"Figure 3.1" of "Elliptic flow of charged particles at midrapidity relative to the spectator plane in Pb-Pb and Xe-Xe collisions"

Measurements of the elliptic flow coefficient relative to the collision plane defined by the spectator neutrons $v_2\{\Psi_\mathrm{SP}\}$ in collisions of Pb ions at center-of-mass energy per nucleon-nucleon pair $\sqrt{s_\mathrm{NN}}=2.76$ TeV and Xe ions at $\sqrt{s_\mathrm{NN}}=5.44$ TeV are reported. The results are presented for charged particles produced at midrapidity as a function of centrality and transverse momentum for the $5-70\%$ and $0.2-6\ \mathrm{GeV}/c$ ranges, respectively. The ratio between $v_2\{\Psi_\mathrm{SP}\}$ and the elliptic flow coefficient relative to the participant plane $v_2\{4\}$, estimated using four-particle correlations, deviates by up to $20\%$ from unity depending on centrality. This observation differs strongly from the magnitude of the corresponding eccentricity ratios predicted by the TRENTo and the elliptic power models of initial state fluctuations that are tuned to describe the participant plane anisotropies. The differences can be interpreted as a decorrelation of the neutron spectator plane and the reaction plane because of fragmentation of the remnants from the colliding nuclei, which points to an incompleteness of current models describing the initial state fluctuations. A significant transverse momentum dependence of the ratio $v_2\{\Psi_\mathrm{SP}\}/v_2\{4\}$ is observed in all but the most central collisions, which may help to understand whether momentum anisotropies at low and intermediate transverse momentum have a common origin in initial state fluctuations. The ratios of $v_2\{\Psi_\mathrm{SP}\}$ and $v_2\{4\}$ to the corresponding initial state eccentricities for Xe--Xe and Pb--Pb collisions at similar initial entropy density show a difference of $(7.0\pm0.9)\%$ with an additional variation of $+1.8\%$ when including RHIC data in the TRENTo parameter extraction. These observations provide new experimental constraints for viscous effects in the hydrodynamic modeling of the expanding quark--gluon plasma produced in heavy-ion collisions at the LHC.本数据集报道了每核子-核子对质心系能量√s_NN=2.76 TeV的铅离子碰撞，以及√s_NN=5.44 TeV的氙离子碰撞中，相对于由旁观中子定义的碰撞平面Ψ_SP的椭圆流系数v₂{Ψ_SP}的测量结果。上述结果针对快度中段产生的带电粒子，分别以5%–70%区间的碰撞中心度、0.2–6 GeV/c区间的横向动量为自变量给出分布特征。利用四粒子关联方法估算得到的、相对于参与者平面的椭圆流系数v₂{4}与v₂{Ψ_SP}的比值，随碰撞中心度变化最大可偏离单位1达20%。该观测结果与经调谐以描述参与者平面各向异性的初始态涨落模型（TRENTo模型与椭圆幂模型）所预测的对应偏心度比值存在显著偏差。这种差异可归因于碰撞核残余物碎裂导致的旁观中子平面与反应平面之间的退关联，这表明当前描述初始态涨落的模型存在不完备性。除最中心碰撞外，所有碰撞区间均观测到v₂{Ψ_SP}/v₂{4}比值存在显著的横向动量依赖性，这或有助于厘清低至中等横向动量下的动量各向异性是否起源于共同的初始态涨落机制。在初始熵密度相近的Xe–Xe与Pb–Pb碰撞中，v₂{Ψ_SP}、v₂{4}分别对应初始态偏心度的比值差异为(7.0±0.9)%；若在TRENTo模型参数提取中纳入相对论重离子对撞机(Relativistic Heavy Ion Collider, RHIC)数据，该比值还会额外变化+1.8%。上述观测结果为大型强子对撞机(Large Hadron Collider, LHC)重离子碰撞中产生的膨胀夸克-胶子等离子体(quark–gluon plasma)的流体力学建模中的粘滞效应提供了新的实验约束。