Examination of flow and nonflow factorization methods in small collision systems
Examination of flow and nonflow factorization methods in small collision systems
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DOI：
10.1103/physrevc.100.024908
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发表时间：
2019-02
影响因子：
3.1
通讯作者：
S. H. Lim;Q. Hu;R. Belmont;K. Hill;J. Nagle;D. Perepelitsa
中科院分区：
文献类型：
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作者：
S. H. Lim;Q. Hu;R. Belmont;K. Hill;J. Nagle;D. Perepelitsa
两个粒子关联被广泛地用于研究重离子碰撞中的流体力学流动模式。在小碰撞系统中，例如$p$$+$$p$和$p$$+$$A$，其中粒子的多重性比在$A$$+$$A$碰撞中小得多，喷流关联、动量守恒、粒子衰变等的非流动效应可能是显著的，即使在粒子之间施加了很大的伪快度差。通过在大型强子对撞机(LHC)上的实验，发展了许多减去两个粒子关联中的非流动贡献的技术，然后用于测量$p$$+$p$和$p$$+$Pb碰撞中的粒子流。最近，相对论重离子对撞机(RHIC)的实验探索了在较低能量的小碰撞系统中采用这些技术的可能性。在本文中，我们使用蒙特卡罗生成器Pythia和HIJING(不包括任何集合流)和AMPT(包含任何集合流)来测试这些技术。我们发现，检验这些检验的结果是至关重要的，无论是对粒子横向动量积分的关联，还是对作为函数的微分关联。我们的结果表明，对于最高能量的$p$$+$$p$碰撞，我们的结果是合理的非流动减法，而如果应用于RHIC的$p$$+$$p$碰撞，则失败。在RHIC的$p$$+$Au碰撞的情况下，HIJING和AMPT的结果都表明，对于$p_{T}\gtrsim1~\rm GeV}/c$，非流动被大大地过度减去，从而低估了椭圆流。
Two particle correlations have been used extensively to study hydrodynamic flow patterns in heavy-ion collisions. In small collision systems, such as $p$$+$$p$ and $p$$+$$A$, where particle multiplicities are much smaller than in $A$$+$$A$ collisions, non-flow effects from jet correlations, momentum conservation, particle decays, etc. can be significant, even when imposing a large pseudorapidity gap between the particles. A number of techniques to subtract the non-flow contribution in two particle correlations have been developed by experiments at the Large Hadron Collider (LHC) and then used to measure particle flow in $p$$+$$p$ and $p$$+$Pb collisions. Recently, experiments at the Relativistic Heavy Ion Collider (RHIC) have explored the possibility of adopting these techniques for small collision systems at lower energies. In this paper, we test these techniques using Monte Carlo generators PYTHIA and HIJING, which do not include any collective flow, and AMPT, which does. We find that it is crucial to examine the results of such tests both for correlations integrated over particle transverse momentum $p_T$ and differentially as a function of $p_T$. Our results indicate reasonable non-flow subtraction for $p$$+$$p$ collisions at the highest LHC energies, while failing if applied to $p$$+$$p$ collisions at RHIC. In the case of $p$$+$Au collisions at RHIC, both HIJING and AMPT results indicate a substantial over-subtraction of non-flow for $p_{T}\gtrsim1~{\rm GeV}/c$ and hence an underestimate of elliptic flow.