■ The isospin dependent nucleon–nucleon inelastic cross section in the nuclear medium
作者: Qingfeng Li, Zhuxia Li（李庆峰，李祝霞）
杂志: Physics Letters B 773 (2017) 557–562
Abstract: The calculation of the energy-, density-, and isospin-dependent Δ production cross sections in nucleon–nucleon (NN) scattering σ ∗NN → NΔ has been performed within the framework of the relativistic BUU approach. The NΔ cross sections are calculated in Born approximation taking into account the effective mass splitting of the nucleons and Δs in asymmetric matter. Due to the different mass splitting for neutron, proton and differently charged Δs, it is shown that, similar to the NN elastic ones, the reductions of NΔ inelastic cross sections in isospin-asymmetric nuclear medium are different from each other for all the individual channels and the effect is largest and of opposite sign for the Δ ++ and Δ− states. This approach is also compared to calculations without effective mass splitting and with splitting derived from Dirac–Brueckerner (DB) calculations. The isospin dependence of the NΔ cross sections is expected to influence the production of π + and π − mesons as well as their yield ratio, and thus affect the use of the latter quantity as a probe of the stiffness of the symmetry energy at supranormal densities.
■ Constraining the high-density nuclear symmetry energy with the transverse-momentum-dependent elliptic flow
作者：Yongjia Wang, Chenchen Guo, Qingfeng Li, Hongfei Zhang, Y. Leifels, and W. Trautmann
杂志：Physical Review C 89, 044603 (2014)
摘要：利用最近更新的极端相对论量子分子动力学模型，研究了400 MeV/nucleon金金碰撞中不同快度区间内自由核子的横动量依赖的椭圆流。发现中子-质子椭圆流的差和比敏感于对称能，特别是在大快度区间内中子-质子椭圆流的比。通过理论模拟与FOPI/LAND给出的横动量依赖的及积分的椭圆流数据比较，提取出对称能随密度的变化趋势接近于线性，这一结果与之前UrQMD及QMD模型给出的结果相一致，而与目前有关文献中使用 pion-/pion+比给出的对称能的密度依赖行为结果不相同。
Abstract: Within the newly updated version of the ultrarelativistic quantum molecular dynamics (UrQMD) model, the transverse-velocity dependence of the elliptic flow of free nucleons from Au+Au collisions at the incident energy 400 MeV/nucleon is studied within different windows of the normalized c.m. rapidity y0. It is found that the elliptic flow difference vn2 −vp2 and ratio vn2/vp2 of neutrons versus protons are sensitive to the density dependence of the symmetry energy, especially the ratio vn2/vp2 at small transverse velocity in the intermediate rapidity intervals 0.4<|y0|<0.6. By comparing either transverse-momentum-dependent or integrated FOPI/LAND elliptic flow data of nucleons and hydrogen isotopes with calculations using various Skyrme interactions, all exhibiting similar values of isoscalar incompressibility but very different density dependences of the symmetry energy, a moderately soft to linear symmetry energy is extracted, in good agreement with previous UrQMD or Tubingen QMD model calculations but contrast with results obtained with π−/π+ yield ratios in theliterature.