NICA 能量下的 QCD 相图：THESEUS 中的 $K^+/π^+$ 尖峰效应与轻团簇
QCD Phase Diagram at NICA energies: $K^+/π^+$ horn effect and light clusters in THESEUS
AI总结：
本文基于三流体流体力学程序 THESEUS 研究 NICA/FAIR 中等能量重离子碰撞，通过冻结面粒子分布修正解释轻团簇产生和 $K^+/π^+$ 比值“尖峰”效应，并指出π介子玻色凝聚可在 NICA 能量范围内产生该信号。
摘要：
我们讨论了基于三流体流体力学程序 THESEUS 的最新进展，目标是将其发展为一个适用于计划中的 NICA 和 FAIR 实验中等能量重离子碰撞的事件生成器。我们采用的策略是：在 QCD 相图中，冻结面上粒子分布的修正可以在突然冻结方案下直接映射到可观测量的修正。我们报告了这些研究在轻团簇（氘核和氚核）产生方面的首批结果，这些结果可以与 HADES 和 NA49 实验的实验数据进行比较，并可用于解释在 $K^+/π^+$ 比值随碰撞能量依赖关系中观测到的“尖峰”效应。QCD 相图中轻团簇产生的介质效应在 NICA 最高能量下可以忽略，但在最低能量下将起主导作用。当由π介子化学势模拟的玻色凝聚开始出现，导致低动量π介子增强（这一现象在 LHC 能量下已被观测到）时，可以在 $K^+/π^+$ 比值中获得一个尖锐的“尖峰”型信号，并且该信号在 NICA 能量范围内就已经会出现。
英文摘要：
We discuss recent progress in the development of the three-fluid hydrodynamics-based program THESEUS towards an event generator suitable for applications to heavy-ion collisions at the intermediate energies of the planned NICA and FAIR experiments. We follow the strategy that modifications of particle distributions at the freeze-out surface in the QCD phase diagram may be mapped directly to the observable ones within a sudden freeze-out scheme. We report first results of these investigations for the production of light clusters (deuterons and tritons) which can be compared to experimental data from the HADES and the NA49 experiment and for the interpretation of the "horn" effect observed in the collision energy dependence of the $K^+/π^+$ ratio. Medium effects on light cluster production in the QCD phase diagram are negligible at the highest NICA energies but shall play a dominant role at the lowest energies. A sharp "horn"-type signal in the $K^+/π^+$ ratio can be obtained when the onset of Bose condensation modelled by a pion chemical potential results in an enhancement of pions at low momenta (which is seen at LHC energies) and would occur already in the NICA energy range.
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