Guy Wilkinson

Abstract:

The partial decay width Rc of the Z into cc̄ quark pair and the number of charm quarks nc per b decay are measured with the DELPHI detector at LEP 1. Particle identification provides clear D0, D+, D+s and Λ+c signatures. The charm hadron production rate is measured in each channel by a fit to the scaled energy, impact parameter information and the invariant mass spectrum. Two measurements of Rc are presented, from the D*+ production rate and from the overall charm counting, including strange charm baryon production, in cc̄ events. The multiplicity nc, which includes hidden cc̄ and strange charm baryon production, is inferred from the charm counting in bb̄ events. The final results are Rc = 0.1665 ± 0.0095 and nc-= 1.166 ± 0.086.

Abstract:

The production of single photons has been studied in the reaction e+e- → γ+invisible particles at centre-of-mass energies of 183 GeV and 189 GeV. A previously published analysis of events with multiphoton final states accompanied by missing energy has been updated with 189 GeV data. The data were collected with the DELPHI detector and correspond to integrated luminosities of about 51 pb-1 and 158 pb-1 at the two energies. The number of light neutrino families is measured to be 2.84 ± 0.15(stat) ± 0.14(syst). The absence of an excess of events beyond that expected from Standard Model processes is used to set limits on new physics as described by supersymmetric and composite models. A limit on the gravitational scale is also determined.

Abstract:

Sleptons, neutralinos and charginos were searched for in the context of scenarios where the lightest supersymmetric particle is the gravitino. It was assumed that the stau is the next-to-ligntest supersymmetric particle. Data collected with the DELPHI detector at a centre-of-mass energy near 189 GeV were analysed combining the methods developed in previous searches at lower energies. No evidence for the production of these supersymmetric particles was found. Hence, limits were derived at 95% confidence level.

Abstract:

Searches for pair production of supersymmctric particles in e+e- collision at centre-of-mass energy of 183 GeV have been performed on DELPHI data under the assumption that R-parity is not conserved. Only one R-parity violating coupling of λ type, which couples the sleptous to the leptons (LLĒ erm), is considered to be dominant at a time. Since in models with R-parity violation any supersymmetic particlr can be the lightest one, searches for charginos, neutralinos, sleptons and squarks have been perfomed both for direct R-parity violating decays and for indirect cascade decays. Morever, it is assumed that the strength of the R-parity violating couplings is such that the lifetimes can be neglected. The present study of supersymmetric particle pair production is used to exclude domains of the Minimal Supersymmetric Standard Model parameter space previously explored under the assumption of R-parity conservation.

Abstract:

Searches for neutral Higgs bosons in the Standard Model and the MSSM have been performed using data collected by the DELPHI experiment at a centre-of-mass energy of 188.7 GeV, corresponding to an integrated luminosity of 158 pb-1. These analyses are used, in combination with our results from lower energies, to set new 95% confidence level lower mass bounds on the Standard Model Higgs boson (94.6 GeV/c2) and on the lightest neutral scalar (82.6 GeV/c2) and neutral pseudoscalar (84.1 GeV/c2) Higgs bosons in a representative scan of the MSSM parameters. The results are also interpreted in the framework of a general two-Higgs doublet model.