QCD Sum Rules And Structure Of Some Hadrons

In this thesis we systematically studied the structure of scalar mesons around 1.5GeV(i.e., K₀^*,α₀ and f₀) as well as the lowest D mesons of 0^- and 0⁺ channels in the framework of QCD sum rules approach.Firstly we gave a systematic description of QCD sum rules approach, in-cluding its principle and necessary mathematical tools. Then we derived the sum rules from light scalar current correlation function. But the results shown that the conventional sum rules can not reveal the realistic mass spectrum of 0+ channel around 1.5GeV. After improving the sum rules by including the instan-ton effects, the masses of scalar meson nonet around 1.5GeV were well separated from each other. The results suggested that f₀(1370), K₀^*(1430) and ao(1450) mesons can be accommodated with the naive quark model, while there is large glueball component inα₀(1500). The underlying structure of f₀(1710) still need further study.Then based on the suggested structure of f₀(1370), K₀^*(1430),α₀(1450) and pion, we calculated their twist-3 light-cone distribution amplitudes by use of QCD sum rules with inclusion of instanton effects. We found that when the instan-ton involved the twist-3 light-cone distribution amplitudes can develop negative values in some longitudinal momentum fraction range. These instanton-involved twist-3 light-cone distribution amplitudes may be helpful to extract reasonable results in hadronic B decay to mesons mentioned above. We conjectured that the exact instanton density, multi-instantons effect and non-valence contribution may have some impacts on the results which will be implemented in future work.Finally the mass spitting of multiplet members in 0^- and 0⁺ channels of D meson were studied within the framework of conventional QCD sum rules. The results indicated that if we appropriately considered the different role of parity-dependent force induced from the complicated nonperturbative QCD vacuum, which represents itself by parity-dependent condensates in 0^- and 0⁺ channels, not only can we give a reasonable explanation to the mass gap of the members in the same channel, but also there will be larger splitting of 0 than 0⁺channel. Results also shown that the operator mixing effects was crucial in realizing correct splitting in 0⁺members. Generalization to higher states and B mesons were also briefly discussed.