δ介子在DDRHF模型内软化对称能的作用
Role of the $δ$ Meson in Softening the Symmetry Energy within the DDRHF Model
AI总结：
本研究在DDRHF模型中引入δ介子，重新调整介子-核子耦合，发现其可有效降低对称能斜率参数L，同时改变中子星质量-半径关系，为控制DDRHF模型同位旋性质提供新自由度。
AI中文摘要：
我们在密度依赖的相对论Hartree-福克（DDRHF）框架内，研究同位旋矢量-标量δ介子对对称能密度依赖性的影响。作为基准，我们通过对核物质饱和性质施加经验约束，生成了1006个包含σ、ω、ρ和π介子的已接受DDRHF参数化方案，得到的对称能斜率参数被限制在相对较大的数值，L≈65至110 MeV。从该集合中随机选取两个代表性参数化方案，记为RHF-NK1和RHF-NK2。以这两个参数化方案为基础，我们引入δ介子，并在相同的饱和性质约束下重新调整介子-核子耦合。数值优化表明，当δ耦合取常数时，能最有效地获得较小的L值，在此情况下，L从约73 MeV降低至32 MeV，而每个核子的结合能、饱和密度、对称能和不可压缩系数几乎保持不变。分道分解显示，这种软化并非仅由δ介子的直接贡献导致，而是由δ、ρ和π介子之间的重新分布，连同同位旋标量福克贡献共同作用的结果。由此得到的中子星质量-半径关系向更小的半径偏移，表明δ介子为控制DDRHF模型的同位旋性质提供了有效的额外自由度。
英文摘要：
We investigate the effects of the isovector-scalar $δ$ meson on the density dependence of the symmetry energy within the density-dependent relativistic Hartree--Fock (DDRHF) framework. As a baseline, we generate $1006$ accepted DDRHF parametrizations including the $σ$, $ω$, $ρ$, and $π$ mesons by imposing empirical constraints on the saturation properties of nuclear matter. The resulting symmetry-energy slope parameters are confined to relatively large values, $L\simeq65$--$110~\mathrm{MeV}$. Two representative parametrizations, denoted RHF-NK1 and RHF-NK2, are randomly selected from this ensemble. Starting from these two parametrizations, we introduce the $δ$ meson and readjust the meson--nucleon couplings under the same saturation-property constraints. The numerical optimization shows that small values of $L$ are obtained most efficiently when the $δ$ coupling is taken to be constant. In this case, $L$ is reduced from approximately $73$ to $32~\mathrm{MeV}$, while the binding energy per nucleon, saturation density, symmetry energy, and incompressibility coefficient remain nearly unchanged. A channel-by-channel decomposition shows that the softening is not caused by the direct $δ$-meson contribution alone, but by a redistribution among the $δ$, $ρ$, and $π$ mesons together with the isoscalar Fock contributions. The resulting neutron-star mass--radius relations shift toward smaller radii, indicating that the $δ$ meson provides an efficient additional degree of freedom for controlling the isovector properties of DDRHF models.
