[en] We analyze the impact of the recent Feynman-diagrammatic (FD) two-loop results for the mass of the lightest CP-even Higgs boson in the MSSM on the theoretical upper bound for m_h as a function of tan β. The results are compared with previous results obtained by renormalization group (RG) methods. The incorporation of dominant FD two-loop corrections into the decay width Γ(h→ff) is also discussed. (orig.)

[en] The partial decay width R_c of the Z into c anti c quark pair and the number of charm quarks n_c per b decay are measured with the DELPHI detector at LEP 1. Particle identification provides clear D⁰, D⁺, D_s⁺ and Λ_c⁺ signatures. The charm hadron production rate is measured in each channel by a fit to the scaled energy, impact parameter information and the invariant mass spectrum. Two measurements of R_c are presented, from the D^*+ production rate and from the overall charm counting, including strange charm baryon production, in c anti c events. The multiplicity n_c, which includes hidden c anti c and strange charm baryon production, is inferred from the charm counting in b anti b events. The final results are R_c=0.1665±0.0095 and n_c=1.166±0.086. (orig.)

[en] We analyze τ lepton decay observables, namely the moments of the hadronic spectral density in the finite energy interval (0,M_τ), within finite order perturbation theory including α_s⁴ corrections. The start of the asymptotic growth of the perturbation theory series is found at this order in a scheme-invariant manner. We establish the ultimate accuracy of finite order perturbation theory predictions and discuss the construction of optimal observables. (orig.)

[en] We analyse CP-violating effects in Z→4 jet decays, assuming the presence of a CP-violating effective triple gluon coupling. We discuss the influence of this coupling on the decay width. Furthermore, we analyse different CP-odd observables and propose strategies of a direct search for such a CP-violating GGG coupling. The present data of LEP 1 should give significant information on the coupling. (orig.)

[en] In the minimal supersymmetric standard model (MSSM) we incorporate the Higgs-boson propagator corrections, evaluated up to two-loop order, into the prediction of Γ(h→ff) and BR(h→ff) for f=b,c, τ. The propagator corrections consist of the full one-loop contribution, including the effects of non-vanishing external momentum, and corrections of O(αα_s) at the two-loop level. The results are supplemented with the dominant one-loop QED corrections and final state QCD corrections from both gluons and gluinos. The effects of the two-loop propagator corrections and of the one-loop gluino contributions are investigated in detail. Our results are compared with the result obtained within the renormalization group approach. Agreement within 10% is found for most parts of the MSSM parameter space. (orig.)

[en] We consider the phenomenology of a class of gauge-mediated supersymmetry (SUSY) breaking (GMSB) models at a e⁺e^- linear collider (LC) with E_c.o.m. up to 500 GeV. In particular, we refer to a high-luminosity (L ∝3 x 10³⁴ cm^-2 s^-1) machine, and use detailed simulation tools for a proposed detector. Our focus is on the case where a neutralino (N₁) is the next-to-lightest SUSY particle (NLSP), for which we determine the relevant regions of the GMSB parameter space. Many observables are calculated and discussed, including production cross sections, NLSP decay widths, branching ratios and distributions, for dominant and rare channels. We sketch how to extract the messenger and electroweak scale model parameters from a spectrum measured via, e.g. threshold-scanning techniques. Several experimental methods to measure the NLSP mass and lifetime are proposed and simulated in detail. We discuss how to determine a narrow range (< or∼5%) for the fundamental SUSY breaking scale √(F), based on the measured m_N1, cτ_N1. Finally, we suggest how to optimise the LC detector performance for this purpose. (orig.)

[en] Precision experiments, such as those performed at LEP and SLC, offer us an excellent opportunity to constrain extended gauge model parameters. To this end, it is often assumed that in order to obtain more reliable estimates, one should include the sizable one-loop standard model (SM) corrections, which modify the Z⁰ couplings as well as other observables. This conviction is based on the belief that the higher order contributions from the ''extension sector'' will be numerically small. However, the structure of higher order corrections can be quite different when comparing the SM with its extension; thus one should avoid assumptions which do not take account of such facts. This is the case for all models with ρ_tree≡M_W²/(M_Z² cos ²Θ_W)≠1. As an example, both the manifest left-right symmetric model and the SU(2)_L x U(1)_Y x U(1) model, with an additional Z' boson, are discussed, and special attention to the top contribution to Δρ is given. We conclude that the only sensible way to confront a model with the experimental data is to renormalize it self-consistently. If this is not done, parameters which depend strongly on quantum effects should be left free in fits, though essential physics is lost in this way. We should note that the arguments given here allow us to state that at the level of loop corrections (indirect effects) there is nothing like a ''model-independent global analysis'' of the data. (orig.)