Hydrodynamic Collectivity in Proton--Proton Collisions at 13 TeV

TL;DR

This study uses a hydrodynamic model to analyze proton-proton collisions at 13 TeV, successfully reproducing many experimental observations but facing challenges in accurately modeling four-particle correlations, highlighting the need for improved initial conditions.

Contribution

It demonstrates the application of the iEBE-VISHNU hydrodynamic model with HIJING initial conditions to pp collisions and identifies limitations in current initial condition modeling for accurate flow fluctuation predictions.

Findings

Model reproduces 2-particle correlations well.

Fails to reproduce negative 4-particle cumulant $c_{2} ext{'} ext{'}$ in high multiplicity pp.

Flow fluctuations linked to initial condition fluctuations.

Abstract

In this paper, we investigate the hydrodynamic collectivity in proton--proton (pp) collisions at 13 TeV, using iEBE-VISHNU hybrid model with HIJING initial conditions. With properly tuned parameters, our model simulations can remarkably describe all the measured 2-particle correlations, including integrated and differential elliptic flow coefficients for all charged and identified hadrons ($K_S^0$, $\Lambda$). However, our model calculations show positive 4-particle cumulant $c_{2}\{4\}$ in high multiplicity pp collisions, and can not reproduce the negative $c_{2}\{4\}$ measured in experiment. Further investigations on the HIJING initial conditions show that the fluctuations of the second order anisotropy coefficient $\varepsilon_{2}$ increases with the increase of its mean value, which leads to a similar trend of the flow fluctuations. For a simultaneous description of the 2- and 4- particle cumulants within the hydrodynamic framework, it is required to have significant improvements on initial condition for pp collisions, which is still lacking of knowledge at the moment.