Guy Wilkinson

Abstract:

The rapidity-rank structure of pp̄ pairs is used to analyze the mechanism of baryon production in hadronic Z0 decay. The relative occurrence of the rapidity-ordered configuration p M p̄, where M is a meson, and that of pp̄ adjacent pairs is compared. The data are found to be consistent with predictions from a mechanism producing adjacent-rank pp̄ pairs, without requiring 'string-ordered' p M p̄ configurations. An upper limit of 15% at 90% confidence is determined for the p M p̄ contribution. (C) 2000 Elsevier Science B.V.

Abstract:

Production of ∑- and Λ(1520) in hadronic Z decays has been measured using the DELPHI detector at LEP. The ∑- is directly reconstructed as a charged track in the DELPHI microvertex detector and is identified by its ∑→nπ decay leading to a kink between the ∑- and π-track. The reconstruction of the Λ(1520) resonance relies strongly on the particle identification capabilities of the barrel Ring Imaging Cherenkov detector and on the ionisation loss measurement of the TPC. Inclusive production spectra are measured for both particles. The production rates are measured to be s 0.081 ± 0.002 ± 0.010, = 0.029 ± 0.005 ± 0.005. The production rate of the Λ(1520) suggests that a large fraction of the stable baryons descend from orbitally excited baryonic states. It is shown that the baryon production rates in Z decays follow a universal phenomenological law related to isospin, strangeness and mass of the particles. © 2000 Elsevier Science B.V.

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 θlepteff= 0.23282 ± 0.00092.