arXiv 2102.052002021-02nucl-thhep-phnucl-ex

核碰撞中简谐流与横动量关联中的非流效应

Non-flow effects in correlation between harmonic flow and transverse momentum in nuclear collisions

Chunjian Zhang, Arabinda Behera, Somadutta Bhatta, Jiangyong Jia

备注：: 9 pages, 8 figures including an appendix, replace with published version
DOI：: 10.1016/j.physletb.2021.136702

摘要：

最近在小碰撞系统中测量了椭圆流$v_2$和平均横向动量$[p_{\mathrm{T}}]$之间的大的反相关信号，与初始几何形状的终态流体动力学响应一致。这种负的$v_2$ - $[p_{\mathrm{T}}]$相关性被预测为正相关的事件非常小的带电粒子多重数$N_{\mathrm{ch}}$由于胶子饱和效应的初始状态动量各向异性。然而，在这些系统中，非流动相关性的作用预计是重要的，这是尚未研究。我们用{ \tt Pythia}和{ \tt Hijing}模型计算了$pp$、$p$ Pb和外围PbPb碰撞中的非流动效应，并与实验结果进行了比较.我们表明，非流动的影响在很大程度上抑制使用快速分离的子事件累积量方法（累积量框架的细节也提供）。剩余非流动的幅度远小于在较高的$N_{\mathrm{ch}}$区域的实验观察，支持最终状态响应的解释。然而，在非常低的$N_{\mathrm{ch}}$区域，剩余非流动的符号和幅度取决于模型细节。因此，目前还不清楚$v_2$ - $[p_{\mathrm{T}}]$的符号变化是否可以作为胶子饱和预测的初始状态动量各向异性的证据。

英文摘要：

A large anti-correlation signal between elliptic flow $v_2$ and average transverse momentum $[p_{\mathrm{T}}]$ was recently measured in small collision systems, consistent with a final-state hydrodynamic response to the initial geometry. This negative $v_2$-$[p_{\mathrm{T}}]$ correlation was predicted to change to positive correlation for events with very small charged particle multiplicity $N_{\mathrm{ch}}$ due to initial-state momentum anisotropies of the gluon saturation effects. However, the role of non-flow correlations is expected to be important in these systems, which is not yet studied. We estimate the non-flow effects in $pp$, $p$Pb and peripheral PbPb collisions using {\tt Pythia} and {\tt Hijing} models, and compare them with the experimental data. We show that the non-flow effects are largely suppressed using the rapidity-separated subevent cumulant method (details of the cumulant framework are also provided). The magnitude of the residual non-flow is much less than the experimental observation in the higher $N_{\mathrm{ch}}$ region, supporting the final-state response interpretation. In the very low $N_{\mathrm{ch}}$ region, however, the sign and magnitude of the residual non-flow depend on the model details. Therefore, it is unclear at this moment whether the sign change of $v_2$-$[p_{\mathrm{T}}]$ can serve as evidence for initial state momentum anisotropies predicted by the gluon saturation.

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