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 Bd0 → 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, ρA12: ℱD* (1) |Vcb| = 0.0392 ± 0.0018 ± 0.0023; ρA12 = 1.32 ± 0.15 ± 0.33 corresponding to a branching fraction: BR(Bd0 → 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.