We propose a model of the nucleon and delta based on the idea that strong QCD forces on length scales -0.2-1 fm result in hidden chiral SU(2) X SU(2) symmetry and that there is a separation of roles between these forces which are also responsible for binding quarks in hadrons and the forces which produce absolute confinement. This leads us to study a linear u model describing the interactions of quarks, u mesons, and pions. We have solved this model in the semiclassical (mean-field) approximation for the hedgehog baryon state. We refer to this solution as a chiral soliton. In the semiclassical approximation the hedgehog state is a linear combination of Nand A. We project this state onto states of good spin and isospin to calculate matrix elements of various operators in these states. Our results are in reasonable agreement with the observed properties of the nucleon. The mesonic contributions to g~ and a ( r N ) are about two to three times too large, suggesting the need for quantum corrections.

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The nature of the nonrelativistic nucleon-nucleon potential in the large-N c limit is discussed. In particular, we address the consistency of the meson-exchange picture of nucleon interactions. It is shown that the nonrelativistic nucleon-nucleon potential extracted from the Feynmann graphs up to and including two-mesonexchange diagrams satisfies the spin-flavor counting rules of Kaplan and Savage and of Kaplan and Manohar, provided the nucleon momenta is of order N c 0 . The key to this is a cancellation of the retardation effect of the box graphs against the contributions of the crossed-box diagram. The consistency requires including ⌬ as an intermediate state.

We study the role of direct (i.e. small-scale) instantons in QCD correlation functions for the nucleon. They generate sizeable, nonperturbative corrections to the conventional operator product expansion, which improve the quality of both QCD nucleon sum rules and cure the long-standing stability problem, in particular, of the chirally odd sum-rule.12.38. Lg, 14.20.Dh Typeset using REVT E X 1 QCD sum rules, introduced by Shifman, Vainshtein and Zakharov (SVZ) [1], provide a systematic, nonperturbative framework for the calculation of hadron properties. They have been intensely studied and applied over the last decade and produced the most exhaustive model-independent analysis of hadron properties to date.The sum-rule approach is based on the comparison of two "dual" descriptions for correlation functions of hadronic currents, in terms of quarks and gluons on the left-hand side (LHS) and in terms of hadrons on the right-hand side (RHS). The RHS uses a simple parametrization of the spectral function in terms of the hadron parameters, such as mass, overlap with the source current and continuum threshold. The QCD calculation on the LHS employs a non-perturbative operator-product expansion (OPE). Long-distance bulk properties of the physical vacuum are efficiently parametrized in terms of the vacuum expectation values of composite quark-gluon operators ("condensates"), which are independent of the hadron considered. The short-distance physics is contained in the perturbatively calculated Wilson coefficients. The inverse renormalization scale of the operators, µ −1 , serves as the dividing line separating long and short distances.Both sides of the sum rules are then Borel-transformed, and the hadron parameters are determined by fitting the two sides in the fiducial region, i.e. in the range of Borel mass values in which both descriptions of the correlator are expected to be adequate. The quality of this fit is the only intrinsic criterion for the accuracy and reliability of the sum rules. If it is met sufficiently well, the resulting hadron parameters will be approximately independent of the Borel mass in the fiducial region. The nucleon sum rules, however, do not show such a stability plateau, despite many improvements over the last decade [2]. It seems that some relevant physics in the fiducial region (around 1 GeV) is missing in the OPE. In this letter we suggest that small-size instantons [3], termed "direct" by SVZ, provide the dominant part of this physics.Instantons [4] are classical solutions of the euclidean Yang-Mills equation. Due to the infrared complexities of QCD, their quantum properties and vacuum distribution cannot yet be derived from first principles. A consistent picture of their importance and bulk 2 features has been established, however, by extensive phenomenological [5,6], analytical [7] and numerical studies (in the instanton liquid model [6] and on the lattice, e.g. in ref.[8]).They indicate, in particular, that the average instanton size ρ c in the vacuum is considerably smaller th...

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