Creating small circular, elliptical, and triangular droplets of quark-gluon plasma

TL;DR

This paper reports the observation of collective flow patterns in small-system collisions, providing evidence for quark-gluon plasma formation in these systems and offering new insights into its properties.

Contribution

It demonstrates the presence of elliptic and triangular flow in small collision systems, supporting the formation of a short-lived QGP droplet and enabling better model discrimination.

Findings

Observation of elliptic and triangular flow in small systems

Hydrodynamical models with QGP formation describe the data

Distinct initial geometries help discriminate models

Abstract

The experimental study of the collisions of heavy nuclei at relativistic energies has established the properties of the quark-gluon plasma (QGP), a state of hot, dense nuclear matter in which quarks and gluons are not bound into hadrons. In this state, matter behaves as a nearly inviscid fluid that efficiently translates initial spatial anisotropies into correlated momentum anisotropies among the produced particles, producing a common velocity field pattern known as collective flow. In recent years, comparable momentum anisotropies have been measured in small-system proton-proton ($p$$+$$p$) and proton-nucleus ($p$$+$$A$) collisions, despite expectations that the volume and lifetime of the medium produced would be too small to form a QGP. Here, we report on the observation of elliptic and triangular flow patterns of charged particles produced in proton-gold ($p$$+$Au), deuteron-gold ($d$$+$Au), and helium-gold ($^3$He$+$Au) collisions at a nucleon-nucleon center-of-mass energy $\sqrt{s_{_{NN}}}$~=~200 GeV. The unique combination of three distinct initial geometries and two flow patterns provides unprecedented model discrimination. Hydrodynamical models, which include the formation of a short-lived QGP droplet, provide a simultaneous description of these measurements.