Femtoscopic signature of strong radial flow in high-multiplicity $pp$ collisions

TL;DR

This paper uses hydrodynamic simulations to analyze pion correlations in high-multiplicity proton-proton collisions, revealing strong radial flow and very small initial fireball sizes, consistent with experimental data.

Contribution

It introduces a method to extract radial flow magnitude from HBT radii in high-multiplicity $pp$ collisions using hydrodynamic models with variable initial conditions.

Findings

Strong radial flow observed in high-multiplicity $pp$ collisions.

Initial fireball size is inferred to be below 1 fm.

Hydrodynamic models match ALICE data on pion correlations.

Abstract

Hydrodynamic simulations are used to calculate the identical pion HBT radii, as a function of the pair momentum $k_{\rm T}$. This dependence is sensitive to the magnitude of the collective radial flow in the transverse plane, and thus comparison to ALICE data enables us to derive its magnitude. By using hydro solutions with variable initial parameters we conclude that in this case fireball explosions start with a very small initial size, well below 1 ${\rm fm}$.