| This thesis deals with the application of QCD sum-rules to investigate the properties of pseudoscalar mesons.; QCD sum-rules generally relate the parameters of hadrons to those of quarks and gluons appearing in Quantum Chromodynamics, thus providing a valuable probe into low-energy hadronic phenomena. At low energies, the perturbative approach to QCD breaks down. Nonperturbative effects are considered through the appearance of condensates of quark-antiquarks and gluons in vacuum.; We have made a detailed study on the decay constants and the SU(2) mass-splittings of the pseudoscalar particles like the K, D and B mesons. We first determine upper bounds on the decay constants by neglecting the hadronic continuum contribution, which is hard to estimate directly. Next, we propose a suitable model for the continuum based on its asymptotic properties. We then evaluate the decay constants by requiring that the sum-rule be satisfied over a wide range of variations of the parameters involved. The calculated values of the decay constants are used to show that a recent proposal for the non-spectator contribution to D decays, through the emission of a hard gluon, is unlikely to work. The decay constants are also used to estimate the (tau)-leptonic decays of F and B(,u).; Finally, the sum-rule approach is extended to investigate the SU(2) mass-splittings of pseudoscalar mesons. The hadronic contribution, arising from the mass difference of the up and down quarks, is calculated this way. The electromagnetic contribution, arising from the exchange of a photon, is estimated by extending Dashen's theorem. With a reasonable value for the up-down quark mass difference, our results are in good agreement with experiments. |
