QCD sum-rule study of scalar mesons

In this thesis, QCD Laplace sum-rules for the light quark q¯q currents are employed to study the properties of the non-strange I = 0 and I = 1 light quark scalar mesons. This QCD sum-rule analysis allows us to interpret the experimentally observed I = 0 and I = 1 scalar mesons. The Holder inequality technique is employed to determine the region of validity for the QCD sum-rule, and a stability analysis of the QCD sum-rule prediction is conducted through a Monte-Carlo uncertainty simulation of uncertainties.; The field theoretical content of the QCD sum rules incorporates purely-perturbative QCD contributions to two-loop order, leading contributions from QCD-vacuum condensates, and the direct single-instanton contributions in the instanton-liquid QCD vacuum model. Single-instanton contributions are the only components of the QCD field theory that distinguish between isospin states, and therefore they are responsible for breaking the mass degeneracy between the lowest-lying isovector and isoscalar mesons. A novel treatment of instanton effects in QCD continuum contribution is included in this thesis. There is also a need to go beyond the narrow resonance approximation for the scalar channels which are likely to exhibit sensitivity to broad resonance structure. A finite-width effect anticipated from physical resonance widths is incorporated for the hadronic content of the I = 0 and I = 1 QCD sum rules.; In the I = 0 channel, our results support interpretation of the f0(980) as the lowest-lying light quark scalar meson, indicating that f0(400 -- 1200) is unnaturally decoupled from a light quark non-strange current. In the I = 1 channel, the results identify a0(1450) as the lowest-lying q¯q resonance, and are indicative of a non-q¯q interpretation for a0(980).