arXiv subjects

James V. Steele

Publications and source records attributed to James V. Steele.

At least 19 recordsLinked to original sources

arXiv2001

Langevin Evolution of Disoriented Chiral Condensate

As the matter produced in a relativistic heavy ion collision cools through the QCD phase transition, the dynamical evolution of the chiral condensate will be driven out of thermal equilibrium. As a prelude to analyzing this evolution, and in particular as a prelude to learning how rapid the cooling must be in order for significant deviations from equilibrium to develop, we present a detailed analysis of the time-evolution of an idealized region of disoriented chiral condensate. We set up a Langevin field equation which can describe the evolution of these (or more realistic) linear sigma model configurations in contact with a heat bath representing the presence of other shorter wavelength degrees of freedom. We first analyze the model in equilibrium, paying particular attention to subtracting ultraviolet divergent classical terms and replacing them by their finite quantum counterparts. We use known results from lattice gauge theory and chiral perturbation theory to fix nonuniversal constants. The result is a theory which is ultraviolet cutoff independent and that reproduces quantitatively the expected equilibrium behavior of the quantum field theory of pions and sigma fields over a wide range of temperatures. Finally, we estimate the viscosity $η(T)$, which controls the dynamical timescale in the Langevin equation, by requiring that the timescale for DCC decay agrees with previous calculations. The resulting $η(T)$ is larger than that found perturbatively. We also determine the temperature below which the classical field Langevin equation ceases to be a good model for the quantum field dynamics.

hep-ph↗

arXiv2000

Effective Field Theory Power Counting at Finite Density

Effective field theory is applied to finite-density systems with an unnaturally large scattering length, such as neutron matter. A new organizational scheme is identified and connected with an expansion in inverse powers of the number of space-time dimensions. This power counting allows for analytic calculations in many-body systems and reproduces the hole-line expansion of traditional nuclear physics.

nucl-th↗

arXiv2000

Meron Pairs and Fermion Zero Modes

Merons, conjectured as a semiclassical mechanism for color confinement in QCD, have been described analytically by either infinite action configurations or an Ansatz with discontinuous action. We construct a smooth, finite action, stationary lattice solution corresponding to a meron pair. We also derive an analytical solution for the zero mode of the meron pair Ansatz, show that it has the qualitative behavior of the exact zero mode of the lattice solution, and propose the use of zero modes to identify meron gauge field configurations in stochastic evaluations of the lattice QCD path integral.

hep-lat↗

arXiv2000

Can Merons Describe Confinement?

Merons, conjectured as a semiclassical mechanism for color confinement in QCD, are topological charge-1/2, singular solutions to the classical Yang-Mills equations of motion. I will discuss how lattice techniques can extend the study of merons to nonsingular stationary solutions without destroying properties believed to be essential for confinement, and how zero modes can be used to identify these gauge field configurations in stochastic evaluations of the lattice QCD path integral.

hep-lat↗

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.

nucl-th↗

arXiv1999

Describing Nuclear Matter with Effective Field Theories

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arXiv1999

The Long and Short of Nuclear Effective Field Theory Expansions

Nonperturbative effective field theory calculations for NN scattering seem to break down at rather low momenta. By examining several toy models, we clarify how effective field theory expansions can in general be used to properly separate long- and short-range effects. We find that one-pion exchange has a large effect on the scattering phase shift near poles in the amplitude, but otherwise can be treated perturbatively. Analysis of a toy model that reproduces 1S0 NN scattering data rather well suggests that failures of effective field theories for momenta above the pion mass can be due to short-range physics rather than the treatment of pion exchange. We discuss the implications this has for extending the applicability of effective field theories.

nucl-th↗

arXiv1999

Convergence Issues in Nucleon-Nucleon Effective Field Theory

The origin of the breakdown scale in an effective field theory treatment of nuclear forces is investigated. Different organizational schemes used in the nonperturbative calculation of nucleon-nucleon scattering seem to break down for different physical reasons. It is argued that the same physics is actually responsible, namely strong two-pion exchange as also observed in the pion scalar form factor. The prospect of extending the applicability of the effective field theory beyond this breakdown point is discussed.

nucl-th↗

arXiv1999

The Role of Nucleons in Electromagnetic Emission Rates

Electromagnetic emission rates from a thermalized hadronic gas are important for the interpretation of dilepton signals from heavy-ion collisions. Although there is a consensus in the literature about rates for a pure meson gas, qualitative differences appear with a finite baryon density. We show this to be essentially due to the way in which the pi-N background is treated in regards to the nucleon resonances. Using a background constrained by unitarity and broken chiral symmetry, it is emphasized that the thermalized hadronic gas can be considered dilute.

hep-ph↗

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

Lifetime of a Disoriented Chiral Condensate

The lifetime of a disoriented chiral condensate formed within a heat bath of pions is calculated assuming temperatures and densities attainable at present and future heavy-ion colliders. A generalization of the reduction formula to include coherent states allows us to derive a formula for the decay rate. We predict the half-life to be between 4 and 7 fm/c, depending on the assumed pion density. We also calculate the lifetime in the presence of higher resonances and baryons, which shortens the lifetime by at most 20%.

nucl-th↗

arXiv1998

Comparison of Regularization Methods for Nucleon-Nucleon Effective Field Theory

The characteristics of a meaningful effective field theory (EFT) analysis are discussed and compared with traditional approaches to NN scattering. A key feature of an EFT treatment is a systematic expansion in powers of momentum, which is demonstrated using an error analysis introduced by Lepage. A clear graphical determination of the radius of convergence for the momentum expansion is also obtained. I 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 other observables are also shown.

nucl-th↗

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.

nucl-th↗

arXiv1997

On-Shell Approach to Pion-Nucleon Physics

We discuss an on-shell approach to pion-nucleon physics that is consistent order by order in a $1/f_π$ expansion with the chiral reduction formula, crossing, and relativistic unitarity. A number of constraints between the on-shell low-energy parameters are derived at tree level in the presence of the pion-nucleon sigma term, and found to be in fair agreement with experiment. We analyze the nucleon form factors, and the $πN\to πN$ scattering amplitude to one-loop, as well as $πN\toππN$ to tree level. We use the latter to derive a new constraint for the pion-nucleon sigma term at threshold. We compare our results to both relativistic and non-relativistic chiral perturbation theory, and discuss the convergence character of the expansion in light of experiment.

hep-ph↗

arXiv1997

Dilepton and Photon Emission Rates from a Hadronic Gas II

We extend our recent analysis of the dilepton and photon emission rates to the case of finite temperature and baryon density, within the context of a density expansion. To leading order, the effects of the baryon density are assessed using data (photon emission) or constraints from broken chiral symmetry (dilepton emission). Next to leading order effects are worked out, and their contribution qualitatively assessed. The opening of the $πN$ cut causes the photon rate to saturate the empirical photon yield of WA80, but may not be enough to fully account for the excess low mass dileptons seen at CERES.

hep-ph↗

arXiv1996

Dilepton and Photon Emission Rates from a Hadronic Gas

We analyze the dilepton and photon emission rates from a hadronic gas using chiral reduction formulas and a virial expansion. The emission rates are reduced to pertinent vacuum correlation functions, most of which can be assessed from experiment. Our results indicate that in the low mass region, the dilepton and photon rates are enhanced compared to most of the calculations using chiral Lagrangians. The enhancement is further increased through a finite pion chemical potential. An estimate of the emission rates is also made using Haag's expansion for the electromagnetic current. The relevance of these results to dilepton and photon emission rates in heavy-ion collisions is discussed.

hep-ph↗

arXiv1995

The Pion-Nucleon Sigma Term and the Goldberger-Treiman Discrepancy

The pion-nucleon sigma term is shown to be equal to the Goldberger-Treiman discrepancy at tree level. Its value estimated this way is very sensitive to the pion-nucleon coupling constant g_{pi NN}. This relation, when combined with the pion-nucleon S-wave scattering lengths, yields a new determination of g_{pi NN} at tree level. The results of a one-loop analysis are also summarized determining an allowed range for the induced pseudoscalar coupling constant g_P.

hep-ph↗

arXiv1995

Master Formula Approach to Chiral Symmetry Breaking: Pi-Pi scattering

The master formula approach to chiral symmetry breaking is used to analyze the phase shifts for $ππ$ scattering in the elastic region. The results are in excellent agreement with the data for the phase shifts up to the K-K bar threshold. Our analysis shows that the pi-pi data near threshold in the scalar channel favors a large quark condensate in the vacuum, i.e. \sim -(240 MeV)^3.

hep-ph↗