Guy Wilkinson

Abstract:

A study of b semileptonic decays into D, Dπ± and D*π± final states is presented. The D0, D+ and D*+ mesons are exclusively reconstructed in Z decay data recorded from 1992 to 1995 in the DELPHI experiment at LEP. The overall branching fractions are measured to be: BR(b→ D0ℓ-νℓX) = (7.04 + 0.34 (stat) + 0.36 (syst.exp) + 0.17 (BRD))%, BR(b→ D+ℓ-νℓX) = (2.72 + 0.19 (stat) + 0.16 (syst.exp) + 0.18 (BRD))%, BR(b→ D*+ℓ-νℓX) = (2.75 + 0.17 (stat) + 0.13 (syst.exp) + 0.09 (BRD))%, where the D0 and D+ results include also contributions from D*0 and D*+ decays. A fit to the distribution of the π± impact parameter to the primary interaction vertex provides a measurement of the b semileptonic branching fractions into the D0π±X, D+π±X and D*+π±X final states. Assuming that single pion decay modes of B mesons dominate, the partial rates for B→Dπℓ-νℓ and B→D*πℓ-νℓ have been obtained, corresponding to a total branching fraction: BR(B→Dπℓ-νℓ) +BR(B→D*πℓ-νℓ) = (3.40 + 0.52 (stat) + 0.32 (syst))%. This result agrees well with the observed difference between the total B semileptonic branching fraction and the sum of the B→Dℓ-νℓ and B→D*ℓ-νℓ branching fractions. © 2000 Elsevier Science B.V.

Abstract:

The τ polarisation has been studied with the e⁺e^- → τ⁺τ^- data collected by the DELPHI detector at LEP in 1993, 1994 and 1995 around the Z resonance firstly through the exclusive decay channels evv̄, μvvμ, πv, ρv, and a1v and secondly with an inclusive hadronic analysis which benefits from a higher efficiency and a better systematic precision. The results have been combined with those previously published on 1990 to 1992 DELPHI data, to produce results which reflect the full LEP-1 statistics. The fit of the τ polarisation dependence on the production angle yielded the polarisation parameters Aτ = 0.1359 ± 0.0096 and Ae = 0.1382 ± 0.0116. From these results the ratio of the vector and axial-vector effective couplings v̄τ/āτ = 0.0683 ± 0.0048 and v̄e/āe = 0.0694 ± 0.0058 have been derived, compatible with e-τ universality. With the assumption of lepton universality, the ratio of vector to axial-vector effective couplings for leptons v̄ι/āι = 0.0687 ± 0.0037 was obtained, implying a value of the effective weak mixing angle sin² θ^lepteff= 0.23282 ± 0.00092.

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.