[en] Searches for pair-production of supersymmetric particles under the assumption that R-parity is violated via a single dominant LL anti E,LQ anti D or anti U anti D anti D coupling are performed using the data collected by the ALEPH collaboration at centre-of-mass energies of 181-184 GeV. The observed candidate events in the data are in agreement with the standard model expectations. Upper limits on the production cross-sections and lower limits on the masses of charginos, sleptons, squarks and sneutrinos are derived. (orig.)

[en] The status of two on-going studies concerning important aspects of the phenomenology of gauge-mediated supersymmetry breaking (GMSB) models at TeV colliders is reported. The first study deals with the characteristics of the light Higgs boson spectrum allowed by the (minimal and non-minimal) GMSB framework. Today's most accurate GMSB model generation and two-loop Feynman-diagrammatic calculation of m_h have been combined. The Higgs masses are shown in dependence of various model parameters at the messenger and electroweak scales. In the minimal model, an upper limit on m_h of about 124 GeV is found for m_t=175 GeV. The second study is focused on the measurement of the fundamental SUSY breaking scale √(F) at the LHC in the GMSB scenario where a stau is the next-to-lightest SUSY particle (NLSP) and decays into a gravitino with cτ_NLSP in the range 0.5 m to 1 km. This implies the measurement of mass and lifetime of long lived sleptons. The identification is performed by determining the time of flight in the ATLAS muon chambers. Accessible range and precision on √(F) achievable with a counting method are assessed. (orig.)

[en] We study the possible signals of the (M+1)SSM with a singlino LSP at LEP2. First we identify regions of the parameter space which are ruled out by negative results of sparticle searches in the context of the MSSM. In the remaining kinematically accessible regions we present total event rates for topologies which require further studies, i.e. estimations of the corresponding efficiencies: various four charged fermion final states with missing energy, possibly with displaced vertices due to a long life-time of the NLSP, the second lightest neutralino. Searches for these unconventional signatures are essential in order to cover the entire kinematically accessible parameter space of the (M+1)SSM with a singlino LSP at LEP2. (orig.)

[en] Searches for pair production of supersymmetric particles in e⁺e^- collisions at centre-of-mass energy of 183 GeV have been performed on DELPHI data under the assumption that R-parity is not conserved. Only one R-parity violating coupling of λ type, which couples the sleptons to the leptons (LL anti E term), is considered to be dominant at a time. Since in models with R-parity violation any supersymmetric particle can be the lightest one, searches for charginos, neutralinos, sleptons and squarks have been performed both for direct R-parity violating decays and for indirect cascade decays. Moreover, it is assumed that the strength of the R-parity violating couplings is such that the lifetimes can be neglected. The present study of supersymmetric particle pair production is used to exclude domains of the minimal supersymmetric standard model parameter space previously explored under the assumption of R-parity conservation. (orig.)

[en] We study the quantum effects on vector-boson pair production in e⁺e^- annihilation induced by the sleptons and squarks of the minimal supersymmetric extension of the standard model (MSSM) in the one-loop approximation. We list full analytic results, and quantitatively analyze the resulting deviation from the standard-model prediction of e⁺e^- → W⁺W^- for the supergravity-inspired MSSM. The latter can be rendered small throughout the whole parameter space by an appropriate choice of renormalization scheme. (orig.)

[en] A selection of di-lepton events with significant missing transverse momentum has been performed using a total data sample of 77.0 pb^-1 at e⁺e^- centre-of-mass energies of 161 GeV, 172 GeV and 183 GeV. The observed numbers of events: four at 161 GeV, nine at 172 GeV, and 78 at 183 GeV, are consistent with the numbers expected from standard model processes, which arise predominantly from W⁺W^- production with each W decaying leptonically. This topology is an experimental signature also for the pair production of new particles that decay to a charged lepton accompanied by one or more invisible particles. Further event selection criteria are described that optimise the sensitivity to particular new physics channels. No evidence for new phenomena is apparent and limits on the production cross-section times branching ratio squared for various new physics processes are presented in a manner intended to minimise the number of model assumptions. (orig.)

[en] In the framework of the minimal supersymmetric standard model we compute the one-loop effective action for the electroweak bosons obtained after integrating out the different sleptons, squarks, neutralinos and charginos, and present the result in terms of the physical sparticle masses. In addition we study the asymptotic behavior of the two-, three- and four-point Green's functions with external electroweak bosons in the limit where the physical sparticle masses are very large in comparison with the electroweak scale. We find that in this limit all the effects produced by the supersymmetric particles can either be absorbed in the standard model parameters and gauge boson wave functions, or else they are suppressed by inverse powers of the supersymmetric particle masses. This work, therefore, completes the proof of decoupling of the heavy supersymmetric particles from the standard ones in the electroweak boson effective action and in the sense of the Appelquist-Carazzone theorem; we started this proof in a previous work. From the point of view of effective field theories this work can be seen as a (partial) proof that the SM can indeed be obtained from the MSSM as the quantum low-energy effective theory of the latter when the SUSY spectra are much heavier than the electroweak scale. (orig.)

[en] We study the helicity amplitudes of the process γγ→ZZ in the standard model at high energy. These amplitudes receive contributions from the W and charged quark and lepton loops, analogous to those encountered in the γγ→γγ, γZ cases studied before. But γγ→ZZ also receives contributions from the Higgs s-channel poles involving the effective Higgs-γγ vertex. At energies g∝300 GeV, the amplitudes in all three processes are mainly helicity-conserving and almost purely imaginary; which renders them a very useful tool in searching for new physics. As an example, a SUSY case is studied, and the signatures due to the virtual effects induced by a chargino-, charged slepton- or a lightest stop-loop in γγ→ZZ, are explored. These signatures, combined with the analogous ones in γγ→γγ and γγ→γZ, should help identifying the nature of possible new physics particles. (orig.)