arXiv 2305.087052023-05nucl-ex

Au+Au 碰撞中 $\sqrt{s_{\rm NN}}=19.6$ 和 $27$ GeV 下 $Λ$ 与 $\barΛ$ 超子的整体极化

Global polarization of $Λ$ and $\barΛ$ hyperons in Au+Au collisions at $\sqrt{s_{\rm NN}}=19.6$ and $27$ GeV

The STAR Collaboration

DOI：: 10.1103/PhysRevC.108.014910

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AI总结：

本文利用 STAR 探测器在 RHIC BES-II 中测量了 19.6 和 27 GeV Au+Au 碰撞中 Λ 与反 Λ 超子的整体极化，未发现二者显著分裂且极化不依赖快度和横动量，并据此给出晚期磁场上限，挑战了现有模型预测。

摘要：

在相对论重离子碰撞中，沿系统角动量方向的 $Λ$ 和 $\barΛ$ 超子整体自旋极化 $P_\mathrm{H}$ 已被发现并在广泛的碰撞能量范围内进行了测量，显示出 $P_\mathrm{H}$ 随 $\sqrt{s_{\rm NN}}$ 降低而增大的趋势。由于碰撞中形成的夸克-胶子等离子体所维持的晚期磁场中 $Λ$ 与 $\barΛ$ 具有不同的磁矩，二者极化之间可能存在分裂。本研究利用 STAR 探测器在 RHIC 束流能量扫描第二阶段的 $\sqrt{s_{\rm NN}}=19.6$ 和 $27$ GeV 碰撞能量下给出的结果未发现显著分裂，在 95% 置信水平下上限分别为 $P_{\barΛ}-P_Λ<0.24$% 和 $P_{\barΛ}-P_Λ<0.35$%。我们推导出对晚期磁场朴素提取的上限，在 $\sqrt{s_{\rm NN}}=19.6$ 和 $27$ GeV 下分别为 $B<9.4\cdot10^{12}$ T 和 $B<1.4\cdot10^{13}$ T，尽管还需要更严格的推导。对 $P_\mathrm{H}$ 的微分测量针对碰撞中心度、横向动量和快度进行。在当前 $|y|<1$ 的接受度和不确定度范围内，本分析未观察到对横向动量和快度的依赖。这些结果对多个现有模型计算提出了挑战，这些计算基于各种不同假设，均预测在此碰撞能量范围内极化对快度有强依赖。

英文摘要：

In relativistic heavy-ion collisions, a global spin polarization, $P_\mathrm{H}$, of $Λ$ and $\barΛ$ hyperons along the direction of the system angular momentum was discovered and measured across a broad range of collision energies and demonstrated a trend of increasing $P_\mathrm{H}$ with decreasing $\sqrt{s_{\rm NN}}$. A splitting between $Λ$ and $\barΛ$ polarization may be possible due to their different magnetic moments in a late-stage magnetic field sustained by the quark-gluon plasma which is formed in the collision. The results presented in this study find no significant splitting at the collision energies of $\sqrt{s_{\rm NN}}=19.6$ and $27$ GeV in the RHIC Beam Energy Scan Phase II using the STAR detector, with an upper limit of $P_{\barΛ}-P_Λ<0.24$% and $P_{\barΛ}-P_Λ<0.35$%, respectively, at a 95% confidence level. We derive an upper limit on the naïve extraction of the late-stage magnetic field of $B<9.4\cdot10^{12}$ T and $B<1.4\cdot10^{13}$ T at $\sqrt{s_{\rm NN}}=19.6$ and $27$ GeV, respectively, although more thorough derivations are needed. Differential measurements of $P_\mathrm{H}$ were performed with respect to collision centrality, transverse momentum, and rapidity. With our current acceptance of $|y|<1$ and uncertainties, we observe no dependence on transverse momentum and rapidity in this analysis. These results challenge multiple existing model calculations following a variety of different assumptions which have each predicted a strong dependence on rapidity in this collision-energy range.

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