ΣN Channels Research Articles

Electric dipole moment of the deuteron in the standard model with NN − ΛN − ΣN coupling

We calculate the electric dipole moment (EDM) of the deuteron in the standard model with |ΔS|=1 interactions by taking into account the NN−ΛN−ΣN channel coupling, which is an important nuclear level systematics. The two-body problem is solved with the Gaussian Expansion Method using the realistic Argonne v18 nuclear force and the YN potential which can reproduce the binding energies of Λ3H, Λ3He, and Λ4He. The |ΔS|=1 interbaryon potential is modeled by the one-meson exchange process. It is found that the deuteron EDM is modified by less than 10%, and the main contribution to this deviation is due to the polarization of the hyperon–nucleon channels. The effect of the YN interaction is small, and treating ΛN and ΣN channels as free is a good approximation for the EDM of the deuteron.

Np → ηd near threshold and the s -wave ηN amplitude

We calculate the process np → ηd near threshold using a separable potential model of the coupled ηN - πN - ππN subystems, and a relativistic three-body calculation for the ηd scattering amplitude. The ππN channels are represented by an effective σN channel, and we compare the case where the σ and π masses are related by mσ = 2mπ and no width is considered, to another where the mass and width of the σ -meson are taken from ππ scattering data. The np → ηd cross-section can be well described up to about 60MeV by models where the real part of the ηN scattering length lies between 0.4≤Re(aηN)≤0.6 fm which allows us to determine the s -wave ηN scattering amplitude for -60≤E≤60 MeV.

ΣΛ Conversion in Λ3H and in other S-shell hypernuclei

The binding energy of the Λ-particle in the J = 1 2 and 3 2 states of Λ 3H and the doublet and quartet Λ-d scattering lengths are calculated with a separable ΛN-ΣN potential matrix, fitted to experimental data in ΛN and ΣN channels. The dependence of the results on ΛΣ coupling is explained in terms of a perturbative treatment of ΛΣ coupling. This perturbative treatment is also applied in a qualitative discussion of the effects of ΛΣ conversion in the J = 0 and 1 states of Λ 4H- Λ 4He and in the J = 1 2 state of Λ 5He. The importance of three-body ΛNN forces caused by ΣΛ coupling is stressed.

One-boson-exchange potential model for the Λp interaction (II)

This paper describes a one-boson-exchange potential model for the Λp interaction, which includes the ΣN channel explicitly. This model is an extension of a model we have previously discussed [1]. The extension consists of the addition of a unitary singlet pseudo-scalar meson ( χ 0) and an octet of scalar bosons. This extension enables us to fit the experimental low-energy Λp data with values of some meson-baryon couplings closer to generally preferred values than was possible with the previous model. In particular, the coupling of the unitary singlet vector meson (ω) to baryons is g ω 2/4 π = 4.5 in this model, whereas it was 24.9 in the earlier model. The present model contains the effects of nonets of pseudoscalar, vector and scalar bosons. The parameters of the model are boson and baryon masses, the couplings of the bosons to baryons and a spin-independent, channel-independent hard-core radius. SU(3) was used to relate the couplings. Four parameters of the model were adjusted to reproduce the most recent low-energy Λp data; all other parameters were fixed beforehand from other considerations. The adjusted parameters are the hard-core radius, the mass and coupling of a unitary singlet scalar boson, and the coupling of the neutral isospin-one member of the octet of scalar bosons to protons. The model provides a very good fit to the experimental Λp cross section data up to and above the ΣN threshold. The ( 3 S 1 − 3 D 1 ) partial cross sections are dominant just below the ΣN threshold, but one is not resonant there as it was the earlier model [1].

Multichannel calculations of hyperon-nucleon cross-sections and phase shifts

We compute the hyperon-nucleon cross-sections by taking care of the coupling between the ΛN and ΣN channels. For this purpose a multichannelND−1 method is used within the framework of the one-boson exchange model. The number of coupling constants that we need is reduced by symmetry arguments and the free parameters left are fixed by fitting the ΛN cross-section data. Good fits are obtained and the predictions of the model concerning the ΣN cross-sections are in good agreement with the experimental data. Furthermore a hyperon-nucleon phase-shift analysis is performed. No indication of a resonance is found; the Λp cross-section enhancement attributed sometimes to a3S1 Λp resonance seems to be accounted for by the coupling effect between ΛN and ΣN channels.

One-boson-exchange potential model for the ΛN interaction

A one-boson-exchange potential model for the ΛN interaction is described. The effect of the ΣN channel is included explicitly. The potential includes contributions from the following bosons: pseudoscalar and vector octets, and scalar and vector unitary singlets. The approximations needed to obtain a potential for a general baryon-baryon interaction are discussed, and certain ambiguities are pointed out. In particular, we consider to what extent the addition of momentum-dependent corrections would be an improvement over the extreme nonrelativistic limit. By numerical calculations, we then adjust the parameters of our model to reproduce experimental low-energy Λp data. We use SU(3) to relate the meson-baryon couplings. The principal search parameters are the mass and coupling of the scalar unitary singlet boson, the vector coupling of the vector unitary singlet boson, and the radius of a phenomenological spin-independent hard core. Λp cross sections are then calculated from zero energy up through the Σ-production threshold. Some of the ambiguities present in a local potential are investigated numerically. For our model we find that the values of the search parameters are not especially sensitive to these ambiguities.

Analysis of K− absorption in deuterium

We have examined theoretically the capture of K− at rest by deuterium, principally in terms of final state YN interactions. In particular we calculated total rates, the momentum spectra associated with the production of Σ−+n+π+ and Λ0+p+π−, and the corresponding angular correlation. (The latter are not found to be useful except perhaps as a check). On the basis of total rates it is known that capture from theP Bohr orbit through thes wave KN channel probably dominates. This assignment is reinforced by the spectrum of the low momentum Λ peak (Λ’s converted from Σ’s). A definitive check can be made when a somewhat better converted Λ spectrum is measured, since one should observe a double peak (with a cusp at the maximum pion momentum associated with production) if this assignment is correct. Analysis of the high pion momentum Λ peak (directly produced Λ’s), using this assignment reveals that the spectrum is insensitive to the YN interaction and yet does not seem to agree with the data. This spectrum shape (and that for Σ−nπ+) depends significantly on the KN → Yπ absorption process off the energy shell, which in turn depends on the KN scattering lengths. Preliminary comparison of data and theory indicates a large negative real part for the isotopic spin one KN scattering length. Finally, from the relative rate for conversion of Σ’s into Λ’s, we deduce a minimum value for the imaginary part of the isotopic spin 1/2p wave σN scattering length. The result, larger than the pion Compton wavelength, indicates a very strong potential in the ΣN channel and/or coupling ΣN and ΛN channels, a potential much stronger than indicated by the universal pion baryon interaction. Capture from theS Bohr orbit is also extensively discussed. In this case, the spectra associated with Σ− and direct Λ production are sensitive to the final YN interaceion.