电磁场与重子阻止在带电流流体动力学模型中的相互作用
The Interplay Between Electromagnetic Fields and Baryon Stopping in a Hydrodynamic Model for Charged Flow
- Utrecht University（乌得勒支大学）
- University of Connecticut（康涅狄格大学）
- Massachusetts Institute of Technology（麻省理工学院）
- Nikhef（荷兰国家核与亚原子物理研究所）
机构由 AI 辅助整理，请以论文原文为准。
AI总结：
本研究提出半解析流体动力学模型，结合旁观者电磁场与Glauber重子阻止，解释Au+Au碰撞中定向流劈裂的符号改变，揭示两者竞争机制。
AI中文摘要：
电荷依赖的定向流为相对论性重离子碰撞中的早期电磁场和重子阻止提供了灵敏的探针。最近的STAR测量显示，定向流劈裂（质子与反质子定向流之差）存在随中心度变化的符号改变，这表明仅电磁效应不足以描述这一可观测量，还必须包含重子阻止——特别是阻止质子分布中在快度上为奇函数且在碰撞参数方向上反射为奇函数的分量。我们开发了一个半解析流体动力学框架，该框架将旁观者诱导的电磁场与基于Glauber的重子阻止描述相结合，并建立在Gubser给出的背景流体动力学流的解析解以及恒定电导率的简化假设之上。对于$\sqrt{s_{NN}}=200$ GeV的Au+Au碰撞，我们发现重子阻止对定向流劈裂产生正贡献，且该贡献随碰撞更外围而减小，而电磁场产生负贡献，且该贡献在更外围碰撞中更大。这两种效应之间的竞争自然地再现了Au+Au碰撞中观测到的随中心度变化的符号改变，描述了$50$--$80\\%$中心度区间内观测到的快度依赖性，并再现了U+U碰撞中观察到的趋势。尽管我们关于解析背景和恒定电导率的简化假设限制了进行定量比较的能力，但我们的模型为输运重子数和旁观者诱导电磁场如何共同塑造电荷依赖的定向流提供了清晰的解释。
英文摘要：
Charge-dependent directed flow provides a sensitive probe of early electromagnetic fields and baryon stopping in relativistic heavy-ion collisions. Recent STAR measurements show a centrality-dependent sign change in the directed flow splitting (the difference between the directed flow of protons and antiprotons), indicating that electromagnetic effects alone are not sufficient to describe this observable and that the baryon stopping $-$ in particular the component of the stopped proton distribution that is odd in rapidity and odd under reflection in the impact parameter direction $-$ must also be included. We develop a semi-analytic hydrodynamic framework that combines spectator-induced electromagnetic fields with a Glauber-based description of baryon stopping, built upon an analytic solution for the background hydrodynamic flow due to Gubser together with the simplifying assumption of a constant electrical conductivity. For Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV, we find that baryon stopping gives a positive contribution to the directed flow splitting that decreases for more peripheral collisions, while electromagnetic fields give a negative contribution that is larger for more peripheral collisions. The competition between these two effects naturally reproduces the observed sign change in Au+Au collisions as a function of centrality, describes the observed rapidity dependence in the $50$--$80\%$ centrality interval, and reproduces trends seen in U+U collisions. Although the simplifying assumptions that we have made regarding the analytic background and constant conductivity limit our ability to make quantitative comparisons, our model provides a transparent explanation of how transported baryon number and spectator-induced electromagnetic fields jointly shape charge-dependent directed flow.
