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.

We present results on vector meson photoproduction via +yp -. Vp in the LBL-SLAC 82" hydrogen bubble chamber exposed to a linearly polarized photon beam at 2.8, 4.7 and 9.3 GeV. We find p" production to have the characteristics of a diffractive process, i.e., a cross section decreasing slowly with energy and a differential cross section with slope of -6.5 GeV -2 . Within errors the p" production amplitudes are entirely due to natural parity exchange. S-channel helicity is conserved to a high degree in the y --+p' transition. We find evidence for small helicity flip amplitudes for 7rr pairs in the ,o" region. Photoproduction of w mesons is separated into its natural (aN) and unnatural (vu) parity exchange contributions. The E -Y and t-dependence and the spin density matrix of the unnatural parity exchange contribution are consistent with an OPE process.The natural parity exchange part has characteristics similar to p" production.At 9.3 GeV the ratio of a@') to aN(w) is -7. The slope of the Q? differential cross section is -4.5 GeVm2, smaller than that of p" and w production. Natural parity exchange is the main contributor to + production. No evidence for higher mass vector mesons is found in 717~~ ~TTT or m final states. The s-and tdependence of Compton scattering as calculated from p, w and 9 photoproduction using VDM agree with experiment, but the predicted Compton cross section is too small by a factor of two.

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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.

DELPHI data collected at centre-of-mass energies up to 208 GeV have been analysed to search for charginos, neutralinos and sfermions in the framework of the Minimal Supersymmetric Standard Model (MSSM) with R-parity conservation. No evidence for a signal was found in any of the channels. The results of each search were used to derive limits on production cross-sections and particle masses. In addition, the combined result of all searches excludes regions in the parameter space of the constrained MSSM, leading to limits on the mass of the Lightest Supersymmetric Particle and other supersymmetric particles.

Single photons detected by the DELPHI experiment at LEP2 in the years 1997-2000 are reanalysed to investigate the existence of a single extra dimension in a modified ADD scenario with slightly warped large extra dimensions. The data collected at centre-of-mass energies between 180 and 209 GeV for an integrated luminosity of ∼650 pb −1 agree with the predictions of the Standard Model and allow a limit to be set on graviton emission in one large extra dimension. The limit obtained on the fundamental mass scale M D is 1.69 TeV/c 2 at 95% CL, with an expected limit of 1.71 TeV/c 2 .

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Soft photons inside hadronic jets converted in front of the DELPHI main tracker (TPC) in events of qq disintegrations of the Z 0 were studied in the kinematic range 0.2 < E γ < 1 GeV and transverse momentum with respect to the closest jet direction p T < 80 MeV/c. A clear excess of photons in the experimental data as compared to the Monte Carlo predictions is observed. This excess (uncorrected for the photon detection efficiency) is (1.17 ± 0.06 ± 0.27) × 10 −3 γ/jet in the specified kinematic region, while the expected level of the inner hadronic bremsstrahlung (which is not included in the Monte Carlo) is (0.340 ± 0.001 ± 0.038) × 10 −3 γ/jet. The ratio of the excess to the predicted bremsstrahlung rate is then (3.4 ± 0.2 ± 0.8), which is similar in strength to the anomalous soft photon signal observed in fixed target experiments with hadronic beams.