arXiv subjects

Frank X. Lee

Publications and source records attributed to Frank X. Lee.

66 records · Page 4Linked to original sources

arXiv2000

N* Mass Spectrum from an Anisotropic Action

Results are reported for N* masses in the 1/2+ and 1/2- sectors on an anisotropic lattice. The gauge action is the usual plaquette plus rectangle type, the quark action is of the D234 type, both having tadpole-improved tree-level coefficients. Clear splittings from the nucleon ground state are observed with smeared operators and 500 configurations. The first excited states in each channel are further isolated, using a correlation matrix and the variational method. The basic pattern of these splittings is consistent with experimental observations.

hep-lat↗

arXiv1998

Negative-Parity Baryon Spectroscopy

Results are reported for the first calculation of the low-lying spin-1/2 odd-parity octet baryon masses using ${\cal O}(a^2)$ improved gluon and fermion actions. Methods for removing even-parity ground-state contaminations from the two-point correlation functions at zero and finite momenta are outlined. We investigate the properties of two odd-parity interpolating fields based on the established interpolating fields for the nucleon ground state. Isolation of the lowest-lying odd-parity state appears to be sufficient to begin exploring odd-parity $N \to N^*$ electromagnetic transition form factors.

hep-lat↗

arXiv1998

Improved Lattice QCD Actions for Hadron Phenomenology

The masses and dispersions of light hadrons are calculated in lattice QCD using an order a^2 tadpole-improved gluon action and the Dchi34 action, an order a^2 tadpole-improved next-nearest-neighbor fermion action originally proposed by Hamber and Wu. Two lattices of constant volume with lattice spacings of approximately 0.40 fm and 0.24 fm are considered. The results reveal some scaling violations at the coarser lattice spacing on the order of 5%. At the finer lattice spacing, the calculated N/rho mass ratio reproduces state-of-the-art results using unimproved actions. Good dispersion and rotational invariance up to momenta p*a the order of 1 are also found. The relative merit of alternative choices for improvement operators is assessed through close comparisons with other plaquette-based tadpole-improved actions.

hep-lat↗

arXiv1998

Higher Nucleon Resonances in Exclusive $(γ, πN)$ Reactions on Nuclei

We report calculations for exclusive pion photoproduction on nuclei beyond the first resonance region in the distorted wave impulse approximation. The elementary operator contains contributions from the resonances $P_{33}(1232)$, $P_{11}(1440)$, $D_{13}(1520)$, $D_{33}(1740)$, $S_{11}(1535)$, $S_{11}(1650)$, and $F_{15}(1680)$, in addition to the usual Born plus vector meson contributions. It gives a good description of single pion photoproduction data up to about 1.1 GeV. Final state interactions are incorporated via optical potentials and are found to be substantial in the coincidence cross sections, but insensitive to the photon asymmetry. Clear signatures of possible medium modifications of the $D_{13}$ and $F_{15}$ resonances are predicted.

nucl-th↗

arXiv1997

Light Hadron Spectroscopy on Coarse Lattices with O(a^2) Mean-Field Improved Actions

The masses and dispersions of light hadrons are calculated in lattice QCD using an O(a^2) tadpole-improved gluon action and an O(a^2) tadpole-improved next-nearest-neighbor fermion action originally proposed by Hamber and Wu. Two lattices of constant volume with lattice spacings of approximately 0.40 fm and 0.24 fm are considered. The results reveal some scaling violations at the coarser lattice spacing on the order of 5%. At the finer lattice spacing, the calculated mass ratios reproduce state-of-the-art results using unimproved actions. Good dispersion and rotational invariance up to momenta of pa ~ 1 are also found. The relative merit of alternative choices for improvement operators is assessed through close comparisons with other plaquette-based tadpole-improved actions.

hep-lat↗

arXiv1997

Determination of Decuplet Baryon Magnetic Moments from QCD Sum Rules

A complete set of QCD sum rules for the magnetic moments of decuplet baryons are derived using the external field method. They are analyzed thoroughly using a Monte-Carlo based procedure. Valid sum rules are identified under the criteria of OPE convergence and ground state dominance and their predictions are obtained. The performances of these sum rules are further compared and a favorable sum rule is designated for each member. Correlations between the input and the output parameters are examined and large sensitivities to the quark condensate magnetic susceptibility $χ$ are found. Using realistic estimates of the QCD input parameters, the uncertainties on the magnetic moments are found relatively large and they can be attributed mostly to the poorly-known $χ$. It is shown that the accuracy can be improved to the 30% level, provided the uncertainties in the QCD input parameters can be determined to the 10% level. The computed magnetic moments are consistent with existing data. Comparisons with other calculations are made.

hep-ph↗

arXiv1997

Magnetic Moments of $Δ^{++}$ and $Ω^-$ from QCD Sum Rules

QCD sum rules for the magnetic moments of $Δ^{++}$ and $Ω^-$ are derived using the external field method. They are analyzed by a Monte-Carlo based procedure, using realistic estimates of the QCD input parameters. The results are consistent with the measured values, despite relatively large errors that can be attributed mostly to the poorly-known vacuum susceptibility $χ$. It is shown that a 30% level accuracy can be achieved in the derived sum rules, provided the QCD input parameters are improved to the 10% level.

hep-ph↗

arXiv1997

Predicative Ability of QCD Sum Rules for Decuplet Baryons

QCD sum rules for decuplet baryon two-point functions are investigated using a comprehensive Monte-Carlo based procedure. In this procedure, all uncertainties in the QCD input parameters are incorporated simultaneously, resulting in realistic estimates of the uncertainties in the extracted phenomenological parameters. Correlations between the QCD input parameters and the phenomenological parameters are studied by way of scatter plots. The predicted couplings are useful in evaluating matrix elements of decuplet baryons in the QCD sum rule approach. They are also used to check a cubic scaling law between baryon couplings and masses, as recently found by Dey and coworkers. The results show a significant reduction in the scaling constant and some possible deviations from the cubic law.

hep-ph↗

arXiv1996

New QCD Sum Rules for Nucleon Axial Vector Coupling Constants

Two new sets of QCD sum rules for the nucleon axial 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 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 coupling constants, as the relative errors are large. The errors range from approximately 50 to 100% compared to the nucleon mass obtained from the same method, which has only 10% to 25% error. The origin of the large errors is examined. Previous analyses of these coupling constants are based on sum rules that have poor OPE 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 new treatment provides exponential suppression of their contributions relative to the ground state. Implications for other nucleon current matrix elements are also discussed.

nucl-th↗

arXiv1996

Light Hadron Masses on Coarse Lattices with Improved Actions

The lattice QCD field is currently undergoing a revolution in the manner in which improvements of the approach are being implemented. We are examining the utility of order-a^2-improved tadpole-improved actions for the light fermion sector. The masses of the low-lying hadrons are calculated using the order-a^2-improved gluon action and the order-a^2-improved 2-link fermion action of Hamber and Wu with further tadpole improvement. Initial results on a 6^3 x 12 lattice at beta = 6.25 and a 10^3 x 16 lattice at beta = 7.0 are presented. The corresponding lattice spacings are approximately 0.40 fm and 0.24 fm respectively. These lattices provide a constant volume in which the scaling of observables is assessed.

hep-lat↗

arXiv1996

New QCD Sum Rules for Nucleon Axial Vector Coupling Constants

New QCD sum rules for the nucleon axial-vector current coupling constants are derived. One set is derived from a generalized correlator of spin-1/2 interpolating fields, and the other with a mixed correlator of spin-1/2 and spin-3/2 fields. Using a recently-developed Monte-Carlo based uncertainty analysis and techniques for determining the optimal interpolating fields, the predicative ability of QCD sum rules for axial-vector current coupling constants are determined. We find the previous studies of this matrix element are based on an invalid sum rule. A combined analysis of this sum rule together with the nucleon mass sum rules yields $g_A=1.48\pm^{1.06}_{0.65}$. The relative error of approximately 60% is large compared to a 10% error for the nucleon mass obtained from the same input parameters.

nucl-th↗

arXiv1995

Abelian Dominance in Chiral Symmetry Breaking

Calculations of the chiral condensate $\langle \barψ ψ\rangle$ 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).

hep-lat↗