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This thesis reports on searches for supersymmetric particles, in particular neutralinos, conducted with the DELPHI detector at the LEP accelerator at CERN. The searches were performed within the framework of the Minimal Supersymmetric Standard Model (MSSM), with R-parity conserved and gravity mediated supersymmetry breaking. The neutralino search covered both Χ02Χ01 and Χ0jΧi0i (j =3,4 and i =1,2) production channels. Data collected in 1999 and 2000 were used, in total about 451 pb-1 at centre-of-mass energies from 192 to 208 GeV. No signal was found in any of the search channels, and the results were interpreted as model independent limits on the production cross-sections of the different topologies. For deriving these limits the data analysed here were combined with those collected at a centre-of-mass energy of 189 GeV (∾ 158 pb-1). The obtained upper limits of the cross-section for ~Χ02Χ01 production, when Χ02→ Z*Χ01, vary with masses of the neutralinos involved and typically range between 0.05 pb and 0.8 pb. The negative results from all sparticle searches were used to exclude regions in the parameter space within a constrained MSSM, with gaugino and sfermion mass unification at the GUT scale, and to set limits on the sparticle masses. The mass of the lightest neutralino is constrained to be larger than 45.5 GeV/c2 for any m0, for tan β ≤ 40 and without sfermion mixing. An interpretation of the results was also performed within the Anomaly Mediated Supersymmetry Breaking model, and the lower mass limit of the lightest neutralino is 68 GeV/c2. All limits are given at 95% confidence level. In the ATLAS combined testbeam 2004, a full slice of the ATLAS detector was tested. The SPS accelerator at CERN was used to create particle beams with energies from 1 to 350 GeV. The beamline instrumentation, such as scintillators, beam chambers and Cherenkov detectors, and extra detectors, are described in this thesis. The data from the testbeam were used to investigate the performance of the hadronic calorimeter for pions at various pseudorapidities and energies. The energy resolutions and linearities obtained were found to be comparable with results from earlier hadronic standalone testbeams. The obtained resolutions were about σ/E = 52 % / sqrt E [GeV] ⊕ 5 %.