arXiv subjects

P. J. Ellis

Publications and source records attributed to P. J. Ellis.

24 records · Page 2Linked to original sources

arXiv1995

Strangeness in Hadronic Stellar Matter

We examine the presence of strangeness-bearing components, hyperons and kaons, in dense neutron star matter. Calculations are performed using relativistic mean field models, in which both the baryon-baryon and kaon-baryon interactions are mediated by meson exchange. Results of kaon condensation are found to be qualitatively similar to previous work with chiral models, if compatibility of the kaon optical potentials is required. The presence of strangeness, be it in the form of hyperons or kaons, implies a reduction in the maximum mass and a relatively large number of protons, sufficient to allow rapid cooling to take place. The need to improve upon the poorly-known couplings of the strange particles, which determine the composition and structure of neutron stars, is stressed. We also discuss generic problems with effective masses in mean field theories.

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arXiv1993

Approximate Treatment of Hermitian Effective Interactions and a Bound on the Error

The Hermitian effective interaction can be well-approximated by (R+R^dagger)/2 if the eigenvalues of omega^dagger omega are small or state-independent(degenerate), where R is the standard non-Hermitian effective interaction and omega maps the model-space states onto the excluded space. An error bound on this approximation is given.

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arXiv1993

An Effective Lagrangian with Broken Scale and Chiral Symmetry Applied to Nuclear Matter and Finite Nuclei

We study nuclear matter and finite nuclei with a chiral Lagrangian which generalizes the linear $σ$ model and also accounts for the QCD trace anomaly by means of terms which involve the $σ$ and $\vmgπ$ fields as well as the glueball field $ϕ$. The form of the scale invariant term leading to an omega meson mass, after symmetry breaking, could involve coupling to $ϕ^2$ or $σ^2$ or some linear combination thereof. In fact an $ω_μω^μϕ^2$ form is strongly favored by the bulk properties of nuclei, which also rather strongly constrain the other parameters. A reasonable description of the closed shell nuclei oxygen, calcium and lead can be achieved, although the spin-orbit splittings are somewhat smaller than the experimental values.

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arXiv1993

Finite Temperature Many-Body Theory with the Lipkin Model

We have compared exact numerical results for the Lipkin model at finite temperature with Hartree-Fock theory and with the results of including in addition the ring diagrams. In the simplest version of the Lipkin model the Hartree-Fock approach shows a ``phase transition" which is absent in the exact results. For more realistic cases, Hartree-Fock provides a very good approximation and a modest improvement is obtained by adding the ring diagrams.

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arXiv1993

Neutron Stars and Nuclei in the Modified Relativistic Hartree Approximation

We have examined the properties of neutron-rich matter and finite nuclei in the modified relativistic Hartree approximation for several values of the renormalization scale, $μ$, around the standard choice of $μ$ equal to the nucleon mass $M$. Observed neutron star masses do not effectively constrain the value of $μ$. However for finite nuclei the value $μ/M=0.79$, suggested by nuclear matter data, provides a good account of the bulk properties with a sigma mass of about 600 MeV. This value of $μ/M$ renders the effective three and four body scalar self-couplings to be zero at 60\% of equilibrium nuclear matter density, rather than in the vacuum. We have also found that the matter part of the exchange diagram has little impact on the bulk properties of neutron stars.

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arXiv1993

Iterative Solution for Effective Interactions in a System with Non-degenerate Unperturbed Energies

We generalize the Lee-Suzuki iteration method for summing the folded diagram series to the case where the unperturbed model-space energies are non-degenerate. A condition is derived for the convergence of the iteration scheme and this depends on the choice of the model space projection operators. Two choices are examined, in the first the projection operators are defined in terms of the unperturbed states and in the second they are defined in terms of the eigenfunctions obtained at each stage of the iteration. As is illustrated by calculations with a simple model, the second procedure gives the better convergence and, by suitable choice of the starting energies, allows the reproduction of any subset of the exact eigenvalues.

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