粗粒输运方法中的Cooper-Frye负贡献
Cooper-Frye Negative Contributions in a Coarse-Grained Transport Approach
摘要：
许多重离子碰撞模型都采用相对论流体力学来描述高密度下的系统演化。Cooper-Frye公式应用于大多数这些模型中，以将流体动力场转化为粒子。然而，从库珀-弗莱公式得到的粒子数并不总是正定的。库珀-弗莱公式的物理负贡献是粒子流向后进入流体动力学区域。我们量化的库珀-弗莱负贡献的粗粒度的运输方法，它允许将它们的实际数量的底层粒子穿越过渡超曲面进行比较。结果表明，在平衡分布函数假设下，该模型的内部交叉点数目远小于Cooper-Frye公式给出的内部交叉点数目。Cooper-Frye负贡献的大小也被研究作为强子质量，碰撞能量范围$E_{\rm lab} = 5-160A$ GeV，和碰撞中心的函数。我们发现，在$E_{\rm lab} = 20A$ GeV碰撞中，中快度时π介子产额的最大负贡献约为13%。
英文摘要：
Many models of heavy ion collisions employ relativistic hydrodynamics to describe the system evolution at high densities. The Cooper-Frye formula is applied in most of these models to turn the hydrodynamical fields into particles. However, the number of particles obtained from the Cooper-Frye formula is not always positive-definite. Physically negative contributions of the Cooper-Frye formula are particles that stream backwards into the hydrodynamical region. We quantify the Cooper-Frye negative contributions in a coarse-grained transport approach, which allows to compare them to the actual number of underlying particles crossing the transition hypersurface. It is found that the number of underlying inward crossings is much smaller than the one the Cooper-Frye formula gives under the assumption of equilibrium distribution functions. The magnitude of Cooper-Frye negative contributions is also investigated as a function of hadron mass, collision energy in the range $E_{\rm lab} = 5-160A$ GeV, and collision centrality. The largest negative contributions we find are around 13% for the pion yield at midrapidity at $E_{\rm lab} = 20A$ GeV collisions.
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