Going against the flow: Revealing the QCD degrees of freedom in hadronic collisions
Year: 2026
Type: article
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.
Source: Physics Letters B
Institutions: Lund University, Universidade de Santiago de Compostela
Language: English
Subfield: Nuclear and High Energy Physics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: diamond
Awards: G1086978037, G1639177200, G1840991089, G2262748287, G3019630846, G3186420570, G3480869486, G3681454997, Development of Event Generators for Heavy Ion Collisions, G4042783231, G4241677620, G425668148, G434337789, G451917667, G5967599077, G6033112984, Rare quark fragmentation for future discoveries, Problems in Quantum Chromo Dynamical Field Theory and Beyond, G7016853551, G7084143925, G712881263, G7567520714, G7974404656, G8037394432, CHARM^2: Measuring charm-(anti)charm correlations in heavy-ion collisions
OpenAlex ID: W7126084386