在LHC质子-质子碰撞中寻找流体动力学小液滴
Searching for small droplets of hydrodynamic fluid in proton--proton collisions at the LHC
摘要：
本文采用iEBE-VISHNU混合模型，在HIJING、super-MC和TRENTo三种不同的初始条件下，研究了$\sqrt{s}=$ 13 TeV质子-质子碰撞高多重数事例的流体动力学集体性.适当调整参数，流体动力学模拟与每个初始模型给出合理的描述测量的两粒子相关性，包括集成和$p_{\rm T}$差分流的所有带电和识别强子。然而，流体动力学模拟未能描述的四粒子累积量$c_2^v\{4\}$的实验测量值的负值。进一步的研究表明，椭圆流$v_2$和初始偏心率$\varepsilon_2$之间的非线性响应在小p-p系统中变得显著。这导致与线性偏心率缩放的较大偏差，并产生额外的流量波动，这导致正的$c_2^v\{4\}$，即使与初始状态相比具有负的$c_2^\varepsilon\{4\}$。我们还首次给出了p-p碰撞中多粒子混合谐波方位角关联的流体力学计算，如归一化非对称累积量$nac_n\{3\}$、归一化对称累积量、$nsc_{2,3}\{4\}$和$nsc_{2,4}\{4\}$。虽然许多定性的功能是由选定的参数的流体动力学模拟再现，测得的负$nsc_{2,3}\{4\}$不能再现。对负$c_2\{4\}$和$nsc_{2,3}\{4\}$的描述失败引发了一个问题，即具有全新初始状态模型的流体力学是否可以解决这个难题，或者流体力学本身可能不是在LHC上观察到的p-p碰撞集体性的赞赏机制。
英文摘要：
In this paper, we investigate the hydrodynamic collectivity in high-multiplicity events of proton-proton collisions at $\sqrt{s}=$ 13 TeV, using iEBE-VISHNU hybrid model with three different initial conditions, namely, HIJING, super-MC and TRENTo. With properly tuned parameters, hydrodynamic simulations with each initial model give reasonable descriptions of the measured two-particle correlations, including the integrated and $p_{\rm T}$-differential flow for all charged and identified hadrons. However, the hydrodynamic simulations fail to describe the negative value of the four-particle cumulant $c_2^v\{4\}$ as measured in experiments. Further investigations show that the non-linear response between the elliptic flow $v_2$ and the initial eccentricity $\varepsilon_2$ becomes significant in the small p-p systems. This leads to a large deviation from linear eccentricity scaling and generates additional flow fluctuations, which results in a positive $c_2^v\{4\}$ even with a negative $c_2^\varepsilon\{4\}$ from the initial state. We also presented the first hydrodynamic calculations of multi-particle mixed harmonic azimuthal correlations in p-p collisions, such as normalized asymmetric cumulant $nac_n\{3\}$, normalized Symmetric-Cumulant, $nsc_{2,3}\{4\}$ and $nsc_{2,4}\{4\}$. Although many qualitative features are reproduced by the hydrodynamic simulations with chosen parameters, the measured negative $nsc_{2,3}\{4\}$ cannot be reproduced. The failure of the description of negative $c_2\{4\}$ and $nsc_{2,3}\{4\}$ triggers the question on whether hydrodynamics with a fundamentally new initial state model could solve this puzzle, or hydrodynamics itself might not be the appreciated mechanism of the observed collectivity in p-p collisions at the LHC.
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