Guy Wilkinson

Abstract:

A measurement of the mass difference, Δmd, between the two physical Bd0 states has been obtained from the analysis of the impact parameter distribution of a lepton emitted at large transverse momentum (pt) relative to the jet axis and from the analysis of the flight distance distribution of secondary vertices tagged by either a high pt lepton or an identified kaon. In the opposite hemisphere of the event, the charge of the initial quark has been evaluated using a high pt lepton, a charged kaon or the mean jet charge. With 1.7 million hadronic Z0 decays recorded by DELPHI between 1991 and 1993, Δmd is found to be: Δmd = 0.531-0.046+0.050 (stat.) ± 0.078 (syst.) ps-1. © Springer-Verlag 1996.

Abstract:

The partial decay width of the Z to bb̄ quark pairs has been measured by the DELPHI detector at LEP. B-hadrons, containing b-quarks, were tagged by several methods using tracks with large impact parameters to the primary vertex complemented sometimes by event shape variables or using leptons with high transverse momentum relative to the hadron. In order to reduce the systematic uncertainties, in all methods the b-tagging efficiency has been extracted directly from the data. Combining all methods, the value: Γbb̄/Γhad = 0.2216 ± 0.0016(stat.) ± 0.0021(syst.) was found, where the cc̄ production fraction was fixed to its Standard Model value. © Springer-Verlag 1996.

Abstract:

Results are presented of a search for pair production of heavy objects decaying into four hadronic jets, as expected for example from associated or pair production of MSSM Higgs bosons, hAor H+H-, using a data sample of 5.9 pb-1 of e+e- collisions at √s = 130-136 GeV collected with the DELPHI detector at LEP in November 1995. The data and expectations from standard processes agree after four-jet selections. An analysis based on b-tagging finds no hA candidate with high mass. A study optimized to search for H+H- events with mass in the 40-50 GeV/c2 range also finds no candidate. Finally a comparison is made with a recent ALEPH analysis which found an excess of four-jet events with high multiplicity and high mass. Such a signal is not observed in the DELPHI data, although a slight excess in the mass region around 105 GeV/c2 is seen.

Abstract:

Rare decays of beauty particles were studied in several charmless modes using the data collected with the DELPHI detector at LEP from t991 to 1994. These decays are mediated by both tree level b → u and one-loop penguin b → s, d transitions. Evidence for charmless decays was obtained in two body hadronic modes. The branching ratios of 30a,s to π+π- or K+π- and B -u to p0π- or K*0π- were found to be (2.8+1.5-1.0 ± 0.2) × 10-5 and (1.7 +1.2-0.8 ± 0.2) × 10-4 respectively. The fraciion of these decays with a charged kaon in the final state that is not from the spectator s quark, was measured to be 0.58 ± 0.18. Upper limits were set at 90% confidence level on the branching ratios for three and four body charmless hadronic decays in the range of (1-3)x 10 -4, for inclusive radiative b → sγ decays at 5.4 × 10-4, for the exclusive radiative decays B0d → K+(892)0γ and B0d φ(1020)γ at 2.1 × 10-4 and 7.0 × 10 -4 respectively, and for dineutrino decays, B0s → svv̄, in the exclusive channels B0d → φ K*(892)0vv̄ and B0s → φ(1020)vv̄ at 1.0 × 10-3 and 5.4 × 10 -3 respectively The limits on dineutrino decays constrain theories with a new U(1) gauge boson coupling predominantly to the third family of fermions. © Springer-Verlag 1996.

Abstract:

Event shape and charged particle inclusive distributions are measured using 750000 decays of the Z to hadrons from the DELPHI detector at LEP. These precise data allow a decisive confrontation with models of the hadronization process. Improved tunings of the JETSET, ARIADNE and HERWIG parton shower models and the JETSET matrix element model are obtained by fitting the models to these DELPHI data as well as to identified particle distributions from all LEP experiments. The description of the data distributions by the models is critically reviewed with special importance attributed to identified particles.