YN Interactions Research Articles

Quark-Model Baryon-Baryon Interaction and Its Applications to Hypernuclei

The quark-model baryon-baryon interaction fss2, proposed by the Kyoto-Niigata group, is a unified model for the complete baryon octet (B_8=N, Lambda, Sigma and Xi), which is formulated in a framework of the (3q)-(3q) resonating-group method (RGM) using the spin-flavor SU_6 quark-model wave functions and effective meson-exchange potentials at the quark level. Model parameters are determined to reproduce properties of the nucleon-nucleon system and the low-energy cross section data for the hyperon-nucleon scattering. Due to the several improvements including the introduction of vector-meson exchange potentials, fss2 has achieved very accurate description of the NN and YN interactions, comparable to various one-boson exchange potentials. We review the essential features of fss2 and our previous model FSS, and their predictions to few-body systems in confrontation with the available experimental data. Some characteristic features of the B_8 B_8 interactions with the higher strangeness, S=-2, -3, -4, predicted by fss2 are discussed. These quark-model interactions are now applied to realistic calculations of few-body systems in a new three-cluster Faddeev formalism which uses two-cluster RGM kernels. As for the few-body systems, we discuss the three-nucleon bound states, the Lambda NN-Sigma NN system for the hypertriton, the alpha alpha Lambda system for 9Be Lambda, and the Lambda Lambda alpha system for 6He Lambda Lambda.

The hypernuclei Λ4He and Λ4H with modern YN interactions

The hypernuclei Λ 4He and Λ 4H provide important information on the hyperon-nucleon interaction. Using the Nijmegen SC YN interactions the 0 + ground and the 1 + excited states are precisely calculated using the Faddeev-Yakubovsky equation. The Λ-Σ admixture is taken into account explicitly. For all these interactions the Λ separation energies disagree with the data.

AB INITIO CALCULATION OF $^5_\Lambda {\rm He}$ WITH EXPLICIT Σ ADMIXTURE

Variational calculations for s-shell hypernuclei are performed by explicitly including Σ degrees of freedom. Two sets of YN interactions (D2 and SC97e(S)) are used. The bound-state solution of [Formula: see text] is obtained by using each of YN potentials, and a large energy expectation value of the tensor ΛN - ΣN transition part is found by using the SC97e(S). The internal energy of 4 He subsystem changes a lot by the presence of a Λ particle with the strong tensor ΛN - ΣN transition potential.

Hyperon-Mixed Neutron Star Matter and Neutron Stars

Effective Σ − n and Σ − Σ − interactions are derived fromthe G-matrix calculations for {n+Σ − } matter and employed in the investigation of hyperon mixing in neutron star matter. The threshold densities ρt(Y ) at which hyperons start to appear are between 2ρ0 and 3ρ0 (where ρ0 is the normal nuclear density) for both Λ and Σ − , and their fractions increase rapidly with baryon density, reaching 10% already for ρ � ρt+ρ0. The mechanism of hyperon mixing and single-particle properties, such as the effective mass and the potential depth, are analyzed taking into account the roles of YN and NN interactions. The resulting equation of state is found to be too soft to sustain the observed neutron star mass Mobs =1 .44M� .W e discuss the reason for this and stress the necessity of the “extra repulsion” for YN and YY interactions to resolve this crucial problem. It is remarked that ρt(Y ) would be as large as 4ρ0 for neutron stars compatible with Mobs. A comment is given regarding the effects on the Y -mixing problem from a less attractive ΛΛ interaction, newly suggested by the NAGARA event.

Ab initio approach to s-shell hypernuclei 3lambdaH, 4lambdaH, 4lambdaHe, and 5lambdaHe with a lambdaN-sigmaN interaction.

Variational calculations for s-shell hypernuclei are performed by explicitly including Sigma degrees of freedom. Four sets of YN interactions [SC97d(S), SC97e(S), SC97f(S), and SC89(S)] are used. The bound-state solution of 5lambdaHe is obtained and a large energy expectation value of the tensor lambdaN-sigmaN transition part is found. The internal energy of the 4He subsystem is strongly affected by the presence of a Lambda particle with the strong tensor lambdaN-sigmaN transition potential.

The hypernuclei (4)(lambda)He and (4)(lambda)H: challenges for modern hyperon-nucleon forces.

The hypernuclei (4)(Lambda)He and (4)(Lambda)H provide important information on the hyperon-nucleon interaction. We present accurate Faddeev-Yakubovsky calculations for the Lambda separation energies of the 0(+) ground and the 1(+) excited states based on the Nijmegen SC YN interactions. We explicitly take the Sigma admixture into account. Mass differences of the baryons and the charge dependence of the interaction are considered. The results show that the Nijmegen models cannot predict all separation energies simultaneously hinting to failures of the current interaction models. It is pointed out that the differences of the Lambda separation energies of (4)(Lambda)He and (4)(Lambda)H are interesting observables to probe the YN interaction models.

Λ13Chypernucleus studied with the13C(K−,π−γ)reaction

The Λ13C hypernucleus was studied by measuring γ rays in coincidence with the 13C(K−,π−) reaction. γ rays from the 1/2− and 3/2− states, which are the partners of the spin-orbit doublet states with a predominant configuration of [12Cg.s.(0+)⊗pΛ], to the ground state were measured. The splitting of the states was found to be ΔE(1/2−−3/2−)=+152±54(stat)±36(syst) keV. This value is 20–30 times smaller than that of single particle states in nuclei around this mass region. The jΛ=lΛ−1/2[(p1/2)Λ] state appeared higher in energy, as in normal nuclei. The value gives new insight into the YN interaction. The excitation energies of the 1/2− and 3/2− states were obtained as 10.982±0.031(stat)±0.056(syst) and 10.830±0.031(stat)±0.056(syst) MeV, respectively. We also observed γ rays from the 3/2+ state, which has a [12C(2+)⊗sΛ] configuration, to the ground state in Λ13C. The excitation energy of the 3/2+ state was obtained as 4.880±0.010(stat)±0.017(syst) MeV. Nuclear γ rays with energies of 4.438 and 15.100 MeV had similar yields, which suggests that a quasifree knockout of a Λ particle is dominant in highly excited regions.Received 11 October 2001DOI:https://doi.org/10.1103/PhysRevC.65.034607©2002 American Physical Society

Observation of Spin-Orbit Splitting inΛSingle-Particle States

The spin-orbit splitting of Lambda single-particle states in (13)(Lambda)C was measured. The 13C(K-,pi(-))(13)(Lambda)C reaction was used to excite both the 1/2(-) and 3/2(-) states simultaneously, which have predominantly 12C(0(+)) x p(Lambda) configuration. gamma rays from the states to the ground state were measured in coincidence with the pi(-)'s, by which ls splitting was found to be 152+/-54(stat)+/-36(syst) keV. The value is 20-30 times smaller than exhibited by the ls splitting in the nuclear shell model. This value gives us new insight into the YN interaction.

Effective YN and YY Interactions and Hyperon-Mixing in Neutron Star Matter: Y Case

Effective ΛN and ΛΛ interactions in dense hyperonic nuclear matter are constructed on the basis of the G-matrix calculation with Nijmegen hard-core potentials. With these effective interactions, the mixing of Λ in neutron star matter and the equation of state are analyzed. The Λ-mixed phase is shown to appear in neutron star cores with a baryon number density ρ > ρt(Λ) � (3 − 5)ρ0, where ρt(Λ) is the threshold density for the Λmixing and ρ0 is the normal nuclear matter density. The density ρt(Λ) depends not only on the ΛN but also on the NN interactions. The three-body force introduced in the NN interaction to reproduce the proper nuclear saturation properties enhances the Λ-mixing and drastically softens the equation of state. The resulting equation of state is not consistent with the observed neutron star mass Mobs =1 .44M� . It is found that this crucial problem can be resolved by the introduction of a three-body repulsion also for the ΛN and ΛΛ interactions. The finite-temperature effect on the Λ-mixing is found to be large, especially at lower densities and is expected to affect the properties of neutron stars at birth.

Effects of instantons on the YN interaction

Neutron inelastic scattering from thulium-169 has been studied for states above 100 keV via the (n,n′γ) reaction at incident energies in the 0.2- to 1.0-MeV range. A high-resolution Ge spectrometer in conjunction with the time-of-flight technique was utilized. Thirty-six gamma-ray transitions from 16 levels were observed. Gamma-ray angular distributions were measured at En=750 keV and excitation functions at 125 degrees were measured in 50 keV steps over the range of incident energies. Differential gamma-ray production cross sections and gamma-ray branching ratios were obtained. Inferred neutron inelastic level cross sections of the four lowest ground-state rotational band (Kπ=1/2+) members are compared to the sum of calculated compound nucleus and direct interaction cross sections. For the remaining levels, measurements are compared to compound nucleus calculations only. The comparison shows generally good agreement particularly near threshold.

Four-body calclation of ^4_ΛH and ^4_ΛHe with realistic YN and NN interactions

Four-body calclation of ^4_ΛH and ^4_ΛHe with realistic YN and NN interactions

Four-body calculation of Λ4H and Λ4He with realistic YN and NN interactions

We carried out four-body calculations of Λ 4H and Λ 4He taking both the 3 N + Λ and 3 N + Σ channels explicity with the use of realistic NN and YN interactions. The Σ-channel component plays an important role in binding energies of the A = 4 hypernuclei though the admixture is approximately 1%. The ΛN - ΣN coupling is found to be of central-force type in the Nijmegen model D and of tensor-force type in the model F.

Single-Particle Potentials and Effective Masses of Hyperons in Hyperonic Nuclear Systems

Hyperon single-particle potentials and effective masses are explored using the nuclear matter G-matrix theory starting from the Nijmegen OBE models. Those of hyperons (Λ, Σ and Ξ) in symmetric matter and neutron matter are calculated to high densities. The results are discussed with regard to the YN interactions and also with regard to the experimental results in the case of Λ. Similar calculations are performed for hyperonic neutron matter, which is composed of neutrons and hyperons (Λ, Σ − or Ξ − ). The roles of Yn and YY interactions are discussed.

Meson-exchange model for the YN interaction

We report on progress in the development of a microscopic model for the hyperon-nucleon interaction within the meson-exchange framework, which incorporates both one-meson as well as correlated two-meson exchange contributions. The main new feature of the model is the exchange of two correlated pions or kaons, both in the scalar-isoscalar and the vector-isovector channels. In the scalar channel it provides the main part of the intermediate-range interaction between two baryons, thereby eliminating the need for the fictitious sigma meson, which has been an unavoidable element of effective one-boson-exchange descriptions.

Soft-core hyperon-nucleon potentials

A new Nijmegen soft-core OBE potential model is presented for the low-energy $\mathrm{YN}$ 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. The potentials are generated by the exchange of nonets of pseudoscalar, vector, and scalar mesons. As is standard in the Nijmegen soft-core models, we also include the $J=0$ contributions from the tensor ${f}_{2}{,f}_{2}^{\ensuremath{'}}{,a}_{2},$ and pomeron Regge trajectories, and use Gaussian form factors to guarantee that the potentials have a soft behavior near the origin. An important innovation with respect to the original soft-core potential is the assignment of the cutoff masses for the baryon-baryon-meson (BBM) vertices in accordance with broken $\mathrm{SU}{(3)}_{F},$ which serves to connect the $\mathrm{NN}$ and the $\mathrm{YN}$ channels. As a novel feature, we allow for medium strong breaking of the coupling constants, using the ${}^{3}{P}_{0}$ model with a Gell-Mann--Okubo hypercharge breaking for the BBM coupling. Charge-symmetry breaking in the $\ensuremath{\Lambda}p$ and $\ensuremath{\Lambda}n$ channels is included as well. We present six hyperon-nucleon potentials which describe the available $\mathrm{YN}$ 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 $\ensuremath{\Sigma}N$ and $\ensuremath{\Lambda}N$ channels. In all cases, we obtained ${\ensuremath{\chi}}^{2}{/N}_{\mathrm{data}}\ensuremath{\approx}0.55$ for 35 $\mathrm{YN}$ data. In particular, we were able to fit the precise experimental datum ${r}_{R}=0.468\ifmmode\pm\else\textpm\fi{}0.010$ for the inelastic capture ratio at rest. For the scalar-meson mixing angle we obtained values ${\ensuremath{\theta}}_{S}=37\ifmmode^\circ\else\textdegree\fi{}--40\ifmmode^\circ\else\textdegree\fi{},$ which points to almost ideal mixing angles for the scalar $q\overline{q}$ states. The G-matrix results indicate that the remarkably different spin-spin terms of the six potentials appear specifically in the energy spectra of $\ensuremath{\Lambda}$ hypernuclei.

Scattering Observables of the NN and YN Interactions in the SU6 Quark Model

Scattering Observables of the NN and YN Interactions in the SU6 Quark Model

Spin-orbit force of instanton-induced interaction in strange and charmed systems

Effects of the spin orbit-force on hadronic systems with strangeness or charm are investigated by a valence quark model with the instanton-induced interaction. By introducing this interaction, the spin-orbit splittings in the negative-parity hyperons becomes 0.14–0.37 times smaller. The flavor-octet baryon mass spectrum and the splittings in the charmed baryons become consistent with the experiments. Although the splitting is also reduced in the flavor-singlet baryons, it still gives two third of the experimental value. The reduction comes from the cancellation between the one-gluon exchange and the instanton-induced interaction, which is channel-specific. In most of the two-baryon channels, the symmetric and antisymmetric spin-orbit force of the YN interaction remains strong after introducing this instanton effect. A few exceptional channels, however, are found where the cancellation affects strongly and the spin-orbit force becomes small.

Baryon-baryon interaction in a quark model

Scattering observables of the NN and YN interactions and nuclear-matter properties are investigated in a recent quark model developed for a unified description of the NN and YN systems. The model is formulated in the (3 q)-(3 q) resonating-group method, and incorporates the full Fermi-Breit interaction with explicit quark-mass dependence and the minimum meson-exchange potentials of scalar and pseudo-scalar meson nonets directly coupled to quarks. Reasonable agreement with the available experimental data is obtained for the NN properties and the low-energy YN cross sections. Predictions for the Σ +p and λp scattering observables at intermediate energies are very much modeldependent, suggesting the importance of further experimental studies of these quantities.

Hypernuclei with meson-exchange hyperon-nucleon interactions

The properties of hypernuclei are studied in relativistic mean field theory, with effective σ coupling constants of hyperons obtained from a dynamic model for the YN interactions arising from the ππ and K K correlations. The properties of Λ-hypernuclei can be well described by fitting the omega coupling constant. A weaker effective ω coupling to the Σ might be required to form a nuclear bound state. The effective potentials of hyperons in hypernuclei are studied, especially the spin-orbit interaction of the hyperons.

The YN Interaction from Hyperon Scattering

The YN Interaction from Hyperon Scattering