在LHC的Pb-Pb碰撞中横向动量关联的流动优势和因子分解
Flow dominance and factorization of transverse momentum correlations in Pb-Pb collisions at the LHC
摘要：
本文首次测量了$\sqrt{s_{_{\rm NN}}} =$ 2.76 TeV的Pb-Pb碰撞中的两粒子横向动量微分关联函数$P_2\equiv\langle Δp_{\rm T} Δp_{\rm T} \rangle /\langle p_{\rm T} \rangle^2$。$P_2$的结果报告为相对赝快度（$Δη$）和方位角（$Δφ$）的两个粒子之间的不同的碰撞中心的函数。对于$|Δη| \geq$ 0.9，发现$Δϕ$依赖性在很大程度上独立于$Δη$。在5%最中心的Pb-Pb碰撞中，两粒子横向动量关联函数在$Δφ= π$附近呈现明显的双峰结构（即，在远侧），这在相同中心性范围内的数量相关中没有观察到，因此提供了三角流在该碰撞中心性中的主导性的指示。作为碰撞中心性的函数进行研究的$P_2$的傅里叶分解表明，基于测量的动量相关性严格由系统的集体运动决定的概念，可以通过流假设很好地再现$|Δη| \geq$ 0.9处的相关性。
英文摘要：
We present the first measurement of the two-particle transverse momentum differential correlation function, $P_2\equiv\langle Δp_{\rm T} Δp_{\rm T} \rangle /\langle p_{\rm T} \rangle^2$, in Pb-Pb collisions at $\sqrt{s_{_{\rm NN}}} =$ 2.76 TeV. Results for $P_2$ are reported as a function of relative pseudorapidity ($Δη$) and azimuthal angle ($Δφ$) between two particles for different collision centralities. The $Δϕ$ dependence is found to be largely independent of $Δη$ for $|Δη| \geq$ 0.9. In 5% most central Pb-Pb collisions, the two-particle transverse momentum correlation function exhibits a clear double-hump structure around $Δφ= π$ (i.e., on the away side), which is not observed in number correlations in the same centrality range, and thus provides an indication of the dominance of triangular flow in this collision centrality. Fourier decompositions of $P_2$, studied as a function of collision centrality, show that correlations at $|Δη| \geq$ 0.9 can be well reproduced by a flow ansatz based on the notion that measured momentum correlations are strictly determined by the collective motion of the system.
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