Young, RossZanotti, JamesKordov, Zeno Rafael2022-08-012022-08-012022https://hdl.handle.net/2440/135884Lattice QCD affords us the unique opportunity to study the quark structure of hadrons non-perturbatively, through the couplings of quark- field operators to hadron mass-eigenstates. In this thesis, we study cases of flavour wavefunction mixing which are induced by broken flavour-symmetry, with particular consideration for the effects of isospin-breaking and electromagnetism. Namely, we investigate mixing between the octet baryons 0 and , and the pseudoscalar mesons 0, and 0, respectively. The latter scenario introduces the computational challenge of calculating disconnected quark-loop diagrams on the lattice, and we have investigated various techniques for improving the calculation. Additionally, we calculate the masses of the pseudoscalar mesons in lattice QCD+QED simulations, and investigate their behaviour with respect to the flavour-symmetry features observed through the state mixing. Finally, we detail and present lattice determinations of the weak decay constants of the pseudoscalar mesons, with the investigation being informed by the aforementioned state mixing. The results obtained from lattice simulations in each investigation are used to t quark mass and charge extrapolations for the relevant quantities.enHadron spectroscopyLattice QCDState mixingMesonBaryonFlavour symmetryIsospin breakingQCD+QEDOperator renormalizationPseudoscalarOctetNonetThesis