Going against the flow: Revealing the QCD degrees of freedom in hadronic collisions

TL;DR

This paper compares hydrodynamic and microscopic models in high-energy collisions, revealing that microscopic models can predict negative elliptic flow in small systems, offering new insights into strong interaction degrees of freedom.

Contribution

It demonstrates that microscopic models can produce negative elliptic flow in small collision systems, challenging hydrodynamic expectations and providing a new way to probe QCD degrees of freedom.

Findings

Microscopic models predict negative elliptic flow in small systems.

Negative elliptic flow linked to string interactions and finite interaction range.

Experimental measurement of flow sign can reveal underlying QCD dynamics.

Abstract

In collisions between heavy nuclei, such as those at the Large Hadron Collider (LHC) at CERN, hydrodynamic models have successfully related measured azimuthal momentum anisotropies to the transverse shape of the collision region. For an elliptically shaped interaction area, the hydrodynamic pressure gradient is greater along the minor axis, resulting in increased particle momentum in that direction - a phenomenon known as positive elliptic flow. In this paper, we demonstrate that in smaller systems, such as proton-proton and peripheral ion-ion collisions, microscopic models for final state interactions, can produce anisotropies where the elliptic flow is negative - that is, the momentum is largest along the major axis, contrary to hydrodynamic predictions. We present results from two distinct microscopic models: one based on repulsion between string-like fields and another based on effective kinetic theory. Negative elliptic flow is a solid prediction of the string interaction model while in the model based on kinetic theory it is linked to a finite interaction range. Consequently, an experimental determination of the sign of elliptic flow, will provide novel insights into the degrees of freedom governing strong nuclear interactions in high-energy collisions and the way in which they interact.