王永佳[2014PRC]:中能金金碰撞中轻质量碎片的集体流
2020-11-29
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  ■ The isospin dependent nucleon–nucleon inelastic cross section in the nuclear medium
  
  核介质中的同位旋依赖核子-核子非弹性散射截面
  
  作者: Qingfeng Li, Zhuxia Li(李庆峰,李祝霞)
  
  杂志: Physics Letters B 773 (2017) 557–562
  
  
  摘要:本文在相对论BUU的框架下计算了核子-核子散射中能量、密度、以及同位旋依赖的Δ产生截面。这个截面的计算是考虑核子及Δ子在同位旋不对称情况下质量劈裂的Born近似取得的。正是由于这些粒子的质量劈裂,结果表明,类似于核子-核子的弹性截面,在同位旋不对称的核介质中每一个特定电荷Δ粒子的出射道的截面介质约化效应均不同。而且,对于Δ++和Δ-态出射而言,其效应最大且效果相反。本文所述方法结果也和不采用质量劈裂及采用DB方法计算的结果做了比较。并且,这种非弹散射截面的同位旋依赖性将会影响π+、π-产额及相应比率的结果,因此,必将会对利用该比率探测高密区对称能软硬程度造成影响。
  
  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
  
  利用横动量依赖的椭圆流约束对称能的高密行为
  
  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.
  
  ■ Collective flow of light particles in Au+Au collisions at intermediate energies
  
  中能金金碰撞中轻质量碎片的集体流
  
  作者:王永佳,郭琛琛, 李庆峰, 张鸿飞,李祝霞,Wolfgang Trautmann
  
  杂志:Physical Review C89,034606 (2014)
  
  摘要:在极端相对论量子分子动力学模型中引入了Skyrme能量密度函数,用更新的模型研究了入射能量在150,250,400 MeV/nucleon的金金碰撞产生的轻质量碎片(质子、中子、氘核、氚核、3He、4He)的直接流和椭圆流。模型模拟的结果与最新的FOPI实验数据进行了比对,发现轻质量碎片的产额、集体流可以被很好的再现。并且细致的研究了状态方程、介质修正的核核碰撞截面及碎片的构建方法对集体流的影响。发现轻质量碎片的集体流非常敏感于介质修正的核核碰撞截面,但不敏感于碎片的构建方法。还发现即便使用最新的实验数据和理论计算,仍然不能提取出更加精确的核物质不可压缩系数的范围。
  
  Abstract: The Skyrme potential energy density functional is introduced into the ultrarelativistic quantum molecular dynamics model and the updated version is applied to studying the directed and elliptic flows of light particles(protons, neutrons, deuterons, tritons, 3He, and4He) in Au+Au collisions at beam energies 150, 250, and 400 MeV/nucleon. The results are compared with the recent FOPI experimental data. It is found that the yields and collective flows of light particles can be described quite well. The influence of the equation of state, medium-modified nucleon-nucleon elastic cross sections (NNECS) and cluster recognition criteria on the directed and elliptic flows is studied in detail. It is found that the flows of light particles are sensitive tothe medium-modified NNECS, but not sensitive to the isospin dependent cluster recognition criteria. It seems difficult, however, even with the new data and calculations, to obtain a more accurate constraint on the nuclear incompressibility K0 than the interval 200–260 MeV.