Searches for Gauge-Mediated Supersymmetry Breaking Topologies in e+e- collisions at LEP2

In gauge-mediated supersymmetry (SUSY) breaking (GMSB) models the lightest supersymmetric particle (LSP) is the gravitino and the phenomenology is driven by the nature of the next-to-lightest SUSY particle (NLSP) which is either the lightest neutralino, the stau or mass degenerate sleptons. Since the NLSP decay length is effectively unconstrained, searches for all possible lifetime and NLSP topologies predicted by GMSB models in e+e- collisions are performed on the data sample collected by OPAL at centre-of-mass energies up to 209 GeV at LEP. Results independent of the NLSP lifetime are presented for all relevant final states including direct NLSP pair-production and, for the first time, also NLSP production via cascade decays of heavier SUSY particles. None of the searches shows evidence for SUSY particle production. Cross-section limits are presented at the 95% confidence level both for direct NLSP production and for cascade decays, providing the most general, almost model independent results. These results are then interpreted in the framework of the minimal GMSB (mGMSB) model, where large areas of the accessible parameter space are excluded. In the mGMSB model, the NLSP masses are constrained to be larger than 53.5 GeV/c^2, 87.4 GeV/c^2 and 91.9 GeV/c^2 in the neutralino, stau and slepton co-NLSP scenarios, respectively. A complete scan on the parameters of the mGMSB model is performed, constraining the universal SUSY mass scale Lambda from the direct SUSY particle searches: Lambda > 40, 27, 21, 17, 15 TeV/c^2 for messenger indices N=1, 2, 3, 4, 5 respectively, for all NLSP lifetimes.Comment: 4 pages, 2 figures. To appear in Proceedings of SUSY06, the 14th International Conference on Supersymmetry and the Unification of Fundamental Interactions, UC Irvine, California, 12-17 June 200

Search for R-Parity Violating Decays of Scalar Fermions at LEP

A search for pair-produced scalar fermions under the assumption that R-parity is not conserved has been performed using data collected with the OPAL detector at LEP. The data samples analysed correspond to an integrated luminosity of about 610 pb-1 collected at centre-of-mass energies of sqrt(s) 189-209 GeV. An important consequence of R-parity violation is that the lightest supersymmetric particle is expected to be unstable. Searches of R-parity violating decays of charged sleptons, sneutrinos and squarks have been performed under the assumptions that the lightest supersymmetric particle decays promptly and that only one of the R-parity violating couplings is dominant for each of the decay modes considered. Such processes would yield final states consisting of leptons, jets, or both with or without missing energy. No significant single-like excess of events has been observed with respect to the Standard Model expectations. Limits on the production cross- section of scalar fermions in R-parity violating scenarios are obtained. Constraints on the supersymmetric particle masses are also presented in an R-parity violating framework analogous to the Constrained Minimal Supersymmetric Standard Model.Comment: 51 pages, 24 figures, Submitted to Eur. Phys. J.

Measurement of the Hadronic Photon Structure Function F_2^gamma at LEP2

The hadronic structure function of the photon F_2^gamma is measured as a function of Bjorken x and of the factorisation scale Q^2 using data taken by the OPAL detector at LEP. Previous OPAL measurements of the x dependence of F_2^gamma are extended to an average Q^2 of 767 GeV^2. The Q^2 evolution of F_2^gamma is studied for average Q^2 between 11.9 and 1051 GeV^2. As predicted by QCD, the data show positive scaling violations in F_2^gamma. Several parameterisations of F_2^gamma are in agreement with the measurements whereas the quark-parton model prediction fails to describe the data.Comment: 4 pages, 2 figures, to appear in the proceedings of Photon 2001, Ascona, Switzerlan

Search for Higgs Bosons in e+e- Collisions at 183 GeV

The data collected by the OPAL experiment at sqrts=183 GeV were used to search for Higgs bosons which are predicted by the Standard Model and various extensions, such as general models with two Higgs field doublets and the Minimal Supersymmetric Standard Model (MSSM). The data correspond to an integrated luminosity of approximately 54pb-1. None of the searches for neutral and charged Higgs bosons have revealed an excess of events beyond the expected background. This negative outcome, in combination with similar results from searches at lower energies, leads to new limits for the Higgs boson masses and other model parameters. In particular, the 95% confidence level lower limit for the mass of the Standard Model Higgs boson is 88.3 GeV. Charged Higgs bosons can be excluded for masses up to 59.5 GeV. In the MSSM, mh > 70.5 GeV and mA > 72.0 GeV are obtained for tan{beta}>1, no and maximal scalar top mixing and soft SUSY-breaking masses of 1 TeV. The range 0.8 < tanb < 1.9 is excluded for minimal scalar top mixing and m{top} < 175 GeV. More general scans of the MSSM parameter space are also considered.Comment: 49 pages. LaTeX, including 33 eps figures, submitted to European Physical Journal

A Measurement of the Product Branching Ratio f(b->Lambda_b).BR(Lambda_b->Lambda X) in Z0 Decays

The product branching ratio, f(b->Lambda_b).BR(Lambda_b->Lambda X), where Lambda_b denotes any weakly-decaying b-baryon, has been measured using the OPAL detector at LEP. Lambda_b are selected by the presence of energetic Lambda particles in bottom events tagged by the presence of displaced secondary vertices. A fit to the momenta of the Lambda particles separates signal from B meson and fragmentation backgrounds. The measured product branching ratio is f(b->Lambda_b).BR(Lambda_b->Lambda X) = (2.67+-0.38(stat)+0.67-0.60(sys))% Combined with a previous OPAL measurement, one obtains f(b->Lambda_b).BR(Lambda_b->Lambda X) = (3.50+-0.32(stat)+-0.35(sys))%.Comment: 16 pages, LaTeX, 3 eps figs included, submitted to the European Physical Journal

Measurement of the Michel Parameters in Leptonic Tau Decays

The Michel parameters of the leptonic tau decays are measured using the OPAL detector at LEP. The Michel parameters are extracted from the energy spectra of the charged decay leptons and from their energy-energy correlations. A new method involving a global likelihood fit of Monte Carlo generated events with complete detector simulation and background treatment has been applied to the data recorded at center-of-mass energies close to sqrt(s) = M(Z) corresponding to an integrated luminosity of 155 pb-1 during the years 1990 to 1995. If e-mu universality is assumed and inferring the tau polarization from neutral current data, the measured Michel parameters are extracted. Limits on non-standard coupling constants and on the masses of new gauge bosons are obtained. The results are in agreement with the V-A prediction of the Standard Model.Comment: 32 pages, LaTeX, 9 eps figures included, submitted to the European Physical Journal

Determination of the b Quark Mass at the Z Mass Scale

In hadronic decays of Z bosons recorded with the OPAL detector at LEP, events containing b quarks were selected using the long lifetime of b flavoured hadrons. Comparing the 3-jet rate in b events with that in d u,s and c quark events, a significant difference was observed. Using Order(alpha_s squared) calculations for massive quarks, this difference was used to determine the b quark mass in the MSbar renormalisation scheme at the scale of the Z boson mass. By combining the results from seven different jet finders the running b quark mass was determined to be mb(MZ) = (2.67 +/- 0.03(stat) +0.29/-0.37(syst) +/- 0.19(theo.)) GeV. Evolving this value to the b quark mass scale itself yields mb(mb) = (3.95 +0.52/-0.62) GeV, consistent with results obtained at the b quark production threshold. This determination confirms the QCD expectation of a scale dependent quark mass. A constant mass is ruled out by 3.9 standard deviations

A Study of One-Prong Tau Decays with a Charged Kaon

From an analysis of the ionisation energy loss of charged particles selected from 110326 e+e- -> tau+tau- candidates recorded by the OPAL detector at e+e- centre-of-mass energies near the Z0 resonance, we determine the one-prong tau decay branching ratios: Br(tau- -> nu_tau K- >=0h0) = 1.528 +- 0.039 +- 0.040 % Br(tau- -> nu_tau K-) = 0.658 +- 0.024 +- 0.029 % where the h0 notation refers to a pi0, an eta, a K^0_S, or a K^0_L, and where the first uncertainty is statistical and the second is systematic.From an analysis of the ionisation energy loss of charged particles selected from 110326 e+e- -> tau+tau- candidates recorded by the OPAL detector at e+e- centre-of-mass energies near the Z0 resonance, we determine the one-prong tau decay branching ratios: Br(tau- -> nu_tau K- >=0h0) = 1.528 +- 0.039 +- 0.040 % Br(tau- -> nu_tau K-) = 0.658 +- 0.024 +- 0.029 % where the h0 notation refers to a pi0, an eta, a K^0_S, or a K^0_L, and where the first uncertainty is statistical and the second is systematic

Multiplicities of π0\pi^{0}, η\eta, K0K^{0} and of charged particles in quark and gluon jets

We compared the multiplicities of pizero, eta, Kzero and of charged particles in quark and gluon jets in 3-jet events, as measured by the OPAL experiment at LEP. The comparisons were performed for distributions unfolded to 100% pure quark and gluon jets, at an effective scale Qjet which took into account topological dependences of the 3-jet environment. The ratio of particle multiplicity in gluon jets to that in quark jets as a function of Qjet for pizero, eta and Kzero was found to be independent of the particle species. This is consistent with the QCD prediction that the observed enhancement in the mean particle rate in gluon jets with respect to quark jets should be independent of particle species. In contrast to some theoretical predictions and previous observations, we observed no evidence for an enhancement of eta meson production in gluon jets with respect to quark jets, beyond that observed for charged particles. We measured the ratio of the slope of the average charged particle multiplicity in gluon jets to that in quark jets, C, and we compared it to a next-to-next-to-next-to leading order calculation. Our result, C=2.27+-0.20(stat+syst),is about one standard deviation higher than the perturbative prediction.We compared the multiplicities of pizero, eta, Kzero and of charged particles in quark and gluon jets in 3-jet events, as measured by the OPAL experiment at LEP. The comparisons were performed for distributions unfolded to 100% pure quark and gluon jets, at an effective scale Qjet which took into account topological dependences of the 3-jet environment. The ratio of particle multiplicity in gluon jets to that in quark jets as a function of Qjet for pizero, eta and Kzero was found to be independent of the particle species. This is consistent with the QCD prediction that the observed enhancement in the mean particle rate in gluon jets with respect to quark jets should be independent of particle species. In contrast to some theoretical predictions and previous observations, we observed no evidence for an enhancement of eta meson production in gluon jets with respect to quark jets, beyond that observed for charged particles. We measured the ratio of the slope of the average charged particle multiplicity in gluon jets to that in quark jets, C, and we compared it to a next-to-next-to-next-to leading order calculation. Our result, C=2.27+-0.20(stat+syst),is about one standard deviation higher than the perturbative prediction