# Observation of partonic flow in proton—proton and proton—nucleus collisions

## TL;DR

Scientists observed quark-gluon plasma-like behavior in small collision systems, suggesting partonic flow occurs even in proton-proton and proton-nucleus collisions.

## Contribution

First observation of quark-level anisotropic flow grouping in high-multiplicity proton–lead and proton–proton collisions.

## Key findings

- Distinctive grouping of anisotropic flow for baryons and mesons observed in high-multiplicity proton–lead and proton–proton collisions.
- Results are consistent with hydrodynamic flow followed by quark coalescence, indicating partonic flow in small systems.

## Abstract

Quantum Chromodynamics predicts a phase transition from hadronic matter to quark–gluon plasma (QGP) at high temperatures and energy densities, where quarks and gluons (partons) are no longer confined within hadrons. The QGP forms in ultrarelativistic heavy-ion collisions. Anisotropic flow coefficients, quantifying the azimuthal expansion of produced matter, probe 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 imparted at the quark level, confirming QGP existence. The observation of QGP-like features in proton–proton and proton–ion collisions has sparked debate about QGP formation in smaller systems. For the first time, we demonstrate 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 including hydrodynamic flow followed by hadron formation via quark coalescence, consistent with the formation of partonic flowing systems in these collisions.

The authors from the ALICE collaboration identify multiple species of mesons and baryons and measure the anisotropic flow with non-flow removal techniques in pp and p-Pb collisions at the LHC, identifying the hallmark of quark flow associated with an expanding quark-gluon plasma.