More than a Lifetime: Towards Precision Predictions for $B$-Meson Decays in the Standard Model and Beyond

This thesis investigates B-meson lifetimes as probes of Standard Model (SM) dynamics and physics beyond the SM (BSM), employing the Heavy Quark Expansion (HQE) and light-cone sum rules (LCSR). Within the HQE, we calculate at leading order in QCD the complete dimension-seven two-quark contributions to the CKM-favoured non-leptonic transitions $b\to c\bar{u}d$ and $b\to c\bar{c}s$, providing analytical Wilson coefficients for the $1\mathrm{/}{m}_b^4$ corrections. We then extend the dimension-six Darwin contribution to a model-independent BSM framework, deriving the Wilson coefficients through order $1\mathrm{/}{m}_b^3$ for a general non-leptonic effective Hamiltonian. We also determine the corresponding dimension-six four-quark weak annihilation contributions. As a special case, our results provide previously missing dimension-five and dimension-six contributions induced by SM QCD-penguin operators. Finally, we investigate B-mesogenesis, which connects the cosmic matter-antimatter asymmetry and dark matter to exotic B-meson decays. We study inclusive decay rates and lifetime ratios within the HQE and exclusive $B^{+}\to p\psi$ decays using LCSR. Comparisons with experimental constraints from BaBar, Belle and Belle II restrict the allowed parameter space. We also identify kinematic regions where power-suppressed contributions exceed the leading terms, highlighting limitations of the HQE. Together, these studies advance the theoretical understanding of heavy-hadron lifetimes, strengthen the bridge between precision flavour physics and physics beyond the SM, and open new avenues for probing the interplay between heavy-quark dynamics and the dark sector.