Baryon-Baryon Interactions

After a short survey of some topics of interest in the study of baryon-baryon scattering, the recent Nijmegen energy dependent partial wave analysis (PWA) of the nucleon-nucleon data is reviewed. In this PWA the energy range for both pp and np is now 0 < Tlab < 350 MeV and a chi^2_{d.o.f.}=1.08 was reached. The implications for the pion-nucleon coupling constants are discussed. Comments are made with respect to recent discussions around this coupling constant in the literature. In the second part, we briefly sketch the picture of the baryon in several, more or less QCD-based, quark-models that have been rather prominent in the literature. Inspired by these pictures we constructed a new soft-core model for the nucleon-nucleon interaction and present the first results of this model in a chi^2 -fit to the new multi-energy Nijmegen PWA. With this new model we succeeded in narrowing the gap between theory and experiment at low energies. For the energies Tlab = 25-320 MeV we reached a record low chi^2_{p.d.p.} = 1.16. We finish the paper with some conclusions and an outlook describing the extension of the new model to baryon-baryon scattering.Comment: 12 pages LaTeX and one postscript figure included. Invited talk presented at the XIVth European Conference of Few-Body Problems in Physics, Amsterdam, August 23-28, 199

On the pion-nucleon coupling constant

In view of persisting misunderstanding about the determination of the pion-nucleon coupling constants in the Nijmegen multienergy partial-wave analyses of pp, np, and pbar-p scattering data, we present additional information which may clarify several points of discussion. We comment on several recent papers addressing the issue of the pion-nucleon coupling constant and criticizing the Nijmegen analyses.Comment: 19 pages, Nijmegen preprint THEF-NYM-92-0

Role of heavy-meson exchange in pion production near threshold

Recent calculations of $s$-wave pion production have severely underestimated the accurately known $pp\rightarrow pp\pi^0$\ total cross section near threshold. In these calculations, only the single-nucleon axial-charge operator is considered. We have calculated, in addition to the one-body term, the two-body contributions to this reaction that arise from the exchange of mesons. We find that the inclusion of the scalar $\sigma$-meson exchange current (and lesser contributions from other mesons) increases the cross section by about a factor of five, and leads to excellent agreement with the data. The results are neither very sensitive to changes in the distorting potential that generates the $NN$ wave function, nor to different choices for the meson-nucleon form factors. We argue that $pp\rightarrow pp\pi^0$\ data provide direct experimental evidence for meson-exchange contributions to the axial current.Comment: 28 Pages, IU-NTC #93-0

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 $\chi^{2}$ per datum of 1.09 for 4301 $pp$ and $np$ data in the range 0--350 MeV.Comment: 36 pages, PHY-7742-TH-9

Description of the two-nucleon system on the basis of the Bargmann representation of the S matrix

For the effective-range function $k\cot \delta$, a pole approximation that involves a small number of parameters is derived on the basis of the Bargmann representation of the $S$ matrix. The parameters of this representation, which have a clear physical meaning, are related to the parameters of the Bargmann $S$ matrix by simple equations. By using a polynomial least-squares fit to the function $k\cot \delta$ at low energies, the triplet low-energy parameters of neutron-proton scattering are obtained for the latest experimental data of Arndt et al. on phase shifts. The results are $a_{t}=5.4030$fm, $r_{t}=1.7494$fm, and $v_{2}=0.163$fm$^{3}$. With allowance for the values found for the low-energy scattering parameters and for the pole parameter, the pole approximation of the function $k\cot \delta$ provides an excellent description of the triplet phase shift for neutron-proton scattering over a wide energy range ($T_{\text{lab}}\lesssim 1000$MeV), substantially improving the description at low energies as well. For the experimental phase shifts of Arndt et al., the triplet shape parameters $v_{n}$ of the effective-range expansion are obtained by using the pole approximation. The description of the phase shift by means of the effective-range expansion featuring values found for the low-energy scattering parameters proves to be fairly accurate over a broad energy region extending to energy values approximately equal to the energy at which this phase shift changes sign, this being indicative of a high accuracy and a considerable value of the effective-range expansion in describing experimental data on nucleon-nucleon scattering. The properties of the deuteron that were calculated by using various approximations of the effective-range function comply well with their experimental values.Comment: 39 pages, 3 figure