Hadron structures and decays within quantum chromodynamics

We have evaluated the meson spectrum using a relativistic model based on reductions of Bethe-Salpeter equation. We included one gluon exchange from Quantum Chromodynamics (QCD) and phenomenological confinement in the interaction kernel. We used an ansatz for the running of the coupling constant with the meson masses. We fitted more than forty states ranging from a hundred MeV to a few GeV using seven parameters, with root mean square deviation from experimental data of 40-50 MeV. This establishes our successful QCD-based model for many applications which we investigate.; We employed our model to examine the strong interaction medium effects on meson properties. We have evaluated the masses of specific mesons as the confinement strength is weakened by the medium. We have shown how these effects may be observed experimentally in specific examples. We have shown how charmonium and {dollar}phi{dollar} meson hadronic decay channels may be closed, and leptonic yield may be enhanced. We demonstrated how radiative decays of charmonium states may also be useful for investigating our predicted in-medium effects.; We next applied our model to investigate B meson decays. We showed the consistency of our model with the heavy quark symmetry, which enabled us to calculate the Isgur-Wise function and extract the Cabibbo-Kobayashi-Maskawa (CKM) mixing matrix element {dollar}Vsb{lcub}cb{rcub}{dollar} from recent experiments. We have evaluated the leptonic decay constants, semileptonic decay form factors and used factorization to calculate the nonleptonic decays of B mesons.; Looking forward to future experiments which will investigate domains within and beyond the standard model, we have applied chiral perturbation theory ({dollar}chi{dollar}PT) to evaluate hyperon nonleptonic decays. We invoked the heavy baryon formalism and included the decuplet baryon contribution. We have obtained the {dollar}Delta I=1/2{dollar} octet amplitudes and the {dollar}Delta I=3/2{dollar} 27-plet amplitudes to one loop. We discussed the convergence of {dollar}chi{dollar}PT amplitudes for both the S-wave and the P-wave for hyperon decays.; We have evaluated CP violation in B decays from a certain anomalous tbW coupling appearing in many versions of models which go beyond the standard model. The effect of this anomalous interaction may be large enough to be seen experimentally in B factories. This may help us in the search for physics beyond the standard model.