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Abstract:

Two measurements of {Mathematical expression} are presented. Both measurements use 250000 Z decays taken with the DELPHI detector in 1991 and rely mainly on the precision of the microvertex detector. One tagging method is as simple as possible so that background rates can be reliably predicted by simulation. The other one uses a more involved tagging technique and reduces the dependence on simulation as much as possible. Combining both results, {Mathematical expression} is found to be 0.2209±0.0041(stat.)±0.0042(syst.)±0.0018 {Mathematical expression}. © 1995 Springer-Verlag.

Abstract:

An analysis of inclusive production of K0 and the meson resonances K*±(892), ρ0(770), f0(975) and f2(1270) in hadronic decays of the Z0 is presented, based on about 973,000 multihadronic events collected by the DELPHI detector at LEP during 1991 and 1992. Overall multiplicities have been determined as 1.962±0.060 K0 mesons, 0.712±0.067 K*±(892) and 1.21±0.15ρ0(770) per hadronic Z0 decay. The average multiplicities of f0(975) for scaled momentum, xp, in the range 0.05≤xp≤0.6 and of f2(1270) for 0.05≤xp≤1.0 are 0.098±0.016 and 0.170±0.043 respectively. The f0(975) and ρ0(770)xp-spectra have similar shapes. The f2(1270)/ρ0(770) ratio increases with xp. The average multiplicities and the differential cross sections are compared with the JETSET Parton Shower model. The model with default parameters fails to reproduce the experimental K0 momentum spectrum at low momentum, describes the K*±(892) and ρ0(770)xp-spectrum shapes, but significantly overestimates their production rates. © 1995 Springer-Verlag.

Abstract:

An excess of events containing, in a jet, a samesign Ξ∓-ℓ∓ pair as compared to those with an oppositesign Ξ∓-ℓ± pair has been observed in an analysis of 1.7 million hadronic Z0 decays collected by the DELPHI detector at LEP between 1991 and 1993 inclusive. The probability for this signal to come from non B-baryon decays is less than 5×10-4. The measured production fraction corresponds to:[Figure not available: see fulltext.] per lepton species, averaged for electrons and muons and assuming that the two channels have an equal contribution. Semileptonic decays of Λb baryons can account for less than 10% of these events and the major part of the signal has to originate from Ξb semileptonic decays. Using the subsample of these events where the Ξ∓ trajectory has been measured in the Vertex Detector, the lifetime of B-baryons producing a Ξ∓ in their semileptonic decay final state is found to be: {Mathematical expression} © 1995 Springer-Verlag.

Abstract:

A study of the production of strange octet and decuplet baryons in hadronic decays of the Z recorded by the DELPHI detector at LEP is presented. This includes the first measurement of the ∑^± average multiplicity. The total and differential cross sections, the event topology and the baryon-antibaryon correlations are compared with current hadronization models. © 1995 Springer-Verlag.

Abstract:

From a sample of 1.5 million Z 0→qq decays collected by the DELPHI detector during 1991, 1992 and 1993, the production cross section for isolated final state photons is measured and is compared with the O(α, α s) matrix element calculations implemented in the EEPRAD and GNJETS Monte Carlo generators. The observed photon yield is used to derive the electroweak couplings of charge 2/3 (u-type) and charge 1/3 (d-type) quarks to the Z 0 boson. The measured values c 2/3=0.91 -0.36 +0.25 and c 1/3=1.62 -0.17 +0.24 are compatible with the Standard Model prediction, c 2/3=1.145 and c 1/3=1.477. © 1996 Springer-Verlag.