[Paper Review] Creating small circular, elliptical, and triangular droplets of quark-gluon plasma

C. Aidala, Y. Akiba+318 more|arXiv (Cornell University)|May 8, 2018

High-Energy Particle Collisions ResearchPhysics and Astronomy39 references108 citations

TL;DR

The paper reports elliptic and triangular flow in small-system p+Au, d+Au, and 3He+Au collisions at 200 GeV, supporting hydrodynamic QGP formation in these systems.

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.

Motivation & Objective

• Motivate investigation of collectivity in small collision systems to test QGP formation in limited volumes.
• Explore how changing the initial geometry (circular, elliptical, triangular) affects flow observables.
• Discriminate between hydrodynamic QGP formation and initial-state momentum correlation models using geometry-varied collisions.
• Quantify v2 and v3 as functions of transverse momentum to assess collective behavior across systems.

Proposed method

• Measure azimuthal distribution dN/dphi of final-state particles in p+Au, d+Au, and 3He+Au collisions at sqrt(sNN) = 200 GeV.
• Decompose azimuthal distributions into a Fourier series to extract v_n(pT) and event-plane angles psi_n.
• Characterize initial geometry with MC Glauber model to compute spatial eccentricities epsilon_n.
• Use hydrodynamic evolution models to connect initial geometry to final-state flow patterns.
• Compare experimental v2 and v3 trends with hydrodynamic predictions across the three systems.

Experimental results

Research questions

• RQ1 Can small collision systems exhibit collective flow patterns similar to large ion collisions?
• RQ2 How do initial geometric configurations (circular, elliptical, triangular) translate into measured v2 and v3 in small systems?
• RQ3 Do hydrodynamic models with QGP formation describe the observed flow better than initial-state momentum correlation scenarios?

Key findings

• Elliptic (v2) and triangular (v3) flow are observed in p+Au, d+Au, and 3He+Au collisions at 200 GeV.
• The initial geometry varies across systems, with epsilon2 and epsilon3 driven by intrinsic geometry in d+Au and 3He+Au and by fluctuations in p+Au.
• Hydrodynamic models that include short-lived QGP droplets can simultaneously describe the measured flow patterns across all three systems.
• The data provide a discriminating test between hydrodynamic QGP formation and non-QGP initial-state momentum correlation explanations.
• The combination of three distinct initial geometries and two flow patterns offers unprecedented model discrimination.