Data from e+e- annihilation into hadrons, taken with the ALEPH detector at the Z resonance, are analyzed. The four-jet rate is studied as a function of the resolution parameter and compared to next-to-leading order calculations combined with resummation of large logarithms. Angular correlations in four-jet events are measured and compared to next-to-leading order QCD predictions. With these observables two different measurements are performed. In a first analysis the strong coupling constant is measured from the four-jet rate yielding alpha_s(M_ Z}) = 0.1170 \pm 0.0001(stat) \pm 0.0014(sys) In a second measurement the strong coupling constant and the \textsc{QCD} colour factors are determined simultaneously from a fit to the four-jet rate and the four-jet angular correlations, giving alpha_s(M_ Z) = 0.119 \pm 0.006(stat) \pm 0.026 (sys) C_A = 2.93 \pm 0.14(stat) \pm 0.58 (sys) C_F} = 1.35 \pm 0.07 (stat) \pm 0.26 (sys) in good agreement with the expectation from QCD.

Events containing only energetic photons are analysed in a sample of 628 pb -1 of data recorded from e+e- collisions at centre-of-mass energies between 189 and 209 GeV by the ALEPH detector at LEP. The $\reenunug$ and $\reeggg$ cross sections are measured and found to be in agreement with the Standard Model predictions. The number of light neutrino generations is determined to be N_\nu = 2.86 \pm 0.09. Upper limits are derived on the cross sections for photon production in the context of several supersymmetric models. Limits are also set on the parameters of models with extra spatial dimensions with contact interactions and with excited electrons.

A detailed numerical and analytical examination of the evolution of stochastic magnetic fields between a putative magnetogenesis era at high cosmic temperatures T~100 MeV–100 GeV and the present epoch is presented. The analysis includes all relevant dissipation processes, such as neutrino- and photon-induced fluid viscosities as well as ambipolar and hydrogen diffusion. A simple and intuitive analytical model matching the results of the three-dimensional MHD simulations allows for the prediction of prerecombination and present day magnetic field correlation lengths and energy densities as a function of initial magnetic field energy density, helicity, and spectral index. Our conclusions are multifold. (a) Initial primordial fields with only a small amount of helicity are evolving into maximally helical fields at the present. Furthermore, the simulations show a self-similarity in the evolution of maximally helical fields implying a seemingly acausual amplification of magnetic fields on large scales is observed. (b) There exists a correlation between the strength of the magnetic field B at the peak of its spectrum and the location of the peak, given at the present epoch by B[approximate]5×10-12 G(L/kpc), where L is the magnetic field correlation length determined by the initial properties of the magnetic field. (c) Concerning studies of the generation of cosmic microwave background (CMBR) anisotropies due to primordial magnetic fields of B~10-9 G on >~10 Mpc scales, such fields are not only impossible to generate in early causal magnetogenesis scenarios but also seemingly ruled out by distortions of the CMBR spectrum due to magnetic field dissipation on smaller scales and the overproduction of cluster-magnetic fields. (d) The most promising detection possibility of CMBR distortions due to primordial magnetic fields may be on much smaller scales at higher multipoles l~106 where the signal is predicted to be the strongest. (e) It seems possible that magnetic fields in clusters of galaxies are entirely of primordial origin, without invoking dynamo amplification. Such fields would be of (precollapse) strength 10-12–10-11 G with correlation lengths in the kpc range and would also exist in voids of galaxies.

Considering a NFW halo of M 31 made of neutralinos, predictions for the \x{03B3} fluxes are computed for different sets of SUSY parameters using the DarkSuspect MC simulation [1] [2]. Then they are compared with the efficiency of CELESTE Cerenkov telescope [3] which has a threshold energy of 30 GeV.

It is widely believed that dark matter exists within galaxies and clusters of galaxies. Under the assumption that this dark matter is composed of the lightest, stable supersymmetric particle, assumed to be the neutralino, the feasibility of its indirect detection via observations of a diffuse gamma-ray signal due to neutralino annihilation within M31 is examined.