Searching for small droplets of hydrodynamic fluid in proton–proton collisions at the LHC

Wenbin Zhao; You Zhou; Koichi Murase; Huichao Song

doi:10.1140/epjc/s10052-020-8376-x

2023 Impact factor 4.2

Particles and Fields

Open Access

Eur. Phys. J. C (2020) 80: 846
https://doi.org/10.1140/epjc/s10052-020-8376-x

Regular Article - Theoretical Physics

Searching for small droplets of hydrodynamic fluid in proton–proton collisions at the LHC

Wenbin Zhao¹^,2, You Zhou³, Koichi Murase⁴ and Huichao Song¹^,2^,4^d

¹ Department of Physics and State Key Laboratory of Nuclear Physics and Technology, Peking University, 100871, Beijing, China
² Collaborative Innovation Center of Quantum Matter, 100871, Beijing, China
³ Niels Bohr Institute, University of Copenhagen, Blegdamsvej 17, 2100, Copenhagen, Denmark
⁴ Center for High Energy Physics, Peking University, 100871, Beijing, China

^d Huichaosong@pku.edu.cn

Received: 23 January 2020
Accepted: 20 August 2020
Published online: 12 September 2020

Abstract

In this paper, we investigate the hydrodynamic collectivity in proton-proton collisions at $\sqrt{s} =$ $\sqrt{s}=$ 13 TeV, using iEBE-VISHNU with three different initial conditions, namely, HIJING, super-MC and TRENTo. With properly tuned parameters, hydrodynamics gives reasonable descriptions of the measured two-particle correlations, including the integrated and $p_{T}$ $p_\mathrm{T}$ -differential flow. However, the hydrodynamic simulations fail to describe the negative four-particle cumulant $c_{2}^{v} {4}$ $c_2^v\{4\}$ as measured in experiments. The four-particle cumulant $c_{2}^{v} {4}$ $c_2^v\{4\}$ is always positive after hydrodynamic evolutions. Further investigations show that the non-linear response between the final $v_{2}$ $$v_2$$ and the initial $ε_{2}$ $\varepsilon _2$ becomes significant in p-p systems. This leads to a large deviation from linear eccentricity scaling and generates additional flow fluctuations, which results in a positive $c_{2}^{v} {4}$ $c_2^v\{4\}$ even with a negative $c_{2}^{ε} {4}$ $c_2^\varepsilon \{4\}$ from the initial state. We also presented the first hydrodynamic calculations of mixed harmonic azimuthal correlations in p-p collisions. Although many qualitative features are reproduced by the hydrodynamic simulations, the measured negative normalized Symmetric-Cumulant $n s c_{2, 3} {4}$ $nsc_{2,3}\{4\}$ cannot be reproduced. Obviously hydrodynamic calculations have a general difficulty to describe the data. It triggers that whether hydrodynamics with a new initial state could solve this puzzle, or hydrodynamics itself is not the appreciated mechanism of the observed collectivity, and the non-hydrodynamic modes become important in p-p collisions at the LHC.

© The Author(s) 2020

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