arXiv subjects

V. G. J. Stoks

Publications and source records attributed to V. G. J. Stoks.

15 recordsLinked to original sources

arXiv1999

Strange nuclear matter within Brueckner-Hartree-Fock Theory

We have developed a formalism for microscopic Brueckner-type calculations of dense nuclear matter that includes all types of baryon-baryon interactions and allows to treat any asymmetry on the fractions of the different species (n, p, $Λ$, $Σ^0$, $Σ^+$, $Σ^-$, $Ξ^-$ and $Ξ^0$). We present results for the different single-particle potentials focussing on situations that can be relevant in future microscopic studies of beta-stable neutron star matter with strangeness. We find the both the hyperon-nucleon and hyperon-hyperon interactions play a non-negligible role in determining the chemical potentials of the different species.

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arXiv1999

Soft-core baryon-baryon potentials for the complete baryon octet

SU(3) symmetry relations on the recently constructed hyperon-nucleon potentials are used to develop potential models for all possible baryon-baryon interaction channels. The main focus is on the interaction channels with total strangeness S=-2, -3, and -4, for which no experimental data exist yet. The potential models for these channels are based on SU(3) extensions of potential models for the S=0 and S=-1 sectors, which are fitted to experimental data. Although the SU(3) symmetry is not taken to be exact, the S=0 and S=-1 sectors still provide the necessary constraints to fix all free parameters. The potentials for the S=-2, -3, and -4 sectors, therefore, do not contain any additional free parameters, which makes them the first models of this kind. Various properties of the potentials are illustrated by giving results for scattering lengths, bound states, and total cross sections.

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arXiv1999

Strange hadron matter and SU(3) symmetry

We calculate saturation curves for strange hadron matter using recently constructed baryon-baryon potentials which are constrained by SU(3) symmetry. All possible interaction channels within the baryon octet (consisting of $N$, $Λ$, $Σ$, and $Ξ$) are considered. It is found that a small $Λ$ fraction in nuclear matter slightly increases binding, but that larger fractions ($>10%$) rapidly cause a decrease. Charge-neutral ${N,Λ,Ξ}$ systems, with equal densities for nucleons and cascades, are only very weakly bound. The dependence of the binding energies on the strangeness per baryon, $f_s$, is predicted for various ${N,Λ,Ξ}$ and ${N,Λ,Σ,Ξ}$ systems. The implications of our results in relativistic heavy-ion collisions and the core of a dense star are discussed. We also discuss the differences between our results and previous hadron matter calculations.

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arXiv1998

Weak capture of protons by protons

The cross section for the proton weak capture reaction $^1H(p,e^+ν_e)^2H$ is calculated with wave functions obtained from a number of modern, realistic high-precision interactions. To minimize the uncertainty in the axial two-body current operator, its matrix element has been adjusted to reproduce the measured Gamow-Teller matrix element of tritium $β$ decay in model calculations using trinucleon wave functions from these interactions. A thorough analysis of the ambiguities that this procedure introduces in evaluating the two-body current contribution to the pp capture is given. Its inherent model dependence is in fact found to be very weak. The overlap integral $Λ^2(E=0)$ for the pp capture is predicted to be in the range 7.05--7.06, including the axial two-body current contribution, for all interactions considered.

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arXiv1998

Soft-core hyperon-nucleon potentials

A new Nijmegen soft-core OBE potential model is presented for the low-energy YN interactions. Besides the results for the fit to the scattering data, which largely defines the model, we also present some applications to hypernuclear systems using the G-matrix method. An important innovation with respect to the original soft-core potential is the assignment of the cut-off masses for the baryon-baryon-meson (BBM) vertices in accordance with broken SU(3)$_F$, which serves to connect the NN and the YN channels. As a novel feature, we allow for medium strong breaking of the coupling constants, using the $^3P_0$ model with a Gell-Mann--Okubo hypercharge breaking for the BBM coupling. We present six hyperon-nucleon potentials which describe the available YN cross section data equally well, but which exhibit some differences on a more detailed level. The differences are constructed such that the models encompass a range of scattering lengths in the $ΣN$ and $ΛN$ channels. For the scalar-meson mixing angle we obtained values $θ_S=37$ to 40 degrees, which points to almost ideal mixing angles for the scalar $q\bar{q}$ states. The G-matrix results indicate that the remarkably different spin-spin terms of the six potentials appear specifically in the energy spectra of $Λ$ hypernuclei.

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arXiv1997

Can the magnetic moment contribution explain the A_y puzzle?

We evaluate the full one-photon-exchange Born amplitude for $Nd$ scattering. We include the contributions due to the magnetic moment of the proton or neutron, and the magnetic moment and quadrupole moment of the deuteron. It is found that the inclusion of the magnetic-moment interaction in the theoretical description of the $Nd$ scattering observables cannot resolve the long-standing $A_y$ puzzle.

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arXiv1996

Meson-baryon coupling constants from a chiral-invariant SU(3) Lagrangian and application to NN scattering

We present a chiral-invariant meson-baryon Lagrangian which describes the interactions of the baryon octet with the lowest-mass meson nonets. The nonlinear realization of the chiral symmetry generates pair-meson interaction vertices. The corresponding pair-meson coupling constants can all be expressed in terms of the meson-nucleon-nucleon pseudovector, scalar, and vector coupling constants, and their corresponding $F/(F+D)$ ratios, and for which empirical estimates are given. We show that it is possible to construct an NN potential of reasonable quality satisfying these theoretical and empirical constraints.

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arXiv1996

Soft two-meson-exchange nucleon-nucleon potentials. I. Planar and crossed-box diagrams

Pion-meson-exchange nucleon-nucleon potentials are derived for two nucleons in the intermediate states. The mesons we include are (i) pseudoscalar mesons: $π, η, η'$; (ii) vector mesons: $ρ, ω, ϕ$; (iii) scalar mesons: $a_{0}(980), \varepsilon(760), f_{0}(975)$; and (iv) the $J=0$ contribution from the Pomeron. Strong dynamical pair suppression is assumed, and at the nucleon-nucleon-meson vertices Gaussian form factors are incorporated into the relativistic two-body framework using a dispersion representation for the pion- and meson-exchange amplitudes. The Fourier transformations are performed using factorization techniques for the energy denominators. The potentials are first calculated in the adiabatic approximation to all planar and crossed three-dimensional momentum-space $π$-meson diagrams. Next, we calculate the $1/M$ corrections.

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arXiv1996

Soft two-meson-exchange nucleon-nucleon potentials. II. One-pair and two-pair diagrams

Two-meson-exchange nucleon-nucleon potentials are derived where either one or both nucleons contains a pair vertex. Physically, the meson-pair vertices are meant to describe in an effective way (part of) the effects of heavy-meson exchange and meson-nucleon resonances. {}From the point of view of ``duality,'' these two kinds of contribution are roughly equivalent. The various possibilities for meson pairs coupling to the nucleon are inspired by the chiral-invariant phenomenological Lagrangians that have appeared in the literature. The coupling constants are fixed using the linear $σ$ model. We show that the inclusion of these two-meson exchanges gives a significant improvement over a potential model including only the standard one-boson exchanges.

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arXiv1996

$ΔI=1$ axial-vector mixing and charge symmetry breaking

Phenomenological Lagrangians that exhibit (broken) chiral symmetry as well as isospin violation suggest short-range charge symmetry breaking (CSB) nucleon-nucleon potentials with a $\mbox{\boldmath $σ$}_1 \!\cdot\!\mbox{\boldmath $σ$}_2$ structure. This structure could be realized by the mixing of axial-vector ($1^+$) mesons in a single-meson exchange picture. The Coleman-Glashow scheme for $ΔI_{z}=1$ charge symmetry breaking applied to meson and baryon $SU(2)$ mass splittings suggests a universal scale. This scale can be extended to $ΔI=1$ nonstrange CSB transitions $\langle a_1^\circ|H_{em}|f_1\rangle$ of size $-0.005$ GeV$^2$. The resulting nucleon-nucleon axial-vector meson exchange CSB potential then predicts $ΔI=1$ effects which are small.

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arXiv1995

The Low-Energy Neutron-Proton Scattering Parameters and the Deuteron

The low-energy parameters describing the np scattering in the 3S1 + 3D1 partial waves, the deuteron parameters, and their relations are discussed. These parameters can be determined quite accurately in the energy-dependent Nijmegen partial-wave analyses of the np data and are also given by the high-quality Nijmegen potentials. The newest values for these parameters are presented.

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arXiv1994

An accurate nucleon-nucleon potential with charge-independence breaking

We present a new high-quality nucleon-nucleon potential with explicit charge dependence and charge asymmetry, which we designate Argonne $v_{18}$. The model has a charge-independent part with fourteen operator components that is an updated version of the Argonne $v_{14}$ potential. Three additional charge-dependent and one charge-asymmetric operators are added, along with a complete electromagnetic interaction. The potential has been fit directly to the Nijmegen $pp$ and $np$ scattering data base, low-energy $nn$ scattering parameters, and deuteron binding energy. With 40 adjustable parameters it gives a $χ^{2}$ per datum of 1.09 for 4301 $pp$ and $np$ data in the range 0--350 MeV.

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arXiv1994

Construction of high-quality Nucleon-Nucleon potential models

We present an updated version (Nijm93) of the Nijmegen soft-core potential, which gives a much better description of the np data than the older version (Nijm78). The chi^2 per datum is 1.87. The configuration-space and momentum-space versions of this potential are exactly equivalent; a unique feature among meson-theoretical potentials. We also present three new NN potential models: a non-local Reid-like Nijmegen potential (NijmI), a local version (NijmII), and an updated regularized version (Reid93) of the Reid soft-core potential. These three potentials all have a nearly optimal chi^2 per datum and can therefore be considered as alternative partial-wave analyses. All potentials contain the proper charge-dependent one-pion-exchange tail. Fortran code for the potentials can be obtained via anonymous ftp from thef-nym.sci.kun.nl

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arXiv1994

Nijmegen Soft-Core Potential Including Two-Meson Exchange

We report on the progress of the construction of the extended soft-core (ESC) Nijmegen potential. Next to the standard one-boson-exchange parts, the model includes the pion-meson-exchange potentials due to the parallel and crossed-box diagrams, as well as the one-pair and two-pair diagrams, vertices for which can be identified with similar interactions appearing in chiral-symmetric Lagrangians. Although the ESC potential is still under construction, it already gives an excellent description of all $N\!N$ scattering data below 350 MeV with $χ^{2}/{\rm datum}=1.3$.

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arXiv1993

Triton calculations with the new Nijmegen potentials

Triton properties are calculated using new nucleon-nucleon potentials, which were fit to the world nucleon-nucleon data. All potentials are charge dependent and explicitly incorporate the mass difference between the charged and neutral pions. Three of these models have a nearly optimal chi**2 per degree of freedom and can therefore be considered as alternative partial wave analyses, which in quality can almost compete with the Nijmegen partial-wave analysis. The triton binding energy obtained with three local models (Nijm II, Reid93, AV18) can be summarized as 7.62 +_ 0.01 MeV, which is nearly 900 keV lower than experiment. The non-local model Nijm I binds by 7.72 MeV.

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