Guy Wilkinson

Abstract:

Searches for pair production of gauginos and squarks in e⁺e^- collisions at a centre-of-mass energy of 189 GeV have been performed on data corresponding to an integrated luminosity of 158 pb^-1 collected by the DELPHI detector at LEP. The data were analyzed under the assumption of non-conservation of R-parity through a single dominant Ū D̄ D̄ coupling between squarks and quarks. Typical final states contain between 4 and 10 jets with or without additional leptons. No excess of data above Standard Model expectations was observed. The results were used to constrain domains of the MSSM parameter space and derive limits on the masses of supersymmetric particles. The following mass limits at 95% CL were obtained from these searches: neutralino mass: mx̃1/⁰ ≥ 32 GeV; chargino mass: mx̃1/⁺ ≥ 94 GeV; stop and sbottom mass (indirect decay) with δAM > 5 GeV: mt̃1 ≥ 74 GeV for φmix = 0 rad, mt̃1 ≥ 59 GeV for φmix = 0.98 rad, mb̃1 ≥ 72 GeV for φmix = 0 rad. The angle φmix is the mixing angle between left and right handed quarks. © 2001 Elsevier Science B.V.

Abstract:

Searches for the Standard Model Higgs boson have been performed in the data collected by the DELPHI experiment at LEP in the year 2000 at centre-of-mass energies between 200 and 209 GeV corresponding to a total integrated luminosity of 224 pb-1. No evidence for a Higgs signal is observed in the kinematically accessible mass range, and a 95% CL lower mass limit of 114.3 GeV/c2 is set, to be compared with an expected median limit of 113.5 GeV/c2 for these data. © 2001 Elsevier Science B.V.

Abstract:

Measurements of on-shell ZZ production are described, using data collected by DELPHI in 1997 and 1998, at centre-of-mass energies √s = 182.6 GeV and 188.6 GeV, respectively. Results obtained in each of the final states qq̄qq̄, μ⁺μ^-qq̄, e⁺e^-qq̄, vv̄qq̄ l⁺l^-l⁺l^-, and vv̄l⁺l^- are presented. The measured cross-sections for on-shell ZZ production via the tree-level doubly-resonant graphs (NC02) are: σNC02 (182.6 GeV) = 0.38 ± 0.18 (stat) ± 0.04 (syst) pb, σNC02 (188.6 GeV) = 0.60±0.13 (stat) ±0.07 (syst) pb. They are consistent with the Standard Model expectations of 0.25 and 0.65 pb at each energy. © 2001 Elsevier Science B.V.

Abstract:

Using data collected in the DELPHI detector at LEP-1, measurements of the inclusive τ branching ratios for decay modes containing one, three, or five charged particles have been performed, giving the following results: B1 ≡ B(τ- → (particle)-≥0π0≥0K0 vτ(v̄)) = (85.316 ± 0.093 ± 0.049)%; B3 ≡ B(τ- → 2h-h+≥0π0≥0K0vτ) = (14.569 ± 0.093 ± 0.048)%; B5 ≡ B(τ- → 3h-2h+≥0π0≥0K0vτ) = (0.115 ± 0.013 ± 0.006)%, where h is either a charged π or K meson. The first quoted uncertainties are statistical and the second systematic.

Abstract:

The semileptonic branching fractions for primary and cascade b decays BR(b → l-), BR(b → c → l+) and BR(b → c̄ → l- were measured in hadronic Z decays collected by the DELPHI experiment at LEP. The sample was enriched in b decays using the lifetime information and various techniques were used to separate leptons from direct or cascade b decays. By fitting the momentum spectra of di-leptons in opposite jets, the average b mixing parameter x̄ was also extracted. The following results have been obtained: (equation presented).