Observation of partonic flow in proton-proton and proton-nucleus collisions

TL;DR

This paper reports the first observation of partonic flow in high-multiplicity proton-proton and proton-lead collisions at the LHC, indicating possible quark-gluon plasma formation in small collision systems.

Contribution

It demonstrates the grouping of anisotropic flow for baryons and mesons in small systems, supported by a hydrodynamic and quark coalescence model, suggesting partonic flow in these collisions.

Findings

Distinctive baryon-meson flow grouping observed

Results consistent with hydrodynamic flow and quark coalescence model

Supports possible QGP formation in small systems

Abstract

Quantum Chromodynamics predicts a phase transition from ordinary hadronic matter to the quark-gluon plasma (QGP) at high temperatures and energy densities, where quarks and gluons (partons) are not confined within hadrons. The QGP is generated in ultrarelativistic heavy-ion collisions. Anisotropic flow coefficients, quantifying the anisotropic azimuthal expansion of the produced matter, provide a unique tool to unravel QGP properties. Flow measurements in high-energy heavy-ion collisions show a distinctive grouping of anisotropic flow for baryons and mesons at intermediate transverse momentum, a feature associated with flow being imparted at the quark level, confirming the existence of the QGP. The observation of QGP-like features in relativistic proton-proton and proton-ion collisions has sparked debate about possible QGP formation in smaller collision systems, which remains unresolved. In this article, we demonstrate for the first time the distinctive grouping of anisotropic flow for baryons and mesons in high-multiplicity proton-lead and proton-proton collisions at the Large Hadron Collider (LHC). These results are described by a model that includes hydrodynamic flow followed by hadron formation via quark coalescence, replicating features observed in heavy-ion collisions. This observation is consistent with the formation of a partonic flowing system in proton-proton and proton-lead collisions at the LHC.