semileptonic branching fraction were measured in Z0 decay into cc̄ events. From the analysis of 3.5 Million Z0 events collected from 1992 to 1995, a sample of charm meson decays with 81% cc̄ purity was selected. The product of the c → D*+ fragmentation probability times the D*+ → D0π+ branching fraction was measured to be: P(c → D*+) · BR(D*+ → D0π+) = 0.174 ± 0.010(stat) ± 0.004(syst). Using the world averaged value for BR(D*+ → D0π+), the fragmentation probability is inferred: P(c → D*+) = 0.255 ± 0.015(stat) ± 0.006(syst) ± 0.005(syst.BR). From the same sample, 1828 ± 51 identified leptons in the opposite hemisphere were selected. From this sample, the charm semileptonic branching fraction was measured to be: BR(c → l+) = 0.0958 ± 0.0042(stat) ± 0.0028(syst).