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
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
Received:
23
January
2020
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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