Investigation of the splitting of quark and gluon jets

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 TeX . The primary splitting of gluons and quarks into subjets agrees with fragmentation models and, for specific regions of the jet resolution TeX , with NLLA calculations. The maximum of the ratio of the primary subjet splittings in quark and gluon jets is TeX . Due to non-perturbative effects, the data are below the expectation at small TeX . The transition from the perturbative to the non-perturbative domain appears at smaller TeX 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

Measurement of the Quark and Gluon Fragmentation Functions in Z0Z^0 Hadronic Decays

The fragmentation functions and multiplicities in $b\overline{b}$ and light quark events are compared. The measured transverse and longitudinal components of the fragmentation function allow the gluon fragmentation function to be evaluated

Search for scalar fermions and long-lived scalar leptons at centre-of-mass energies of 130 GeV to 172 GeV

Data taken by DELPHI during the 1995 and 1996 LEP runs have been used to search for the supersymmetric partners of electron, muon and tau leptons and of top and bottom quarks. The observations are in agreement with standard model predictions. Limits are set on sfermion masses. Searches for long lived scalar leptons from low scale supersymmetry breaking models exclude stau masses below 55~GeV/c$^2$ at the 95\% confidence level, irrespective of the gravitino mass

Measurements of the leptonic branching fractions of the τ\tau

Data collected with the DELPHI detector from 1993 to 1995 combined with previous DELPHI results for data from 1991 and 1992 yield the branching fractions B({\tau \rightarrow \mbox{\rm e} \nu \bar{\nu}}) = (17.877 \pm 0.109_{stat} \pm 0.110_{sys} )\% and $B({\tau \rightarrow \mu \nu \bar{\nu}}) = (17.325 \pm 0.095_{stat} \pm 0.077_{sys} )\%$

Search for neutral and charged Higgs bosons in e+e−e^+ e^- collisions at s=161GeV\sqrt{s}= 161 GeV and 172GeV172 GeV

A search for neutral and charged Higgs bosons has been performed in the data collected by the {\sc DELPHI} detector at centre-of-mass energies of 161~GeV and 172~GeV. The analysis assumes either the pair-production of charged Higgs bosons, ${\mathrm H}^{\pm}$, or the production of the lightest neutral Higgs boson, h, with either a Z or a neutral pseudoscalar Higgs boson, A. All final state topologies expected from the decay of h and A %neutral Higgs particles into hadrons or a pair of $\tau$ leptons, and from the decay of ${\mathrm H}^{\pm}$ %charged Higgs bosons into a pair of quarks or a $\tau \nu_{\tau}$ pair have been considered. %In the case of the associated production with a Z boson, all decay %channels of the Z have been considered. %The case of a %Higgs boson decaying into invisible products has also been studied. %The results are interpreted %in the framework of the Standard Model and of its minimal supersymmetric %extension for the neutral Higgs bosons. %Results for the charged Higgs particles are deduced in the more %general two-doublet scheme. Lower limits at the 95\% confidence level have been obtained on the Higgs boson masses. The limits are 66.2~GeV/$c^2$ for h in the Standard Model, 59.5~GeV/$c^2$ for h and 51.0~GeV/$c^2$ for A in the minimal supersymmetrpseudosca

Measurement of the e+ e- ---> gamma gamma (gamma) cross-section at the LEP energies

The total and the differential cross-sections for the reaction $e^+e^-\rightarrow \gamma\gamma(\gamma)$ have been measured with the DELPHI detector at LEP % with at centre-of-mass energies from 1 30 %, 136, 161, 172 and to 183 GeV for an integrated luminosity of 78.19.~pb$^{-1}$. %CR An agreement with the QED predictions was found and consequently The results agree with the QED prediction s. The lower limits (obtained including previously published results at the \Zzero energies) on the QED cutoff parameters are $\Lambda_{+} > 253$~GeV and $\Lambda_{-} > 225$~GeV and the lower bo und on the mass of an excited electron with an effective coupling constant $\lambda_{\gamma} = 1$ is 231~GeV/$c^2$. %Upper limits on the branching ratios for the decays ${\rm Z}^0\rightarrow$, ${\rm Z}^0\rightarrow \pi^0\gamma$, % ${\rm Z}^0\rightarrow \eta\gamma$ %and ${\rm Z}^0\rightarrow \gamma\gamma\gamma$ have been determined to be 2.5 % $\times$ $10^{-5}$, 2.5 $\times$ $10^{-5}$, % 4.0 $\times$ $10^{-5}$, %and 9.1 $\times$ $10^{-4}$ respectively. All the limits are at the 95$\%$ confidence level

A precise measurement of the partial decay width ratio Rb0=Γbb‾/ΓhadR_{b}^{0} = \Gamma_{b\overline{b}}/\Gamma_{had}

The partial decay width of the Z to \bb quark pairs has beenmeasured by the DELPHI detector at LEP using data taken in the years1992 to 1995.Decays of b-hadrons were tagged by several methods usingtracks with large impact parameters and/or reconstructed secondary vertices, complemented by event shape variables.Combining these methods in a multivariate analysis the value\frac{\Gamma({\mathrm Z \rightarrow \bb}) }{ \Gamma({\mathrm {Z \rightarrow had}}) } = 0.21634 \pm 0.00067({\mathrm {stat}}) \pm 0.00060 ({\mathrm {syst}})was obtained, where the ${\mathrm c} \bar {\mathrm c}$ production fractionwas fixed to its Standard Model value