Measurement of the Hadronic Photon Structure Function F2gamma at LEP2

The hadronic structure of the photon F2gamma is measured as a function of Bjorken x and of the photon virtuality Q2 using deep-inelastic scattering data taken by the OPAL detector at LEP at e+e- centre-of-mass energies from 183 to 209 GeV. Previous OPAL measurements of the x dependence of F2gamma are extended to an average Q2 of =780GeV2 using data in the kinematic range 0.15 < x < 0.98. The Q2 evolution of F2gamma is studied for 12.1 < 780GeV2 using three ranges of x. As predicted by QCD, the data show positive scaling violations in F2gamma for the central x region 0.10-0.60. Several parameterisations of F2gamma are in qualitative agreement with the measurements whereas the quark-parton model prediction fails to describe the data.Comment: 23 pages, 5 figures, Submitted to Phys. Lett.

Search for Neutral Higgs Boson in CP-Conserving and CP-Violating MSSM Scenarios

This report summarizes the final results from the OPAL collaboration on searches for neutral Higgs bosons predicted by the Minimal Supersymmetric Standard Model (MSSM). CP-conserving and for the first time at LEP CP-violating scenarios are studied. New scenarios are also included, which aim to set the stage for Higgs searches at future colliders. The results are based on the data collected with the OPAL detector at e+e- centre-of-mass energies up to 209 GeV. The data are consistent with the prediction of the Standard Model with no Higgs boson produced. Model-independent limits are derived for the cross-section of a number of events topologies motivated by prediction of the MSSM. Limits on Higgs boson masses and other MSSM parameters are obtained for a number of representative MSSM benchmark scenarios. For example, in the CP-conserving scenario mh-max where the MSSM parameters are adjusted to predict the largest range of values for mh at each tan beta, and for a top quark mass of 174.3 GeV, the domain 0.7<tan beta<1.9 is excluded at the 95% confidence level and Higgs boson mass limits of mh>84.5 GeV and ma>85.0 GeV are obtained. For the CP-violating benchmark scenario CPX which, by construction, enhances the CP-violating effects in the Higgs sector, the domain tan beta<2.8 is excluded but no universal limit can be set on the Higgs boson masses.Comment: 62 pages, 28 figures, Submitted to Eur. Phys. J.

Search for pair-produced leptoquarks in e+e- interactions at sqrt(s) = 183 GeV

A search for pair-produced leptoquarks has been performed using a sample of e+e- collision events collected by the OPAL detector at LEP at e+e- centre-of-mass energies of about 183 GeV. The data sample corresponds to an integrated luminosity of 55.9 pb-1. The leptoquarks were assumed to be produced via couplings to the photon and the Z0 and then to decay within a single fermion generation. No evidence for contributions from leptoquark pair production processes was observed. Lower limits on scalar and vector leptoquark masses are obtained. The existing limits are improved in the region of large decay branching ratio to quark-neutrino.Comment: 26 pages, LaTeX, including 9 eps figs, submitted to European Physical Journal

Measurement of the Production Rate of Charm Quark Pairs from Gluons in Hadronic Z0 Decays

The rate of secondary charm-quark-pair production has been measured in 4.4 million hadronic Z0 decays collected by OPAL. By selecting events with three jets and tagging charmed hadrons in the gluon jet candidate using leptons and charged D* mesons, the average number of secondary charm-quark pairs per hadronic event is found to be (3.20+-0.21+-0.38)x10-2.Comment: 22 pages, 4 figures,submitted to Euro. Phys. J.

A Combination of Preliminary Electroweak Measurements and Constraints on the Standard Model

This note presents a combination of published and preliminary electroweak results from the four LEP collaborations ALEPH, DELPHI, L3 and OPAL based on electron-positron collision data taken at centre-of-mass energies above the Z-pole, 130 \GeV to 209 \GeV (\LEPII), as prepared for the 2006 summer conferences. Averages are derived for di-fermion cross sections and forward-backward asymmetries, photon-pair, W-pair, Z-pair, single-W and single-Z cross sections, electroweak gauge boson couplings, W mass and width and W decay branching ratios. An investigation of the interference of photon and Z-boson exchange is presented, and colour reconnection and Bose-Einstein correlation analyses in W-pair production are combined. The main changes with respect to the experimental results presented in 2005 are new preliminary combinations of final {\LEPII} results on the mass and width of the W boson. Including the precision electroweak measurements performed at the Z pole published recently, the results are compared with precise electroweak measurements from other experiments, notably CDF and D{\O}at the Tevatron. Constraints on the input parameters of the Standard Model are derived from the results obtained in high-$Q^2$ interactions, and used to predict results in low-$Q^2$ experiments, such as atomic parity violation, M{\"o}ller scattering, and neutrino-nucleon scattering.Comment: 173 pages; v2: Update of titlepage and addition of two reference

Combination procedure for the precise determination of Z boson parameters from results of the LEP experiments

The precise determination of the Z boson parameters from the measurements at the Z resonance by the four collaborations ALEPH, DELPHI, L3 and OPAL in e+e- collisions at the large electron positron collider LEP at CERN is a landmark for precision tests of the electroweak theory. The four experiments measured quantities which were used to extract the mass and width of the Z boson, the hadronic cross-section at the pole of the resonance, the ratio of hadronic and leptonic decay widths, and the leptonic forward-backward asymmetries at the pole. The combination procedure based on these parameters is presented in this paper.Comment: 41 pages, 4 figure

Searches for the Neutral Higgs Bosons of the MSSM: Preliminary Combined Results Using LEP Data Collected at Energies up to 209 GeV

In the year 2000 the four LEP experiments collected data at centre-of-mass energies between 200 and 209 GeV, integrating approximately 870 pb-1 of luminosity, with about 510 pb-1 above 206 GeV. The LEP working group for Higgs boson searches has combined these data with data sets collected previously at lower energies. In representative scans of the parameters of the Minimal Supersymmetric Standard Model (MSSM), the mass limits mh>91.0 GeV and mA>91.9 GeV are obtained for the light CP-even and the CP-odd neutral Higgs boson, respectively. For a top quark mass less than or equal to 174.3 GeV, assuming that the stop quark mixing is maximal, and choosing conservative values for other SUSY parameters affecting the Higgs sector, the range 0.5<tanbeta<2.4 is excluded. Additionally, the results of flavour-independent searches for hadronically decaying Higgs bosons are included, allowing exclusion of MSSM models with suppressed decays of the Higgs bosons to pairs of b quarks.Comment: 20 Pages, 8 figures. Submitted to EPS'01 in Budapest and Lepton-Photon '01 in Rome. Figures supplied as .eps files, including separate color version

Search for the Standard Model Higgs Boson at LEP

The four LEP collaborations, ALEPH, DELPHI, L3 and OPAL, have collected 2465 pb-1 of e+e- collision data at energies between 189 and 209 GeV, of which 542 pb-1 were collected above 206 GeV. Searches for the Standard Model Higgs boson have been performed by each of the LEP collaborations. Their data have been combined and examined for their consistency with the Standard Model background and various Standard Model Higgs boson mass hypotheses. A lower bound of 114.1 GeV has been obtained at the 95% confidence level for the mass of the Higgs boson. The likelihood analysis shows a preference for a Higgs boson with a mass of 115.6 GeV. At this mass, the probability for the background to generate the observed effect is 3.4%.Comment: 27 pages, 13 figures. Figures supplied as .eps files. This note summarizes the LEP combined SM Higgs search results to be presented at EPS01 in Budapest, and at Lepton-Photon '01 in Rome. Submitted to CERN-EP preprints First update (v2): remove extraneous .eps files Second update (v3): Some minor corrections. 1-CLb(mh=115.6) -> 3.4% (was 3.5% before), "test signal curve" in fig. 1 updated. Cosmetic changes to figures v4: change word "revise" to "review" (was written assuming British meaning of the word

Flavour Independent Search for Hadronically Decaying Neutral Higgs Bosons at LEP

Hadronic decays of Higgs bosons, not necessarily into b-quarks, have been searched for using data collected at LEP-2. Such searches are complementary to the usual Standard Model Higgs searches and lead to more model-independence. Preliminary results obtained by the four LEP collaborations are presented for the hZ production mechanism (using the q-qbarq-qbar, q-qbarnu-nubar and q-qbarl+l- channels) and combined for the first time.Comment: 7 pages, 2 figures. Figures are supplied as .eps files. Sumbitted to the EPS '01 conference in Budapest, and the Lepton-Photon '01 conference in Rom

Searches for Invisible Higgs bosons: Preliminary combined results using LEP data collected at energies up to 209 GeV

In the year 2000 the four LEP experiments have collected data at energies between 200 and 209 GeV, for approximately 868 pb-1 integrated luminosity. The LEP working group for Higgs boson searches has combined these data with earlier data sets collected at lower centre-of-mass energies to search for a neutral CP-even Higgs boson, produced at the Standard Model rate, decaying into "invisible" particles. No statistically significant excess has been observed when compared to the Standard Model background prediction, and assuming that the Higgs boson decays only into such states a lower bound has been set on its mass at 95% confidence level of 114.4 GeV.Comment: 7 pages, 5 figures, figures supplied as .eps files. Submitted to the EPS '01 conference in Budapest and the Lepton-Photon '01 conference in Rome. Version 2 adds theoretical citation