Aspects of Higgs Physics and New Physics at the LHC

We study Higgs physics and related issues in the Standard Model (SM) and beyond. In the first part, we present theoretical predictions for Higgs pair production in the SM including higher order corrections in Quantum Chromodynamics (QCD) and the uncertainties on the respective cross sections from various sources. The second part covers supersymmetry. In the framework of the minimal supersymmetric extension of the SM, we investigate decays of light top squarks in the kinematic region where a flavour-changing decay into a charm or up quark and a neutralino or a four-body decay occurs. Furthermore, we discuss the Higgs sector in the next-to-minimal supersymmetric extension of the SM with complex parameters. In particular, we calculate the Higgs boson masses at one-loop order. In addition, we present Higgs boson production cross sections and decay widths. The third part introduces Composite Higgs Models as plausible candidates for physics beyond the SM. We calculate the leadingorder cross section for single and double Higgs production via gluon fusion obtained by means of the low-energy theorem (LET) and compare it to the result computed with full mass dependence of the new heavy vector-like fermions. In addition, we calculate the next-to-leading order QCD corrections for double Higgs production in the LET approximation for Composite Higgs Models. Furthermore, we study the effects of fermionic bottom partners on Higgs and electroweak precision observables.