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

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.