arXiv subjects

J. A. McNeil

Publications and source records attributed to J. A. McNeil.

9 recordsLinked to original sources

arXiv2009

Separable Expansions of V_{low} for 2- and 3-Nucleon Systems

We present an alternative organizational scheme for developing effective theories of 2- and 3-body systems that is systematic, accurate, and efficient with controlled errors. To illustrate our approach we consider the bound state and scattering properties of the 2- and 3-nucleon systems. Our approach combines the computational benefits of using separable potentials with the improved convergence properties of potentials evolved with a renormalization group procedure. Long ago Harms showed that any potential can be expanded in a series of separable terms, but this fact is only useful if the expansion can be truncated at low order. The separable expansion provides an attractive organizational scheme that incorporates the two body bound state in the leading term while allowing for systematic corrections thereafter. We show that when applied to a renormalization group-evolved potential, the separable expansion converges rapidly, with accurate results for both 2- and 3-body scattering processes using only two separable terms.

nucl-th↗

arXiv2009

Convergence of 2-Body Effective Range Expansions for $nd$ Quartet Scattering

We examine the convergence properties of the 2-nucleon Effective Range Expansion as used in Effective Theories (ET-ERE's) for 3-nucleon calculations. We accomplish this by accounting for the 2-body dynamics with a simple rank-1 separable 2-body potential where the finite range effects can be incorporated systematically in both the 2- and 3-body problems. We make our initial comparisons in the simple context of the $^3S_1$ 2-nucleon channel and the $^4S_{3/2}$ 3-nucleon channel. We find that convergence problems for some of the 3-nucleon scattering amplitudes using the ET-ERE can be traced to its poor account of finite range effects that soften the momentum dependence of the deuteron propagator in the Faddeev kernel. In contrast, our simple separable potential with dipole form factors works very well in all cases considered.

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arXiv1998

Chiral baryon in the coherent pair approximation

We revisit the work of K. Goeke, M. Harvey, F. Grümmer, and J. N. Urbano (Phys. Rev. {\bf D37}, 754 (1988)) who considered a chiral model for the nucleon based on the linear sigma model with scalar-isoscalar scalar-isovector mesons coupled to quarks and solved using the coherent-pair approximation. In this way the quantum pion field can be treated in a non-perturbative fashion. In this work we review this model and the coherent pair approximation correcting several errors in the earlier work. We minimize the expectation value of the chiral hamiltonian in the ansatz coherent-pair ground state configuration and solve the resulting equations for nucleon quantum numbers. We calculate the canonical set of nucleon observables and compare with the Hedgehog model and experiment. Using the corrected equations yield slightly different values for nucleon observables but do not correct the large virial deviation in the $π$-nucleon coupling. Our results therefore do not significantly alter the conclusions of Goeke, et al..

hep-ph↗

arXiv1997

Renormalization Group Flow Equations for the Sigma Model

We present a nonperturbative renormalization group solution of the Gell-Mann--Levy $σ$-model which was originally proposed as a phenomenological description of the dynamics of nucleons and mesons. In our version of the model the fermions are interpreted as quarks which interact via the $σ$ and $π$ mesons. We derive and numerically solve renormalization group (RG) flow equations to leading order in a derivative expansion to study the behavior of the model as it evolves from high to low momentum scales. We develop an expansion in chiral-symmetry-breaking which enables us to track this symmetry breaking with the evolution of the scale. We use infrared observables to constrain the phenomenology allowing predictions of other quantities such as $π-π$ scattering lengths. The results show improvement over the tree level calculation and are consistent with experiment and the results of alternate theoretical approaches such as chiral perturbation theory and lattice gauge theory.

hep-ph↗

arXiv1995

Derivative Expansion of Renormalization Group Effective Potentials for Scalar Field Theories

This paper has been withdrawn to address an omission. It will be resubmitted in the near future.

hep-th↗

arXiv1994

Monte Carlo and Renormalization Group Effective Potentials in Scalar Field Theories

We study constraint effective potentials for various strongly interacting $ϕ^4$ theories. Renormalization group (RG) equations for these quantities are discussed and a heuristic development of a commonly used RG approximation is presented which stresses the relationships among the loop expansion, the Schwinger-Dyson method and the renormalization group approach. We extend the standard RG treatment to account explicitly for finite lattice effects. Constraint effective potentials are then evaluated using Monte Carlo (MC) techniques and careful comparisons are made with RG calculations. Explicit treatment of finite lattice effects is found to be essential in achieving quantitative agreement with the MC effective potentials. Excellent agreement is demonstrated for $d=3$ and $d=4$, O(1) and O(2) cases in both symmetric and broken phases.

hep-lat↗

arXiv1994

Goldstone Theorem in the Gaussian Functional Approximation to the Scalar $ϕ^{4}$ Theory

We verify the Goldstone theorem in the Gaussian functional approximation to the $ϕ^{4}$ theory with internal O(2) symmetry. We do so by reformulating the Gaussian approximation in terms of Schwinger-Dyson equations from which an explicit demonstration of the Goldstone theorem follows directly.

hep-th↗

arXiv1993

Chiral Baryon with Quantized Pions

We study a hybrid chiral model for the nucleon based on the linear sigma model with explicit quarks. We solve the model using a Fock-space configuration consisting of three quarks plus three quarks and a pion as the ground state ansatz in place of the ``hedgehog'' ansatz. We minimize the expectation value of the chiral hamiltonian in this ground state configuration and solve the resulting equations for nucleon quantum numbers. We calculate the canonical set of nucleon observables and compare with previous work.

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arXiv1992

Relativistic Nuclear Matter with Self-Consistent Correlation Energy

We study relativistic nuclear matter in the $σ- ω$ model including the ring-sum correlation energy. The model parameters are adjusted self-consistently to give the canonical saturation density and binding energy per nucleon with the ring energy included. Two models are considered, mean-field-theory where we neglect vacuum effects, and the relativistic Hartree approximation where such effects are included but in an approximate way. In both cases we find self-consistent solutions and present equations of state. In the mean-field case the ring energy completely dominates the attractive part of the energy density and the elegant saturation mechanism of the standard approach is lost, namely relativistic quenching of the scalar attraction. In the relativistic Hartree approach the vacuum effects are included in an approximate manner using vertex form factors with a cutoff of 1 - 2 GeV, the range expected from QCD. Due to the cutoff, the ring energy for this case is significantlysmaller, and we obtain self-consistent solutions which preserve the basic saturation mechanism of the standard relativistic approach.

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