Guy Wilkinson

Abstract:

A test of the benchmark QED process e⁺e^- → γγ(γ) is reported, using the data collected with the DELPHI detector at LEP 2. The data analysed were recorded at centre-of-mass energies ranging from 161 GeV to 208 GeV and correspond to a total integrated luminosity of 656.4 pb^-1. The Born cross-section for the process e ⁺e^- → γγ(γ) was determined, confirming the validity of QED at the highest energies ever attained in electron-positron collisions. Lower limits on the parameters of a number of possible deviations from QED, predicted within theoretical frameworks expressing physics beyond the Standard Model, were derived. © Springer-Verlag/ Società Italiana di Fisica 2004.

Abstract:

Data from Z decays in DELPHI have been searched for Bd ⁰ → D*⁺ℓ^-νℓ with the D*⁺ decaying to D⁰π⁺ and D⁰ → K^-π⁺, K^-π ⁺π⁺π^- or K^-π ⁺(π⁰). These events are used to measure the CKM matrix element |Vcb| and the form factor slope, ρA1 ²: ℱD* (1) |Vcb| = 0.0392 ± 0.0018 ± 0.0023; ρA1² = 1.32 ± 0.15 ± 0.33 corresponding to a branching fraction: BR(Bd ⁰ → D*⁺ℓ^-νℓ) = (5.90 ± 0.22 ± 0.50)%. Combining these and previous DELPHI measurements gives: ℱD* (1) |Vcb| = 0.0377 ± 0.0011 ± 0.0019, ρA1² = 1.39 ± 0.10 ± 0.33 and BR(Bd⁰ → D* ⁺ℓ^-νℓ) = (5.39 ± 0.11 ± 0.34)% Using ℱD* (1) = 0.91 ± 0.04, yields: |Vcbl = 0.0414 ± 0.0012(stat.) ± 0.0021(syst.) ± 0.0018(theory). The b-quark semileptonic branching fraction into a D*⁺ emitted from higher mass charmed excited states has also been measured to be: BR(b → D*⁺Xℓ^-ν ℓ) = (0.67 ± 0.08 ± 0.10)%.

Abstract:

Oscillations in the Bs⁰-Bs⁰ system were studied in events selected from about 4.3 million hadronic Z ⁰ decays registered by DELPHI between 1992 and 2000. This paper presents updates of two published analyses ([11,12]). The first analysis, which utilizes leptons emitted with large momentum transverse to a jet, was improved by means of a better algorithm for the vertex reconstuction and a new algorithm for flavour-tagging at production time. The second analysis, which utilizes Ds-lepton events, was improved by optimizing the treatment of proper time resolution. No signal of B⁰ oscillations was observed and limits on the mass difference between the physical B⁰ states were obtained to be: δms > 8.0ps^-1 at the 95% C.L. with a sensitivity of δms = 9.1 ps^-1 in the high pt lepton analysis and δms < 4.9 ps-1 at the 95% C.L. with a sensitivity of δms = 8.6 ps^-1 in the D s-lepton analysis. Previously published results on these analyses are superseded. The combination of these results with those obtained in other independent analyses previously performed in DELPHI (Ds-hadron, exclusive Bs⁰, inclusive vertex) gives: δm, > 8.5 ps^-1 at the 95% C.L. with a sensitivity of δms = 12.0 ps-1.