arXiv subjects

Derek B. Leinweber

Publications and source records attributed to Derek B. Leinweber.

At least 163 records · Page 9Linked to original sources

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

Not Strange but Bizarre Physics from the SAMPLE Experiment

Since the report of the SAMPLE Collaboration suggesting the strange-quark contribution to nucleon magnetic moments, G_M^s(0), may be greater than zero, numerous models have appeared supporting positive values for G_M^s(0). In this paper the bizarre physics associated with G_M^s(0) > 0 is illustrated. Using new lattice QCD results, our best estimate for G_M^s(0) shifts slightly from G_M^s(0) = -0.75 +/- 0.30 mu_N, to G_M^s(0) = -0.62 +/- 0.26 mu_N.

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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↗

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.

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arXiv1996

QCD Sum Rules for Skeptics

A new Monte-Carlo based uncertainty analysis is introduced to quantitatively determine the predictive ability of QCD sum rules. A comprehensive analysis of ground state rho-meson and nucleon spectral properties is performed. Many of the findings contradict the conventional wisdom of both practitioners and skeptics alike. Associations between the phenomenological fit parameters are particularly interesting as they reveal how the sum rules resolve the spectral properties. The use of derivative sum rules for the determination of rho-meson spectral properties is shown to be a very unfavorable approach. Most prior nucleon sum rule analyses are based on a sum rule which is found to be invalid; the results are suspect, and should be reevaluated. The ``Ioffe formula'', argued by many to qualitatively encapsulate a description of the nucleon mass in terms of the chiral symmetry breaking order parameter is misleading at best. QCD Sum Rules are found to be self-consistent without contributions from direct instantons. This implies that instanton effects are adequately accounted for in the nonperturbative vacuum condensates. This in-depth examination of QCD sum rule self consistency paints a favorable picture for further quantitative refinements of the QCD sum rule approach.

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arXiv1996

Background-Field Formalism in Quantum Systems

The background-field formalism is used extensively in fundamental approaches to QCD to explore hadronic matrix elements of various currents. While the lattice QCD approach is formulated in the fully-interacting Hilbert space, which includes both QCD and background field interactions, the QCD sum-rule formalism is traditionally developed in the pure-QCD Hilbert space. The latter approach encounters difficulties with excited state contaminations which are not exponentially suppressed and requires extrapolations to isolate the desired physics. Proponents of the pure-QCD Hilbert space formalism used in the QCD sum-rule approach have criticized the lattice QCD approach as neglecting important physics. In this letter, the equivalence of the two approaches is established and the flaws in the former criticisms are resolved. Finally, the application of the fully-interacting Hilbert space formalism to the study of electromagnetic polarizabilities is outlined.

hep-ph↗

arXiv1996

Rho-Omega Mixing via QCD Sum Rules with Finite Mesonic Widths

Based on the analysis of both Borel and Finite-Energy QCD sum rules, the inclusion of finite mesonic widths leads to a dramatic effect on the predictions for the momentum dependence of the rho-omega mixing matrix element. It is shown that the rho-omega mixing matrix element traditionally discussed in the literature, has the same sign and similar magnitude in the space-like region as its on-shell value. This contrasts the zero-width result where the mixing matrix element is typically of opposite sign in the space-like region.

nucl-th↗

arXiv1996

New QCD Sum Rules for Nucleons in Nuclear Matter

Two new QCD sum rules for nucleons in nuclear matter are obtained from a mixed correlator of spin-1/2 and spin-3/2 interpolating fields. These new sum rules, which are insensitive to the poorly known four-quark condensates, provide additional information on the nucleon scalar self-energy. These new sum rules are analyzed along with previous spin-1/2 interpolator-based sum rules which are also insensitive to the poorly known four-quark condensates. The analysis indicates consistency with the expectations of relativistic nuclear phenomenology at nuclear matter saturation density. However, a weaker density dependence near saturation is suggested. Using previous estimates of in-medium condensate uncertainties, we find $M^* = 0.64^{+0.13}_{-0.09}$ GeV and $Σ_v = 0.29^{+0.06}_{-0.10}$ GeV at nuclear matter saturation density.

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↗

arXiv1996

Mesonic Width Effects on the Momentum Dependence of the Rho-Omega Mixing Matrix Element

It is shown, in a model independent way, that the large difference in rho and omega widths gives rise to a new source of momentum dependence for the rho-omega mixing matrix element. The q^2 dependence arising due to the meson widths leads to a significant alteration of the result obtained in the zero-width approximation usually discussed in the literature. The origin of this strong momentum dependence lies in the difference between the rho and omega meson widths.

nucl-th↗

arXiv1995

QCD Equalities for Baryon Current Matrix Elements

An examination of the symmetries manifest in the QCD path integral for current matrix elements reveals various equalities among the quark sector contributions. QCD equalities among octet baryon magnetic moments lead to a determination of the disconnected sea-quark contribution to nucleon magnetic moments, which is the most reliable determination in the literature. Matching QCD equalities to recent calculations of decuplet baryon magnetic moments in chiral perturbation theory (ChiPT) reveals an equivalence between ChiPT to order $p^2$ and the simple quark model with an explicit disconnected sea-quark contribution. New insights into SU(3)-flavor symmetry breaking, sea contributions and constituent quark composition are obtained. The strangeness contribution to nucleon magnetic moments is found to be large at $G_M^s(0) = -0.75 \pm 0.30\ μ_N$. The QCD equalities must be followed by any model which hopes to capture the essence of nonperturbative QCD. Not all models are in accord with these symmetries.

hep-ph↗

arXiv1995

New QCD Sum Rules for In-Medium Nucleons

New QCD sum rules for nucleons in nuclear matter are derived from a mixed correlation function of spin-1/2 and spin-3/2 interpolating fields. These sum rules allow a determination of the scalar self-energy of the nucleon independent of the poorly known four-quark condensates. An analysis of these new sum rules in concert with previous nucleon sum rules from spin-1/2 interpolators indicates consistency with the expectations of relativistic mean-field phenomenology. We find $M^* = 0.68 \pm 0.08$ GeV and $Σ_v = 0.31 \pm 0.06$ GeV at nuclear matter saturation density.

nucl-th↗

arXiv1995

Valid QCD Sum Rules for Vector Mesons in Nuclear Matter

QCD sum rules for vector mesons (rho, omega, phi) in nuclear matter are reexamined with an emphasis on the reliability of various sum rules. Monitoring the continuum contribution and the convergence of the operator product expansion plays a crucial role in determining the validity of a sum rule. The uncertainties arising from less than precise knowledge of the condensate values and other input parameters are analyzed via a Monte-Carlo error analysis. Our analysis leaves no doubt that vector-meson masses decrease with increasing density. This resolves the current debate over the behavior of the vector-meson masses and the sum rules to be used in extracting vector meson properties in nuclear matter. We find a ratio of rho-meson masses of $m_{ρ^*}/m_ρ= 0.78\pm0.08$ at nuclear matter saturation density.

nucl-th↗

arXiv1995

Testing QCD Sum Rule Techniques on the Lattice

Results for the first test of the ``crude'' QCD continuum model, commonly used in QCD Sum Rule analyses, are presented for baryon correlation functions. The QCD continuum model is found to effectively account for excited state contributions to the short-time regime of two-point correlation functions and allows the isolation of ground state properties. Confusion in the literature surrounding the physics represented in point-to-point correlation functions is also addressed. These results justify the use of the ``crude'' QCD continuum model and lend credence to the results of rigorous QCD Sum Rule analyses.

nucl-th↗

arXiv1995

Nucleon Properties from Unconventional Interpolating Fields

Interpolating fields, used to excite hadrons from the QCD vacuum in nonperturbative field-theoretic investigations of hadron properties, are explored with an emphasis on unconventional nucleon interpolators. The QCD continuum model for excited state contributions to QCD correlation functions is a central element in extracting the physics contained in these alternate correlation functions. The analysis confirms the independence of nucleon properties obtained from different interpolating fields. However, this independence comes about in a trivial manner. These results provide a resolution to the long standing debate over the optimal nucleon interpolating field to be used in QCD Sum Rule analyses.

nucl-th↗

arXiv1994

Essential Strangeness in Nucleon Magnetic Moments

Effective quark magnetic moments are extracted from experimental measurements as a function of the strangeness magnetic moment of the nucleon. Assumptions made in even the most general quark model analyses are ruled out by this investigation. Ab initio QCD calculations demand a non-trivial role for strange quarks in the nucleon. The effective moments from QCD calculations are reproduced for a strangeness magnetic moment contribution to the proton of 0.11 $μ_N$, which corresponds to $F_2^s(0) = -0.33\ μ_N$.

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