■ The isospin dependent nucleon–nucleon inelastic cross section in the nuclear medium
作者: Qingfeng Li, Zhuxia Li（李庆峰，李祝霞）
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
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
作者：王永佳，郭琛琛， 李庆峰， 张鸿飞，李祝霞，Wolfgang Trautmann
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.
■ Ultrarelativistic quantum molecular dynamics calculations of two-pion Hanbury-Brown–Twiss
作者：李庆峰， G. Graef （德国）, M. Bleicher （德国）
摘要：本文中我们利用一个微观的输运模型---极端相对论量子分子动力学模型---研究了最新LHC能区能量在2.76 TeV时中心铅-铅碰撞后的两pion介子的HBT关联。Pratt-Bertsch HBT半径的横动量依赖形式是通过把在径向协动系统内的关联子做一个三维的高斯拟合来抽取出来的。我们定性符合了LHC-ALICE的实验结果。但是，横向的R_O半径比实验值大了约50%。我们也将LHC能区的计算和实验结果同RHIC能区的STAR结果做了比较。在这两个能区中,我们均发现计算的R_O/R_S比值总是比实验值大，这意味着模型中的粒子出射强度没有实验观察到的猛烈。
Abstract: Two-pion Hanbury-Brown–Twiss (HBT) correlations for central Pb-Pb collisions at the Large-Hadron-Collider (LHC) energy of√sNN = 2.76 TeV are investigated with the microscopic transport model ultrarelativistic quantum molecular dynamics. The transverse momentum dependence of the Pratt-Bertsch HBT radii is extracted from a three-dimensional Gaussian fit to the correlator in the longitudinal comoving system.Qualitative agreement with theALICE data is obtained, howeverRout is overpredicted by nearly 50%. The LHC results are also compared to data from the STAR experiment at RHIC. For both energies we find that the calculated RO : RS ratio is always larger than data, indicating that the emission in the model is less explosive than observed in the data.
■ Fusion hindrance in reactions with very heavy ions: Border between normal and hindered fusion
作者：沈彩万, D. Boilley(法国), 李庆峰，沈军杰，Y. Abe(日本)
Abstract: The fusion hindrance in heavy-ion collisions is studied in the framework of the two-center liquid drop model. It appears that the neck and the radial degrees of freedom might both be hampered by an inner potential barrier on their path between the contact configuration to the compound nucleus. Heavy-ion reactions with and without the two kinds of fusion hindrance are classified through systematic calculations. It is found that the number of reactions without radial fusion hindrance is much smaller than that without neck fusion hindrance, and for both kinds of fusion hindrance the number of reactions without fusion hindrance at small mass-asymmetry parameter α is smaller than that at large α. In the formation of a given compound nucleus, if a reaction with αc is not hindered, then other reactions with α>αc are also not hindered, as is well known experimentally.
■ Nonequilibrium dynamics in heavy-ion collisions at low energies available at the GSI Schwerionen Synchrotron
作者：李庆峰，沈彩万，郭琛琛，王永佳，李祝霞，J. Lukasik(波兰), W. Trautmann（德国）
Abstract: The updated ultrarelativistic quantum molecular dynamics (UrQMD) model, a microscopic transport model, is used to study the directed and elliptic collective flows and the nuclear stopping in Au+Au collisions at incident energies covered by INDRA and lower-energy FOPI experiments. It is seen clearly that these observables are sensitive to both the potential terms (including isoscalar and isovector parts as well as the momentum dependent term) in the equation of state (EoS) and the collision term [including the Pauli blocking and the medium-modified nucleon-nucleon elastic cross section (NNECS)]. The momentum modifications of both, themean-field potentials and the density dependent NNECS, are found to affect the collectivity of heavy-ion collisions. At INDRA energies ( 150 MeV/nucleon) the dynamic transport with a soft EoS with momentum dependence and with
the momentum-modified density-dependent NNECS describes the directed flow exhibited by hydrogen isotopes (Z = 1) emitted at midrapidity fairly well.
■ Diffusion over a fluctuating barrier in underdamped dynamics
Abstract: We apply a Langevin model by imposing additive and multiplicative noises to study thermally activated diffusion over a fluctuating barrier in underdamped dynamics. The barrier fluctuation is characterized by Gaussian colored noise with exponential correlation. We present the exact solutions for the first and second moments. Furthermore, we use direct simulations to calculate the asymptotic probability for a Brownian particle passing over the fluctuating barrier. The results indicate that the correlation of the fluctuating barrier is crucial for barrier crossing dynamics.
■ Fusion hindrance in the neck evolution of symmetric nuclear reactions
Abstract: Fusion hindrance in the radial degree of freedom for massive nuclear reactions is known for a long time. However the present work shows that the fusion hindrance also exists in the neck evolution. We calculate the potential at different distances and different neck parameters by the two-center liquid drop model and then check whether fusion hindrance exists in the neck evolution by examing the sign of slope of potential vs. the neck parameter. The area of fusion hindrance in the neck evolution is shown.