The four LEP collaborations, ALEPH, DELPHI, L3 and OPAL, have searched for the neutral Higgs bosons which are predicted by the Minimal Supersymmetric Standard Model (MSSM). The data of the four collaborations are statistically combined and examined for their consistency with the background hypothesis and with a possible Higgs boson signal. The combined LEP data show no significant excess of events which would indicate the production of Higgs bosons. The search results are used to set upper bounds on the cross-sections of various Higgs-like event topologies. The results are interpreted within the MSSM in a number of "benchmark" models, including CP-conserving and CP-violating scenarios. These interpretations lead in all cases to large exclusions in the MSSM parameter space. Absolute limits are set on the parameter tan β and, in some scenarios, on the masses of neutral Higgs bosons.

Narrow fractions of polystyrene molecules in the form of uncatenated rings (cycles) were synthesized by reacting bifunctional living linear precursors with an appropriate coupling agent at very low concentrations. The cyclic molecules were separated from the simultaneously formed linear polycondensates by fractionate precipitation. The molecular weights of the cycles ranged from 11100 to 185000, thus encompassing the critical molecular weight for entanglements in linear polystyrene. The ring-like nature of these fractions has been investigated by a variety of techniques, including the limiting viscosity number in a good and in a solvent as well as neutron scattering in deuteriated cyclohexane. These measurements, part of which are reported here in some detail, display a gratifying agreement with the theoretical predictions reported earlier for uncatenated cyclic polymers. The zero-shear melt viscosities of these cyclic fractions and several others prepared by Fetters and Hostetter to extend the molecular weight range to 390000 were measured over a wide range of temperatures and compared with the viscosities of linear polystyrenes of similar molecular weights. Above the critical molecular weight for entanglement coupling, no major differences were found between the temperature dependence or the molecular weight dependence of the cyclic polymers and those of their linear counterparts. For the same molecular weight, however, the cycles exhibit somewhat lower melt viscosity values than do the linear molecules. The results are critically compared with those reported by Roovers on similar polystyrene cycles and briefly discussed in terms of recent molecular theories based on snake-like motion (reptation) of chains along a curvilinear tube formed by the constraints of the surrounding entangled matrix.

Link to publication Citation for published version (APA):Abreu, P., Boudinov, E., Holthuizen, D. J., Kjaer, N. J., Kluit, P. M., Mulders, M. P., ... van Eldik, J. E. (1997). Search for neutral heavy leptons produced in $Z$ decays. Zeitschrift für Physik. C, Particles and Fields, 74, 57. DOI: 10.1007/s002880050370 General rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible.Download date: 09 May 2018 Z. Phys. C 74, 57-71 (1997) ZEITSCHRIFT FÜR PHYSIK C Abstract. Weak isosinglet Neutral Heavy Leptons (ν m ) have been searched for using data collected by the DEL-PHI detector corresponding to 3.3 × 10 6 hadronic Z 0 decays at LEP1. Four separate searches have been performed, for short-lived ν m production giving monojet or acollinear jet topologies, and for long-lived ν m giving detectable secondary vertices or calorimeter clusters. No indication of the existence of these particles has been found, leading to an upper limit for the branching ratio BR(Z 0 → ν m ν) of about 1.3 × 10 −6 at 95% confidence level for ν m masses between 3.5 and 50 GeV/c 2 . Outside this range the limit weakens rapidly with the ν m mass. The results are also interpreted in terms of limits for the single production of excited neutrinos.

For over 30 years complexes with the general formula [NiPh(P,O)L] (L = tertiary phosphine; P,O = chelating phosphanylenolato ligand) have been used as highly efficient oligomerisation catalysts suitable for the production of linear alpha-olefins. The same complexes, which are usually referred to as SHOP-type catalysts (SHOP = Shell Higher Olefin Process) can also be used as ethylene polymerisation catalysts, provided they are treated with a phosphine scavenger that selectively removes the tertiary phosphine ligand (L). This Perspective examines the impact of various parameters (influence of the substituents, backbone size, solvent, use of co-catalysts, etc.) on the catalytic outcome of the complexes. Overall, this review shows that the selectivity and activity of the catalyst may be tuned efficiently through directed modification of the P,O chelator.