Guy Wilkinson

Abstract:

This paper presents results on charged Higgs boson production, based on LEP data collected at √s = 172 GeV, that complement the previous DELPHI results obtained at centre of mass energies up to 161 GeV. The charged Higgs bosons are assumed to be pair produced and to decay either into a quark pair or into τVτ. The three different possible final states are included in the analysis. Data from ring imaging Cherenkov and microvertex detectors are used to identify the quarks as a cs pair. The number of candidates found is compatible with the background expected from standard processes. Combining the results of the present analysis with those of the previous analysis at lower energies, a new lower mass limit for the charged Higgs boson is set at 54.5 GeV/c². © 1998 Elsevier Science B.V.

Abstract:

The value of the b quark mass at the M2 scale defined in the MS renormalization scheme, mb(MZ), was determined using 2.8 million hadronic Z decays collected during 1992-1994 by the DELPHI detector to be mb(MZ) = 2.67 ± 0.25 (stat.) ± 0.34 (frag.) ± 0.27 (theo.) GeV/c². The analysis considers NLO corrections to the three-jet production rate including mass effects, and the result obtained agrees with the QCD prediction of having a running b quark mass at an energy scale equal to MZ. This is the first time that such a measurement is performed far above the bb̄ production threshold. The study also verifies the flavour independence of the strong coupling constant for b and light quarks within 1% accuracy. © 1998 Elsevier Science B.V.

Abstract:

The data collected by DELPHI in 1996 have been used to measure the average charged particle multiplicities and dispersions in qq̄ events at centre-of-mass energies of √s = 161 GeV and √s = 172 GeV, and the average charge multiplicity in WW events at √s = 172 GeV. The multiplicities in qq̄ events are consistent with the evolution predicted by QCD. The dispersions in the multiplicity distributions are consistent with Koba-Nielsen-Olesen (KNO) scaling. The average multiplicity of charged particles in hadronic W decays has been measured for the first time; its value, 19.23 ± 0.74(stat + syst), is consistent with that expected for an e⁺e^- interaction at a centre-of-mass energy equal to the W mass. The charged particle multiplicity in W decays shows no evidence of effects of colour reconnection between partons from different W's at the present level of statistics. © 1998 Elsevier Science B.V.

Abstract:

The splitting processes in identified quark and gluon jets are investigated using longitudinal and transverse observables. The jets are selected from symmetric three-jet events measured in Z decays with the DELPHI detector in 1991-1994. Gluon jets are identified using heavy quark anti-tagging. Scaling violations in identified gluon jets are observed for the first time. The scale energy dependence of the gluon fragmentation function is found to be about two times larger than for the corresponding quark jets, consistent with the QCD expectation CA/CF. The primary splitting of gluons and quarks into subjets agrees with fragmentation models and, for specific regions of the jet resolution y, with NLLA calculations. The maximum of the ratio of the primary subjet splittings in quark and gluon jets is 2.77 ± 0.11 ± 0.10. Due to non-perturbative effects, the data are below the expectation at small y. The transition from the perturbative to the non-perturbative domain appears at smaller y for quark jets than for gluon jets. Combined with the observed behaviour of the higher rank splittings, this explains the relatively small multiplicity ratio between gluon and quark jets.

Abstract:

From a data sample of 9.98 pb^-1 integrated luminosity, collected by DELPHI at a centre-of-mass energy of 172 GeV, 118 events were selected as W-pair candidates. From these, the branching fraction Br(W → qq̄) was measured to be 0.660^+0.036-0.037 (stat.) ± 0.009(syst.) and the cross-section for the doubly resonant process e⁺e^- → W⁺W^- to be 11.58^+1.44-1.35 (stat.) ± 0.32(syst.) pb. The mass of the W boson, obtained from direct reconstruction of the invariant mass of the fermion pairs in the decays WW → ℓvqq̄ and WW → qq̄qq̄, was determined to be mW = 80.22±0.41(stat.)±0.04(syst.)±0.05(int.)±0.03(LEP) GeV/c², where "int." denotes the uncertainty from interconnection effects like colour reconnection and Bose-Einstein interference. Combined with the W mass obtained from the cross-sections measured by DELPHI at threshold, a value of mW = 80.33 ± 0.30(stat.) ± 0.05(syst.) ± 0.03(int.) ± 0.03(LEP) GeV/c² was found.