重离子碰撞中参与者和旁观者角动量的解相关
Decorrelation of participant and spectator angular momenta in heavy-ion collisions
摘要：
高能重离子碰撞包含巨大的角动量$|\vec{J}|$，它是相对论重离子对撞机（RHIC）和大型强子对撞机（LHC）实验跨越的碰撞能量范围$\sqrt{s_\mathrm{NN}}$中的$\mathcal{O}(10^3-10^6\hbar)$。一小部分$\vec{J}$被转移到重叠碰撞区，这是不可或缺的测量，如涡驱动强子自旋对齐与$\hat{J}$。实验利用前向和后向碰撞核子$\hat{J}_\mathrm{spec}$的取向估计了碰撞重叠区$\hat{J}_\mathrm{part}$内参与核子的$\hat{J}$取向。使用两个模型，我们研究了$\hat{J}_\mathrm{part}$和$\hat{J}_\mathrm{spec}$之间的解相关，驱动角动量守恒和事件的事件波动，以及由椭圆重叠区域的方向和$\hat{J}_\mathrm{part}$之间的解相关。在这两种情况下都观察到了$\sqrt{s_\mathrm{NN}}$依赖的去相关，并且该去相关足够大，可以作为实验观察由$\vec{J}$驱动的现象时使用的重要校正因子。
英文摘要：
High-energy heavy-ion collisions contain enormous angular momentum, $|\vec{J}|$, which is $\mathcal{O}(10^3-10^6\hbar)$ in the range of collision energy, $\sqrt{s_\mathrm{NN}}$, spanned experimentally by the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC). A fraction of $\vec{J}$ is transferred to the overlapping collision region, which is indispensable for measuring observables such as vorticity-driven hadron spin alignment with $\hat{J}$. Experiments estimate the orientation of $\hat{J}$ of the participant nucleons within the collision overlap region, $\hat{J}_\mathrm{part}$, by using that of the forward- and backward-going spectating nucleons $\hat{J}_\mathrm{spec}$. Using two models, we study the decorrelation between $\hat{J}_\mathrm{part}$ and $\hat{J}_\mathrm{spec}$, driven both by angular-momentum conservation and event-by-event fluctuations, as well as by the decorrelation between the orientation of the elliptic overlap region and the $\hat{J}_\mathrm{part}$. $\sqrt{s_\mathrm{NN}}$-dependent decorrelation is observed in both of these cases and is large enough to be an important corrective factor used when experimentally observing phenomena driven by $\vec{J}$.
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