利用手征SU(3)夸克模型研究轻双Λ超核^6_ΛΛHe,^5_ΛΛHe,^5_ΛΛH,^4_ΛΛHe,和^4_ΛΛH的结合能

在手征SU(3)夸克集团模型下, 通过共振群方法(RGM)计算了双Λ超核^6_ΛΛHe,^5_ΛΛHe,^5_ΛΛH,^4_ΛΛHe,和^4_ΛΛH的结合能. 结果表明, 假定双Λ超核具有双Λ集团和壳心核集团构成的两集团结构, 得到的^6_ΛΛHe超核的结合能与实验值基本吻合, 表明手征SU(3)夸克集团模型不仅能较好地描述重子谱、N-N和Y-N相互作用及轻Λ超核的结合能, 也能较好地描述Y-Y相互作用及双Λ超核^6_ΛΛHe,^5_ΛΛHe,^5_ΛΛH,^4_ΛΛHe,和^4_ΛΛH的结合能, 指出了它们存在的可能性.     

Impurity effects of the Λhyperon in the hypernuclear systems ${}_{{\rm{\Lambda }}}^{25}\mathrm{Mg}$ and ${}_{{\rm{\Lambda }}}^{29}\mathrm{Si}$

Based on the beyond-mean-field Skyrme–Hartree–Fock model, impurity effects of the Λhyperon in the hypernuclear systems ${}_{\,{\rm{\Lambda }}}^{25}$ Mg and ${}_{\,{\rm{\Lambda }}}^{29}$ Si are investigated, respectively. Four cases, in which the Λhyperon occupies the single-particle orbitals ${\rm{\Lambda }}$[000]${\tfrac{1}{2}}^{+}$, ${\rm{\Lambda }}$[110]${\tfrac{1}{2}}^{-}$, ${\rm{\Lambda }}$[101]${\tfrac{3}{2}}^{-}$ and ${\rm{\Lambda }}$[101]${\tfrac{1}{2}}^{-}$, are focused. In each case, the potential energy surface and the energy curves projected on certain angular momenta are employed to show the influence of the Λhyperon on the nuclear core. Beside the shrinkage effect that is induced by the Λhyperon occupying the s_Λ orbital, it is found that the Λhyperon on the p_Λ orbital, ${\rm{\Lambda }}$[110]${\tfrac{1}{2}}^{-}$, drives the nuclear core toward a prolate shape, while the ones on the other two p_Λ orbitals, ${\rm{\Lambda }}$[101]${\tfrac{3}{2}}^{-}$ and ${\rm{\Lambda }}$[101]${\tfrac{1}{2}}^{-}$, drive the nuclear core toward an oblate shape. The energy spectra and the corresponding intra-band E2 transition rates for the rotational bands are given as a prediction for future experiments.     

夸克平均场模型的有限核及超核研究

夸克平均场模型采用组分夸克模型描述重子,已被用于研究有限核及超核的性质.介子平均场直接与核子内的组分夸克相互作用,从而改变了核介质内重子的性质.夸克平均场模型能够给出令人满意的球型核及超核的性质,该模型也预言了核介质中核子体积的膨胀及核子有效质量的降低.     

A theoretical overview of hypernuclear weak decay

The physics of the weak decay of hypernuclei is briefly reviewed from a theoretical point of view. Special regard is devoted to the recent progress concerning the determination of the non-mesonic decay widths and the asymmetry parameters. Convincing evidence has been achieved for a solution of the long-standing puzzle on the ratio Γ_n/Γ_p. Very recently, it has been shown that the exchange of a ππ pair in the weak mechanism plays a crucial role in explaining the discrepancies between theory and experiment on the decay asymmetries.     

Skyrme Hartree-Fock模型对中等质量超形变超核的研究（英文）

采用Skyrme Hartree-Fock 模型和微观的核子-超子相互作用对Ar 同位素和40Ca 及相应超核的超形变态进行了研究,计算结果表明,基态的Λ 超子分离能比激发态的大。这一结果与反对称化分子动力学模型的结果一致,而与相对论平均场的结果相反。这区别主要来源于不同模型中不同的相互作用导致的核心核及相应超核密度分布的不同。The superdeformed (SD) states in medium heavy hypernuclei with core nuclei of Ar isotopes and 40Ca are studied in the frame of Skyrme Hartree-Fock (SHF)+ BCS model together with a microscopic ΛN interaction. The calculation indicates that the Λ separation energy S_Λ of ground state is larger than that of SD state. The result is consistent with the antisymmetrized molecular dynamics (AMD) calculation, but inconsistent with that of relativistic mean field (RMF). The difference comes from the different interaction and density distribution in the core nuclei and the corresponding hypernuclei.     

相对论平均场理论对∧,Ξ和∑超核的研究(英文)

基于相对论平均场理论,系统地研究了单∧,Ξ和∑超核,超子-核子相互作用通过拟合实验数据以及参照之前理论工作来确定。以16O为核芯,通过加入不同类型超子（∧,Ξ0,-和∑+,0,-）,比较了超子的平均势场和单粒子能级,并研究了对核芯的杂质效应。整体上看,∧和Ξ0超子在大块性质上类似;Ξ0超子平均势场最浅;库仑相互作用对于带电超子Ξ-,∑+和∑-非常重要。作为杂质,原子核中加入超子会使整个体系更束缚。然而,不同超子对原子核的半径有不同效应。此外,讨论了ωYY张量耦合,发现其对超子的能级劈裂有显著影响,甚至导致Ξ超子中出现能级反转现象。Single ∧, Ξ, and ∑ hypernuclei are systematically studied within the framework of relativistic mean-field (RMF) model with YN interactions being constrained according to the experimental data and previous theoretical efforts. By adding a hyperon to 16O, the mean-field potentials and single-particle levels for hyperons (∧, Ξ0,-, and ∑+,0,-) are compared and the impurity effects on the nuclear core are examined. In general, the ∧ and ∑0 hyperons show similar behaviors in bulk properties since both of them are electroneutral and with similar coupling constants; Ξ0 hyperon owns the shallowest mean-field potential well; and Coulomb interactions play vital roles in the charged Ξ-, ∑-, and ∑+ hyperons. As an impurity, the intruded single-hyperon makes the nuclear system more bound in most cases due to the attractive NY interaction. However, very different effects on the nucleon radii are observed for different hyperons. Besides, the effects of the ωYY tensor couplings on the spin-orbit splitting are discussed, and remarkable influences are found which even change the level ordering of Ξ hyperon.     

∧超核中单粒子谱的自旋及赝自旋对称性研究(英文)

原子核中单粒子谱的自旋和赝自旋对称性对核结构的研究具有重要意义。本文基于相对论平均场理论,以132Sn,_∧133Sn,及_2∧134Sn为例,研究了∧超核中单∧谱和单中子谱的自旋和赝自旋对称性。研究发现,单∧谱的自旋对称性保持得相当好,与实验观测一致;而其赝自旋对称性只是近似保持,与核子谱的情况类似。此外,还研究了∧超子对中子谱的杂质效应,发现∧超子使中子谱的自旋对称性变差,赝自旋对称性变好。Spin and pseudospin symmetries in the single-particle spectra of atomic nuclei are of great significance for the study of nuclear structure. In this work, taking 132Sn, _∧133Sn, and _2∧134Sn as examples, the spin and pseudospin symmetries in ∧ hypernuclei are studied by using the relativistic mean-field model. For the single-∧ spectra, results show that the spin symmetry maintains well while the pseudospin symmetry is approximately conserved. Besides, as impurities, the ∧ hyperons worsen the spin symmetry of single-neutron spectra while improve the pseudospin symmetry.     

基于手征SU(3)夸克集团模型的YN相互作用和轻超核研究

采用基于手征SU(3)夸克集团模型的YN相互作用位,计算了轻Λ超核5ΛHe,4ΛHe和4ΛH的能谱.结果显示,近似的等效定域ΛN相互作用位不能给出正确的超核结合能,而非定域ΛN位能给出超核5ΛHe结合能的合理范围以及超核4ΛHe和4ΛH能级的正确顺序.这表明YN相互作用的非定域性起着重要的作用.计算中采用了2套模型参数,计算结果正好给出实验值的上下限.这表明超核的实验数据可以提供更多的信息来进一步约束YN相互作用,以弥补YN散射实验数据的不足.利用超核实验数据来优化选择YN相互作用参数是有可能做得到的.     

Production of hypernuclei with the reaction

We present results on the production of bound states of Θ⁺ in nuclei using the (K⁺,π⁺) reaction. By taking into account the states obtained within a wide range of strength of the Θ⁺ nucleus optical potential, plus the possibility to replace different nucleons of the nucleus, we obtain an excitation spectra with clearly differentiated peaks. The magnitude of the calculated cross sections is well within reachable range.     

Relativistic Hypernuclei: Old Problems and New Prospects

The first experiments with relativistic hypernuclei (Bowen; Khorozov and Lukstins) were done many years ago. They demonstrated not only that such extremely difficult experiments are manageable but also their great advantage, a possibility of observing and studying independently the production and weak decay of hypernuclei: the points of the production and decay of relativistic hypernuclei are separated by tens of centimeters instead of some microns in classical experiments. At the same time these first experiments revealed a huge problem with selection of the proper trigger.Recently we proposed to explore a unique feature of the ⁹Be nucleus: after removing a neutron from its ground state several groups of alpha-particles appear from different excited states of a residual nucleus ⁸Be. Detection of the correlated pair of -particles produced in a vacuum volume at a distance of some 40 cm from target is an unambiguous signal of nonmesonic decay of hypernucleus ¹⁰ Be(++n+n) or ¹⁰ Be(++n+p). In this particular case it is possible to take exclusive decay rates (on different excited states of ⁸Be^*). This open a way for a phenomenological analysis of matrix elements of the four-baryon weak interaction.The experiment is approved for new accelerator Nuclotron at JINR, Dubna.