$^{208}$ Pb可能的八极形变和超中心$v_2$到$v_3$之谜
Possible octupole deformation of $^{208}$Pb and the ultracentral $v_2$ to $v_3$ puzzle
摘要：
最近的测量已经确定了超中心重离子碰撞对非球形核形变参数的敏感性。在${}^{ 129}$碰撞的情况下，核轮廓的四极形变导致在最中心的碰撞中椭圆流的增强。在${}^{ 208}$ Pb碰撞中，相似的中心性存在差异，其中椭圆流被流体动力学模型过度预测或三角流被预测不足;这被称为超中心碰撞中的$v_2$到$v_3$之谜。受低能核结构计算的启发，我们考虑了$^{208}$ Pb核可能具有梨形形变（八极）的可能性，这种形变在中心PbPb碰撞中具有增加三角流的作用.利用ALICE和ATLAS的最新数据，我们重新审视了超中心碰撞中的$v_2$ -到- $v_3$之谜，包括最近测量的三角累积量比$v_3\left\{4\right\}/v_3\left\{2\right\}$的新约束，并比较了两种不同的流体动力学模型。我们发现，虽然八极形变会稍微改善$v_2$和$v_3$之间的比率，它是在一个显着更差的三角流累积量比的代价。事实上，后一个观测结果更倾向于没有八极形变，对于${}^{ 208}$ Pb有$β_3\lesssim 0.0375$，因此即使在最高对撞机能量下也与双魔核的预期一致。$v_2$到$v_3$的难题仍然是流体动力学模型的一个挑战。
英文摘要：
Recent measurements have established the sensitivity of ultracentral heavy-ion collisions to the deformation parameters of non-spherical nuclei. In the case of ${}^{ 129}$Xe collisions, a quadrupole deformation of the nuclear profile led to an enhancement of elliptic flow in the most central collisions. In ${}^{ 208}$Pb collisions a discrepancy exists in similar centralities, where either elliptic flow is over-predicted or triangular flow is under-predicted by hydrodynamic models; this is known as the $v_2$-to-$v_3$ puzzle in ultracentral collisions. Motivated by low-energy nuclear structure calculations, we consider the possibility that $^{208}$Pb nuclei could have a pear shape deformation (octupole), which has the effect of increasing triangular flow in central PbPb collisions. Using the recent data from ALICE and ATLAS, we revisit the $v_2$-to-$v_3$ puzzle in ultracentral collisions, including new constraints from recent measurements of the triangular cumulant ratio $v_3\left\{4\right\}/v_3\left\{2\right\}$ and comparing two different hydrodynamic models. We find that, while an octupole deformation would slightly improve the ratio between $v_2$ and $v_3$, it is at the expense of a significantly worse triangular flow cumulant ratio. In fact, the latter observable prefers no octupole deformation, with $β_3\lesssim 0.0375$ for ${}^{ 208}$Pb, and is therefore consistent with the expectation for a doubly-magic nucleus even at top collider energies. The $v_2$-to-$v_3$ puzzle remains a challenge for hydrodynamic models.
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