Peter Renton

Abstract:

A test of the QED process e+e- → γγ(γ) is reported. The data analysed were collected with the DELPHI detector in 1998 and 1999 at the highest energies achieved at LEP, reaching 202 GeV in the centre-of-mass. The total integrated luminosity amounts to 375.7 pb-1. The differential and total cross-sections for the process e+e- → γγ were measured, and found to be in agreement with the QED prediction. 95% confidence level (C.L.) lower limits on the QED cut-off parameters of Λ+ > 330 GeV and λ- > 320 GeV were derived. A 95% C.L. lower bound on the mass of an excited electron of 311 GeV/c2 (for λγ = 1) was obtained, s-channel virtual graviton exchange was searched for, resulting in 95% C.L. lower limits on the string mass scale, M(S): M(S) > 713 GeV/c2 (λ = 1) and M(S) > 691 GeV/c2 (γ = -1). (C) 2000 Published by Elsevier Science B.V.

Abstract:

The rapidity-rank structure of pp̄ pairs is used to analyze the mechanism of baryon production in hadronic Z0 decay. The relative occurrence of the rapidity-ordered configuration p M p̄, where M is a meson, and that of pp̄ adjacent pairs is compared. The data are found to be consistent with predictions from a mechanism producing adjacent-rank pp̄ pairs, without requiring 'string-ordered' p M p̄ configurations. An upper limit of 15% at 90% confidence is determined for the p M p̄ contribution. (C) 2000 Elsevier Science B.V.

Abstract:

Production of ∑- and Λ(1520) in hadronic Z decays has been measured using the DELPHI detector at LEP. The ∑- is directly reconstructed as a charged track in the DELPHI microvertex detector and is identified by its ∑→nπ decay leading to a kink between the ∑- and π-track. The reconstruction of the Λ(1520) resonance relies strongly on the particle identification capabilities of the barrel Ring Imaging Cherenkov detector and on the ionisation loss measurement of the TPC. Inclusive production spectra are measured for both particles. The production rates are measured to be s 0.081 ± 0.002 ± 0.010, = 0.029 ± 0.005 ± 0.005. The production rate of the Λ(1520) suggests that a large fraction of the stable baryons descend from orbitally excited baryonic states. It is shown that the baryon production rates in Z decays follow a universal phenomenological law related to isospin, strangeness and mass of the particles. © 2000 Elsevier Science B.V.

Abstract:

The τ polarisation has been studied with the e+e- → τ+τ- data collected by the DELPHI detector at LEP in 1993, 1994 and 1995 around the Z resonance firstly through the exclusive decay channels evv̄, μvvμ, πv, ρv, and a1v and secondly with an inclusive hadronic analysis which benefits from a higher efficiency and a better systematic precision. The results have been combined with those previously published on 1990 to 1992 DELPHI data, to produce results which reflect the full LEP-1 statistics. The fit of the τ polarisation dependence on the production angle yielded the polarisation parameters Aτ = 0.1359 ± 0.0096 and Ae = 0.1382 ± 0.0116. From these results the ratio of the vector and axial-vector effective couplings v̄τ/āτ = 0.0683 ± 0.0048 and v̄e/āe = 0.0694 ± 0.0058 have been derived, compatible with e-τ universality. With the assumption of lepton universality, the ratio of vector to axial-vector effective couplings for leptons v̄ι/āι = 0.0687 ± 0.0037 was obtained, implying a value of the effective weak mixing angle sin2 θlepteff= 0.23282 ± 0.00092.

Abstract:

This paper describes a measurement of the Michel parameters, (Formula presented.), and the average (Formula presented.) helicity, (Formula presented.), in τ lepton decays together with the first measurement of the tensor coupling in the weak charged current. The (Formula presented.) pairs were produced at the LEP e⁺e⁻ collider at CERN from 1992 through 1995 in the DELPHI detector. Assuming lepton universalityin the decays of the τ the measured values of the parameters were: (Formula presented.). The strength of the tensor coupling was measured to be κ^Wτ = −0.029 ± 0.036 ± 0.018. The first error is statistical and the second error is systematic in all cases. The results are consistent with the V −A structure of the weak charged current in decays of the τ lepton.