Search for supersymmetry in $\tau$ final states at ATLAS and constraints on new physics using electroweak precision data

In this thesis, various models beyond the Standard Model (SM) offering different solutions to some of the shortcomings of the SM are studied: Supersymmetry, the two Higgs doublet model, and models with warped extra dimensions. A search for events with large missing transverse momentum, jets, and at least two τ leptons using 2 fb−1 of proton-proton collision data recorded at √s = 7TeV with the ATLAS detector at the Large Hadron Collider is performed. No excess above the SM background expectation is observed and a 95% CL upper limit on the visible cross section for new phenomena is set. A 95% CL lower limit of 32TeV is set on the Gauge Mediated Supersymmetry Breaking scale independent of the ratio of tanβ. These limits provide the most stringent tests to date in a large part of the considered parameter space. By using the results of the oblique vacuum polarization parameters from a fit to the electroweak precision data indirect constraints on model parameters are set. While in the two Higgs doublet model as well as in models with warped extra dimensions a heavy Higgs boson is compatible with the electroweak precision data, models with warped extra dimensions featuring custodial symmetry cannot accommodate a heavy Higgs.