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arXiv 0802.1190nucl-th
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核和超核中自旋轨道相互作用的注记

Note on spin-orbit interactions in nuclei and hypernuclei

N. Kaiser, W. Weise

摘要:

详细比较了$Λ$超核与普通核的自旋轨道相互作用。我们认为自旋轨道相互作用主要有三个贡献:1)一个涉及标量和矢量平均场的短程分量; 2)一个由二阶π交换张量力产生的“错号”自旋轨道项; 3)一个由二π交换和虚$Δ(1232)$ -等压线激发引起的三体项(a la Fujita-Miyazawa).对于原子核中的核子,与π介子交换动力学相关的长程粒子趋于抵消,主要为短程起源的自旋轨道机制留下空间(例如,相对论标量和矢量平均场项的参数化)。与此相反,在超核中没有类似的$2π$交换三体对$Λ$超子的贡献,导致短程(相对论平均场)分量和由二级$π$与中间$Σ$超子交换产生的“错号”自旋轨道相互作用之间几乎完全抵消。这些短程分量和长程分量之间的不同平衡机制能够同时解释普通核中非常强的自旋轨道相互作用和$Λ$超核中非常弱的自旋轨道分裂。

英文摘要:

A detailed comparison is made between the spin-orbit interactions in $Λ$ hypernuclei and ordinary nuclei. We argue that there are three major contributions to the spin-orbit interaction: 1) a short-range component involving scalar and vector mean fields; 2) a ''wrong-sign'' spin-orbit term generated by the pion exchange tensor force in second order; and 3) a three-body term induced by two-pion exchange with excitation of virtual $Δ(1232)$-isobars (a la Fujita-Miyazawa). For nucleons in nuclei the long-range pieces related to the pion-exchange dynamics tend to cancel, leaving room dominantly for spin-orbit mechanisms of short-range origin (parametrized e.g. in terms of relativistic scalar and vector mean fields terms). In contrast, the absence of an analogous $2π$-exchange three-body contribution for $Λ$ hyperons in hypernuclei leads to an almost complete cancellation between the short-range (relativistic mean-field) component and the ''wrong-sign'' spin-orbit interaction generated by second order $π$-exchange with an intermediate $Σ$ hyperon. These different balancing mechanisms between short- and long-range components are able to explain simultaneously the very strong spin-orbit interaction in ordinary nuclei and the remarkably weak spin-orbit splitting in $Λ$ hypernuclei.

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