Symmetry Energy from Nuclear Reactions

自然界中存在的原子核往往具有相同数量的中子和质子。对于更大的核质量，这归因于对称能，这是与中子-质子不平衡相关的核能的贡献，不平衡越大，对称能就越大。具有大的不平衡的原子核可以衰变为能量更有利的产物，当这种不平衡过度时，原子核甚至不能形成。理解对称能对于从现有原子核的性质推断出中子星是必不可少的，中子星在引力的作用下保持稳定，同时处于黑洞坍缩的边缘。在他们的努力中，反演器将依靠他们互补的专业知识，并将受益于国家超导回旋加速器实验室（NSCL）核理论小组其他成员的核结构专业知识，以及部分设在密歇根州立大学的核天体物理联合研究所（JINA）成员的核天体物理专业知识。强调与所谓的核对称能有关的方面。对称能控制着中子-质子不平衡的核性质的平均演化，考虑到现代现有和计划中的加速器可以加速短寿命同位素，现在更容易通过实验改变。对称能对于从原子核的性质外推到中子星的性质是很重要的。后者是宏观的核系统，在坍缩成黑洞的边缘，通过与对称能对核密度的依赖性相关的重力和压力的反向作用而变得稳定。利用重核中心碰撞的观测量，特别是带电π介子比和集体流，研究了密度高于正常核的对称能。研究人员将通过将α粒子的动态产生纳入碰撞的传输理论来改进这些中心碰撞的理论描述。由于每个核子的结合能很大，质子和中子数相等，α粒子一方面产生得相当多，另一方面，它们显著改变了周围环境中的相对中子-质子不平衡，因此它们很重要。将通过同时解释弹性散射和电荷交换反应的数据来研究正常和低于正常密度的对称能，从而了解具有中子-质子不平衡的核中中子和质子分布的差异。 最后，研究人员将从依赖于基本相互作用的多体理论中提取中子和质子的光学势，特别注意这些势对中子-质子不平衡的依赖性。基本提取有助于减少外周反应实验解释中的歧义。

Atomic nuclei occurring in nature tend to have the same number of neutrons and protons. For larger nuclear masses, this is attributed to the symmetry energy, a contribution to the nuclear energy associated with neutron-proton imbalance, that becomes larger the larger the imbalance. Nuclei with large imbalance can decay to more energetically favored products and, when that imbalance is excessive, the nuclei cannot even be formed. Understanding the symmetry energy is essential for extrapolating from the properties of existing nuclei to neutron stars kept stable by gravity while at the verge of collapse to black holes. In their efforts, the inverstigators will rely on their complementary expertise and will benefit from the expertise in nuclear structure of other members of the National Superconducting Cyclotron Laboratory (NSCL) Nuclear Theory Group and from the expertise in nuclear astrophysics of the members of the Joint Institute for Nuclear Astrophysics (JINA), housed partly at Michigan State University.This project will advance our understanding of nuclear reactions, emphasizing aspects pertaining to the so-called nuclear symmetry energy. The symmetry energy governs average evolution of nuclear properties with neutron-proton imbalance that is now easier to vary experimentally given the modern existing and planned accelerators that can accelerate short-lived isotopes. The symmetry energy is important for extrapolating from the properties of the nuclei to those of neutron stars. The latter are macroscopic nuclear systems made stable, right at the verge of collapse into a black hole, by opposing forces of gravity and pressure tied to the dependence of symmetry energy on nuclear density. The symmetry energy at densities higher than normal for nuclei is studied using observables from central collisions of heavy nuclei, in particular charged pion ratios and collective flow. The investigators will improve on theoretical descriptions of those central collisions, by incorporating dynamic production of alpha particles into transport theory for the collisions. Because of large binding energy per nucleon and equal proton and neutron numbers, alpha particles are, on one hand, rather copiously produced and, on the other, they significantly change relative neutron-proton imbalance in their surroundings; hence their importance. The symmetry energy at normal and subnormal densities will be studied by simultaneously interpreting data from elastic scattering and charge exchange reactions, thus learning about differences in the neutron and proton distributions in those nuclei that have neutron-proton imbalance. Finally, the investigators will extract optical potentials of neutrons and protons from many-body theory relying on fundamental interactions, paying particular attention to the dependence of those potentials on neutron-proton imbalance. The fundamental extraction can help to reduce ambiguities in the interpretations of peripheral reaction experiments.