Cooling is used as a filter on a set of gluon fields sampling the Wilson action to selectively remove essentially all fluctuations of the gluon field except for the instantons. The close agreement between quenched lattice QCD results with cooled and uncooled configurations for vacuum correlation functions of hadronic currents and for density-density correlation functions in hadronic bound states provides strong evidence for the dominant role of instantons in determining light hadron structure and quark propagation in the QCD vacuum.

Point-to-point vacuum correlation functions for spatially separated hadron currents are calculated in quenched lattice QCD on a 16 3 × 24 lattice with 6/g 2 = 5.7. The lattice data are analyzed in terms of dispersion relations, which enable us to extract physical information from small distances where asymptotic freedom is apparent to large distances where the hadronic resonances dominate. In the pseudoscalar, vector, and axial vector channels where experimental data or phenomenological information are available, semiquantitative agreement is obtained. In the nucleon and delta channels, where no experimental data exist, our lattice data complement experiments. Comparison with approximations based on sum rules and interacting instantons are made, and technical details of the lattice calculation are described.

Point-to-point correlation functions of hadron currents in the QCD vacuum are calculated on a lattice and analyzed using dispersion relations, providing physical information down to small spatial separations. Qualitative agreement with phenomenological results is obtained in channels for which experimental data are available, and these correlation functions are shown to be useful in exploring approximations based on sum rules and interacting instantons.PACS numbers: 12.38.Gc

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Several new developments in the calculation and interpretation of hadron densitydensity correlation functions are presented. The asymptotic behavior of correlation functions is determined from a tree diagram path integral. A method is developed to use this behavior to correct for leading image contributions on a finite periodic spatial lattice and to correct for the finite temporal extent of the lattice. Equal time correlation functions are shown to determine a sum of the ground state rms radius plus a polarization correction, and it is shown how to extract the hadron polarizability from unequal time correlation functions. Image-corrected correlation functions calculated in quenched lattice QCD are presented and used to determine the size of the pion and nucleon.

We present an investigation of various gauge invariant de nitions of the qq BetheSalpeter (BS) amplitude for mesons in lattice QCD, and compare them to the Coulomb and Landau gauge BS amplitudes. We show that the gauge invariant BS amplitude is considerably broadened by the use of \fat" gauge links (constructed by smearing the links of the original lattice). A qualitative demonstration of the Lorentz contraction of the pion wavefunction at non-zero momentum is given. We also calculate density-density correlations and discuss the limitations in extracting the charge radius of the pion from these observables. Lastly, the polarization dependence of the BS amplitude for the meson is exhibited, and we extract the relative admixture of l = 0 and l = 2 states showing that simple hydrogen-like wavefunctions provide a good t. March 1993

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We describe an e cient method to compute the volume of hexahedral cells used in three-dimensional hydrodynamics simulations. We consider two common methods for treating the hexahedron using triangular boundaries.Motivation