Frank X. Lee

Hadron masses from novel fat link fermion actions

The hadron mass spectrum is calculated in lattice QCD using a novel fat-link clover fermion action in which only the irrelevant operators in the fermion action are constructed using smeared links. The simulations are performed on a 16{sup 3} x 32 lattice with a lattice spacing of a=0.125 fm. We compare actions with n=4 and 12 smearing sweeps with a smearing fraction of 0.7. The n=4 Fat-Link Irrelevant Clover (FLIC) action provides scaling which is superior to mean-field improvement, and offers advantages over nonperturbative 0(a) improvement, including a reduced exceptional configuration problem.

Quasi-free η photoproduction from nuclei

Abstract Quasi-free η photoproduction from nuclei is studied in the Distorted-Wave Impulse Approximation (DWIA). The elementary η production operator contains Born terms, vector meson and nucleon resonance contributions and provides an excellent description of the recent low-energy Mainz measurements on the nucleon. The resonance sector includes the S 11 (1535), P 11 (1440) and D 13 (1520) states whose couplings are fixed by independent electromagnetic and hadronic data. Different models for the ηN t -matrix are used to construct a simple ηA optical potential based on a tϱ approximation. We find that the exclusive A ( γ , ηN ) B process can be used to study medium modifications of the N ∗ resonances, particularly if the photon asymmetry can be measured. The inclusive A ( γ , η ) X reaction is compared to new data obtained on 12 C, 40 Ca, and is found to provide a clear distinction between different models for the ηN t -matrix, and we find that the imaginary part of the ηN scattering length is only about 0.12 fm.

New QCD sum rules for nucleon axial-vector coupling constants

Two new sets of QCD sum rules for the nucleon axial-vector coupling constants are derived using the external-field technique and generalized interpolating fields. An in-depth study of the predicative ability of these sum rules is carried out using a Monte Carlo{endash}based uncertainty analysis. The results show that the standard implementation of the QCD sum rule method has only marginal predicative power for the nucleon axial-vector coupling constants, as the relative errors are large. The errors range from approximately 50{percent} to 100{percent} compared to the nucleon mass obtained from the same method, which has only a 10{percent}{endash}25{percent} error. The origin of the large errors is examined. Previous analyses of these coupling constants are based on sum rules that have poor operator product expansion convergence and large continuum contributions. Preferred sum rules are identified and their predictions are obtained. We also investigate the new sum rules with an alternative treatment of the problematic transitions which are not exponentially suppressed in the standard treatment. The alternative treatment provides exponential suppression of their contributions relative to the ground state. Implications for other nucleon current matrix elements are also discussed. {copyright} {ital 1997} {ital The American Physical Society}

Abelian dominance of chiral symmetry breaking in lattice QCD

Calculations of the chiral condensate {l_angle}{bar {chi}}{chi}{r_angle} on the lattice using staggered fermions and the Lanczos algorithm are presented. Four gauge fields are considered: the quenched non-Abelian field, an Abelian projected field, and monopole and photon fields further decomposed from the Abelian field. Abelian projection is performed in the maximal Abelian gauge and in the Polyakov gauge. The results show that monopoles in the maximal Abelian gauge largely reproduce the chiral condensate values of the full non-Abelian theory, in both SU(2) and SU(3) color. {copyright} {ital 1996 The American Physical Society.}

Chiral extrapolation beyond the power-counting regime

Chiral effective field theory can provide valuable insight into the chiral physics of hadrons when used in conjunction with nonperturbative schemes such as lattice quantum chromodynamics (QCD). In this discourse, the attention is focused on extrapolating the mass of themeson to the physical pion mass in quenched QCD. With the absence of a known experimental value, this serves to demonstrate the ability of the extrapolation scheme to make predictions without prior bias. By using extended effective field theory developed previously, an extrapolation is performed using quenched lattice QCD data that extends outside the chiral power-counting regime. The method involves an analysis of the renormalization flow curves of the low-energy coefficients in a finite-range regularized effective field theory. The analysis identifies an optimal regularization scale, which is embedded in the lattice QCD data themselves. This optimal scale is the value of the regularization scale at which the renormalization of the low-energy coefficients is approximately independent of the range of quark masses considered. By using recent precision, quenched lattice results, the extrapolation is tested directly by truncating the analysis to a set of points above 380 MeV, while temporarily disregarding the simulation results closer to the chiral regime. This tests the ability of the method to make predictions of the simulation results, without phenomenologically motivated bias. The result is a successful extrapolation to the chiral regime.

Hadron masses from a novel fat-link fermion action

J.M. Zanotti, S. Bilson-Thompson, F.D.R. Bonnet, D.B. Leinweber, A.G. Williams and J.B. Zhang, W. Melnitchouk, F.X. Lee

Abelian dominance in chiral symmetry breaking

Calculations of the chiral condensate 〈 ψ ¯ ψ 〉 on the lattice using staggered fermions and the Lanczos algorithm are presented. Three gauge fields are considered: the quenched non-Abelian field, the Abelian field projected in the maximal Abelian gauge, and the monopole field further decomposed from the Abelian field. The results show that the Abelian monopoles largely reproduce the chiral condensate values of the full non-Abelian theory, both in SU(2) and in SU(3).

Chiral symmetry breaking and cooling in lattice QCD

Chiral symmetry breaking is calculated as a function of cooling in quenched lattice QCD. A nonzero signal is found for the chiral condensate beyond 100 cooling steps, suggesting that there is chiral symmetry breaking associated with instantons. Quantitatively, the chiral condensate in cooled gauge field configurations is small compared to the value without cooling. {copyright} {ital 1996 The American Physical Society.}

Photo- and Electroproduction of Eta Mesons on Nucleons and Nuclei

Eta photo- and electroproduction off the nucleon is investigated in an approach that contains Born terms, vector meson and nucleon resonance contributions. In a comparison with the new Mainz data we find a large sensitivity on the elementary ηN N coupling. Our analysis results in a pseudoscalar ηN N coupling with a coupling constant of g ηN N 2 /4π=0.4. Furthermore, we also study coincidence cross sections for eta electroproduction and present calculations for structure functions and kinematical conditions that are most sensitive to the S 11(1535) and the D 13(1520) resonances. Finally, we show results on the inclusive eta photoproduction off complex nuclei with a very good agreement with recent data from Mainz.