Regular Article - Theoretical Physics
Searching for small droplets of hydrodynamic fluid in proton–proton collisions at the LHC
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
Accepted: 20 August 2020
Published online: 12 September 2020
In this paper, we investigate the hydrodynamic collectivity in proton-proton collisions at 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 -differential flow. However, the hydrodynamic simulations fail to describe the negative four-particle cumulant as measured in experiments. The four-particle cumulant is always positive after hydrodynamic evolutions. Further investigations show that the non-linear response between the final and the initial 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 even with a negative 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 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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