| In this thesis two different topics in high energy physics are discussed.; The first part is based on fundamental work by Seiberg who uncovered the strong coupling dynamics for some supersymmetric gauge field theories. We generalize his results and find the classification of all supersymmetric gauge field theories with a certain quantum modification of their vacua. Necessary conditions for the existence of these theories are proven. The physics of these theories is described in detail. Most of the results are based on our ability to predict the superpotential. Results regarding the Kahler potential are far more difficult to obtain. Only weak coupling Kahler potentials for some models are known. We calculate the Kahler potentials at weak coupling for supersymmetric Quantum Chromo Dynamics with Nf = Nc + 1 and Nf = Nc.; In the second part we introduce a short distance operator product expansion, which allows us to calculate a host of rare, exclusive decays. We apply the expansion to the decays B+/-→D* +/-s e+e- , B0→J/yh,D0 ,D*0e+e- and B+/-→p+/-e+ e- . Strong reduced matrix elements have to be calculated. We show that in some cases they can be calculated explicitly using factorization and Ward identities. In other cases they can be related to each other using heavy quark spin symmetries, chiral symmetries and non-relativistic Quantum Chromo Dynamics spin symmetries, which reduces the number of independent reduced matrix elements. The decay rates of B+/-→D* +/-s e+e- are proportional to V2ub and the shape of the differential decay rates of B+/-→p+/-e+ e- are sensitive to cos alpha. These processes can, therefore, be used to obtain limits on these parameters. |
