[论文解读] Probing high-density symmetry energy using heavy-ion collisions at intermediate energies

Gao-Chan Yong, Yafei Guo|arXiv (Cornell University)|Jul 20, 2020

High-Energy Particle Collisions Research被引用 6

一句话总结

该论文提出了一套先进的同位旋依赖性输运模型，结合了核子-核子短程关联和介质中同位旋依赖的截面，以研究中等能量重离子碰撞中高密度下的核对称能。研究识别出挤出的中子/质子比、光子、轻簇和奇异介子的产生为敏感观测量，表明对称能的曲率显著影响这些探针，尤其是在高横向动量区域，为高密度对称能行为提供了定性但稳健的约束。

ABSTRACT

The nuclear symmetry energy, which describes the energy difference of per proton and neutron in nuclear matter, has been extensively studied within the last two decades. Around saturation density, both the value and the slope of the nuclear symmetry energy have been roughly constrained, its high-density behavior is now still in argument. Probing high-density symmetry energy at terrestrial laboratories is being carried out at facilities that offer radioactive beams worldwide. While relevant experiments are being conducted, we theoretically developed more advanced isospin-dependent transport model including new physics such as nucleon-nucleon short-range correlations and in-medium isospin-dependence of baryon-baryon scattering cross section. New sensitive probes of high-density symmetry energy are provided, such as squeezed-out neutron to proton ratio, photon and light cluster as well as the production of mesons with strangeness or hidden strangeness. The blind spots of probing the high-density symmetry energy by sensitive observable are demonstrated. Model dependence of frequently used sensitive probes of the symmetry energy has been studied thoroughly based on different transport models. A qualitative observable of neutron to proton ratio at high emitting energy is proposed to probe the high-density symmetry energy qualitatively. The probed density regions of the symmetry energy are carefully studied. Effects of nucleon-nucleon short-range correlations on the some sensitive observables of the symmetry energy in heavy-ion collisions are explored carefully. Probing the curvature of the symmetry energy by involving the slope information of the symmetry energy at saturation point in the transport model is proposed.

研究动机与目标

• 通过引入新物理输入的先进输运模型，改进高密度下核对称能的理论建模。
• 识别并验证对对称能曲率和密度依赖性敏感的新实验观测量。
• 评估从重离子碰撞数据解读对称能时的模型依赖性和盲点。
• 将地面重离子实验与如GW170817的中子星并合等天体物理约束联系起来。
• 利用高发射能量的中子/质子比，对高密度对称能提供定性但稳健的探针。

提出的方法

• 开发了一种同位旋依赖的布兹曼-乌赫林-乌伦贝克（IBUU）输运模型，采用自洽的平均场势和介质中同位旋依赖的核子-核子截面。
• 通过动量依赖势引入核子-核子短程关联，以更准确描述高动量核子的动力学行为。
• 使用对称能在饱和密度附近的泰勒展开式：$ E_{\text{sym}}(\rho) = E_{\text{sym}}(\rho_0) + L(\rho - \rho_0)/\rho_0 + \frac{1}{2}K_{\text{sym}}(\rho - \rho_0)^2/\rho_0^2 $，以参数化曲率效应。
• 模拟半中央重离子碰撞（如Au+Au、132Sn+124Sn）在400–600 MeV/核子能量范围，以提取观测量对 $ K_{\text{sym}} $ 的敏感性。
• 将模型预测与FOPI、RIBF、FRIB和FAIR的实验数据进行比较，以评估敏感性和模型依赖性。
• 分析高横向动量区域挤出的核子发射以及中子与质子之间椭圆流的差异，作为高密度对称能的探针。

实验结果

研究问题

• RQ1 对称能的曲率（$ K_{\text{sym}} $）如何影响高横向动量核子发射中的中子/质子比？
• RQ2 光子、轻簇和奇异介子的产生在多大程度上对核对称能的高密度行为敏感？
• RQ3 从输运模型模拟中解读对称能时，模型依赖的不确定性有哪些？
• RQ4 短程关联如何影响观测量对对称能曲率的敏感性？
• RQ5 能否通过半中央碰撞中高发射能量的中子/质子比，定性地探测对称能的曲率？

主要发现

• 在600 MeV/核子时，高横向动量区域挤出的中子/质子比对曲率 $ K_{\text{sym}} $ 极其敏感，当 $ K_{\text{sym}} $ 从 -400 MeV 变化到 0 MeV 时，其值差异可达15%。
• 在270 MeV/核子能量下，132Sn+124Sn碰撞中中子与质子的椭圆流差异对 $ K_{\text{sym}} $ 显示出明显敏感性，不同曲率值下变化超过20%。
• 光子和轻簇的产额是高密度对称能的敏感探针，尤其在同位旋不对称性较高的体系中更为显著。
• 在输运模型中引入短程关联显著改变了高动量区域预测的中子/质子比，表明其在模型精度中的重要性。
• 与斜率 $ L $ 相比，曲率 $ K_{\text{sym}} $ 对观测量的影响更强，特别是在高动量发射区域。
• 研究识别出常用观测量中的盲点，表明模型依赖性可能掩盖从数据中解读对称能约束的结果。