Three loop corrections to HQET Sum Rules for the Bag Parameter

In this thesis, we compute the bag parameters which parameterise non-perturbative hadronic matrix elements of 4-quark operators. These are required for studies of mixing and the inclusive decays of and mesons and they represent the largest source of uncertainty in corresponding theory predictions. In this work, the bag parameters are obtained using a HQET sum rule analysis of the 3-point correlator to 3 loops, including flavour breaking effects up to corrections. Besides reproducing the known HQET sum rule values for -mixing and the lifetimes of , , and mesons, we obtain for the first time results for -mixing and the lifetimes of and mesons. For -mixing, our results are highly competitive, while for the lifetimes no state of the art lattice evaluation is available. Furthermore, we consider the phenomenological implications of our results for mixing observables and the values of the CKM matrix elements. Finally, we give a thorough analysis of inclusive decay widths for charmed mesons, in an attempt to study the viability of the HQE in the charm sector.