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R. J. Furnstahl

Publications and source records attributed to R. J. Furnstahl.

At least 127 records · Page 7Linked to original sources
arXiv2002

Neutron Radii in Mean-Field Models

Bulk nuclear observables such as charge radii and binding energies are well described by both nonrelativistic and covariant mean-field models. However, predictions of neutron radii, which are not tightly constrained by reliable data, vary significantly. The nature of this variation is investigated using correlations between basic properties of the models and the neutron skin thickness in lead. The results suggest that conventional covariant models are too limited. The study is guided by principles and insights of effective field theory (EFT), such as power counting, and the relation of mean-field models to a more general EFT approach to nuclei is discussed.

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arXiv2001

Recent Developments in the Nuclear Many-Body Problem

The study of quantum chromodynamics (QCD) over the past quarter century has had relatively little impact on the traditional approach to the low-energy nuclear many-body problem. Recent developments are changing this situation. New experimental capabilities and theoretical approaches are opening windows into the richness of many-body phenomena in QCD. A common theme is the use of effective field theory (EFT) methods, which exploit the separation of scales in physical systems. At low energies, effective field theory can explain how existing phenomenology emerges from QCD and how to refine it systematically. More generally, the application of EFT methods to many-body problems promises insight into the analytic structure of observables, the identification of new expansion parameters, and a consistent organization of many-body corrections, with reliable error estimates.

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arXiv2001

Are Occupation Numbers Observable?

The question of whether occupation numbers and momentum distributions of nucleons in nuclei are observables is considered from an effective field theory perspective. Field redefinitions lead to variations that imply the answer is negative, as illustrated in the interacting Fermi gas at low density. Implications for the interpretation of (e,e'p) experiments with nuclei are discussed.

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arXiv2000

Field Redefinitions at Finite Density

The apparent dependence of nuclear matter observables on off-shell properties of the two-nucleon potential is re-examined in the context of the effective field theory (EFT) approach. Finite density (thermodynamic) observables are invariant under field redefinitions, which extends the well-known theorem about the invariance of S-matrix elements. Simple examples demonstrate how field redefinitions can shift contributions between purely off-shell two-body interactions and many-body forces, leaving both scattering and finite-density observables unchanged. If only the transformed two-body potentials are kept, however, the nuclear matter binding curves will depend on the off-shell part (generating ``Coester bands''). The correspondence between field redefinitions and unitary transformations, which have traditionally been used to generate ``phase-equivalent'' nucleon-nucleon potentials, is also demonstrated.

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arXiv2000

Effective Field Theory for Dilute Fermi Systems

The virtues of an effective field theory (EFT) approach to many-body problems are illustrated by deriving the expansion for the energy of an homogeneous, interacting Fermi gas at low density and zero temperature. A renormalization scheme based on dimensional regularization with minimal subtraction leads to a more transparent power-counting procedure and diagrammatic expansion than conventional many-body approaches. Coefficients of terms in the expansion with logarithms of the Fermi momentum are determined by the renormalization properties of the EFT that describes few-body scattering. Lessons for an EFT treatment of nuclear matter are discussed.

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arXiv2000

Quantum Hadrodynamics: Evolution and Revolution

The underlying philosophy and motivation for quantum hadrodynamics (QHD), namely, relativistic field theories of nuclear phenomena featuring manifest covariance, have evolved over the last quarter century in response to successes, failures, and sharp criticisms. A recent revolution in QHD, based on modern effective field theory and density functional theory perspectives, explains the successes, provides antidotes to the failures, rebuts the criticisms, and focuses the arguments in favor of a covariant representation.

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arXiv1999

Large Lorentz Scalar and Vector Potentials in Nuclei

In nonrelativistic models of nuclei, the underlying mass scales of low-energy quantum chromodynamics (QCD) are largely hidden. In contrast, the covariant formulations used in relativistic phenomenology manifest the QCD scales in nuclei through large Lorentz scalar and four-vector nucleon self-energies. The abundant and varied evidence in support of this connection and the consequences are reviewed.

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arXiv1999

Parameter Counting in Relativistic Mean-Field Models

Power counting is applied to relativistic mean-field energy functionals to estimate contributions to the energy from individual terms. New estimates for isovector, tensor, and gradient terms in finite nuclei are shown to be consistent with direct, high-quality fits. The estimates establish a hierarchy of model parameters and identify how many parameters are well constrained by bulk nuclear observables. We conclude that four (possibly five) isoscalar, non-gradient parameters, one gradient parameter, and one isovector parameter are well determined by the usual bulk nuclear observables.

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arXiv1999

Perturbative Effective Field Theory at Finite Density

An accurate description of nuclear matter starting from free-space nuclear forces has been an elusive goal. The complexity of the system makes approximations inevitable, so the challenge is to find a consistent truncation scheme with controlled errors. Nonperturbative effective field theories could be well suited for the task. Perturbative matching in a model calculation is used to explore some of the issues encountered in extending effective field theory techniques to many-body calculations.

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arXiv1999

Describing Nuclear Matter with Effective Field Theories

An accurate description of nuclear matter starting from free-space nuclear forces has been an elusive goal. The complexity of the system makes approximations inevitable, so the challenge is to find a consistent truncation scheme with controlled errors. The virtues of an effective field theory approach to this problem are discussed.

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arXiv1998

Removing Pions from Two-Nucleon Effective Field Theory

Two-nucleon effective field theory (EFT) beyond momenta of order the pion mass is studied for both cutoff regularization and dimensional regularization with power divergence subtraction (PDS). Models with two mass scales illustrate how effects of long-distance pion physics must be removed from the coefficients that encode short-distance physics. The modified effective range expansion shows that treating pions perturbatively, as in the PDS power counting, limits the radius of convergence of the EFT. Predictions from both regularization schemes with one-pion contributions are compared to data. The breakdown of the effective field theory occurs for momenta of order 300 MeV, even using the modified effective range expansion. This behavior is shown to be consistent with that expected from two-pion contributions.

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arXiv1998

Regularization Methods for Nucleon-Nucleon Effective Field Theory

Attempts to apply effective field theory (EFT) methods to nonrelativistic nucleon-nucleon (NN) scattering have raised questions about the nature and limitations of an EFT expansion when used nonperturbatively. We discuss the characteristics of a meaningful EFT analysis and compare them with traditional approaches to NN scattering. A key feature of an EFT treatment is a systematic expansion in powers of momentum, which we demonstrate using an error analysis introduced by Lepage. A clear graphical determination of the radius of convergence for the momentum expansion is also obtained. We use these techniques to compare cutoff regularization, two forms of dimensional regularization, and the dibaryon approach, using a simple model for illustration. The naturalness of the parameters and predictions for bound-state energies are also shown.

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arXiv1998

Pion-Nucleus Scattering at Medium Energies with Densities from Chiral Effective Field Theories

Recently developed chiral effective field theory models provide excellent descriptions of the bulk characteristics of finite nuclei, but have not been tested with other observables. In this work, densities from both relativistic point-coupling models and mean-field meson models are used in the analysis of meson-nucleus scattering at medium energies. Elastic scattering observables for 790 MeV/$c$ $π^{\pm}$ on $^{208}$Pb are calculated in a relativistic impulse approximation, using the Kemmer-Duffin-Petiau formalism to calculate the $π^{\pm}$ nucleus optical potential.

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arXiv1997

The Nuclear Spin-Orbit Force in Chiral Effective Field Theories

A compelling feature of relativistic mean-field phenomenology has been the reproduction of spin-orbit splittings in finite nuclei after fitting only to equilibrium properties of infinite nuclear matter. This successful result occurs when the velocity dependence of the equivalent central potential that leads to saturation arises primarily because of a reduced nucleon effective mass. The spin-orbit interaction is then also specified when one works in a four-component Dirac framework. Here the nature of the spin-orbit force in more general chiral effective field theories of nuclei is examined, with an emphasis on the role of the tensor coupling of the isoscalar vector meson (omega) to the nucleon.

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arXiv1997

Relativistic Point-Coupling Models as Effective Theories of Nuclei

Recent studies have shown that concepts of effective field theory such as naturalness can be profitably applied to relativistic mean-field models of nuclei. Here the analysis by Friar, Madland, and Lynn of naturalness in a relativistic point-coupling model is extended. Fits to experimental nuclear data support naive dimensional analysis as a useful principle and imply a mean-field expansion analogous to that found for mean-field meson models.

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arXiv1997

The Skyrme Energy Functional and Naturalness

Recent studies show that successful relativistic mean-field models of nuclei are consistent with naive dimensional analysis and naturalness, as expected in low-energy effective field theories of quantum chromodynamics. The nonrelativistic Skyrme energy functional is found to have similar characteristics.

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arXiv1996

Vacuum Nucleon Loops and Naturalness

Phenomenological studies support the applicability of naturalness and naive dimensional analysis to hadronic effective lagrangians for nuclei. However, one-baryon-loop vacuum contributions in renormalizable models give rise to unnatural coefficients, which indicates that the quantum vacuum is not described adequately. The effective lagrangian framework accommodates a more general characterization of vacuum contributions without reference to a Dirac sea of nucleons.

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