Guy Wilkinson

Abstract:

Three jet events arising from decays of the Z boson, collected by the DELPHI detector, were used to measure differences in quark and gluon fragmentation. Gluon jets were anti-tagged by identifying b quark jets. Unbiased quark jets came from events with two jets plus one photon. Quark and gluon jet properties in different energy ranges were compared for the first time within the same detector. Quark and gluon jets of nearly the same energy in symmetric three jet event topologies were also compared. Using three independent methods, the average value of the ratio of the mean charged multiplicities of gluon and quark jets is 〈 r 〉= 1.241 ± 0.015 (stat.) ± 0.025 (syst.). Gluon jets are broader and produce fragments with a softer energy spectrum than quark jets of equivalent energy. The string effect has been observed in fully symmetric three jet events. The measured ratio Rγ of the charged particle flow in the qq̄ inter-jet region of the qq̄g and qq̄γ samples agrees with the perturbative QCD expectation. The dependence of the mean charged multiplicity on the hadronic center-of-mass energy was analysed in photon plus n-jet events. The value for αs(MZ) determined from these data using a QCD prediction with corrections at leading and next-to-leading order is αs(MZ) = 0.116 ± 0.003 (stat.) ± 0.009 (syst.). © Springer-Verlag 1996.

Abstract:

The interference between initial and final state radiation in the process e+e- → μ+μ- at √s ≈ MZ has been studied by measuring the forward-backward asymmetry as a function of the acoplanarity angle between the final state muons. The interference is expected to be sensitive to the space-time separation of the initial and final state radiation. The measured asymmetry distribution has been compared to theoretical predictions using the KORALZ generator, with and without script O sign(α) interference. The magnitude of the interference between initial and final state radiation was found to be of the order predicted and to follow the expected distribution. Using the theoretical predictions, a value of ΓZ = 2.50 ± 0.21 (stat.) ±0.06 (syst.) GeV . has been extracted. The interpretation of this result is discussed. There is an additional uncertainty in the estimate of ΓZ from as yet uncalculated higher order interference terms. By assuming a value of ΓZ consistent with the world average, the data were used to estimate the size of these uncalculated corrections. © Springer-Verlag 1996.

Abstract:

This paper presents an update of the measurement of the mean lifetime of the Bs0 meson. Combining Ds - ℓ, Ds - h, φ - ℓ and inclusive Ds final states from the 3.2 million hadronic Z decays collected by DELPHI between 1991 and 1994, the Bs0 mean lifetime was measured to be: τ(Bs0) = 1.67 ± 0.14ps. © Springer-Verlag 1996.

Abstract:

The inclusive production of the neutral vector mesons K*0(892) and φ(1020), and of the tensor meson K2*0(1430), in hadronic decays of the Z has been measured by the DELPHI detector at LEP. The average production rates per hadronic Z decay have been determined to be 0.77 ± 0.08 K*0(892), 0.104 ± 0.008 φ(1020) and 0.079 ± 0.040 K2*0(1430). The ratio of the tensor-to-vector meson production yields, 〈K2*0(1430)〉/〈K *0(892)〉 = 0.10± 0.05, is smaller than the 〈f2(1270)〉/〈ρ0(770)〉 and 〈f2′(1525)〉 /〈φ(1020)〉 ratios measured by DELPHI. The production rates and differential cross sections are compared with the predictions of JETSET 7.4 tuned to the DELPHI data and of HERWIG 5.8. The K*0(892) and φ(1020) data are compatible with model predictions, but a large disagreement is observed for the K2*0(1430).

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.