Collectivity in Small QCD Systems

TL;DR

This paper reviews recent experimental evidence of collective behavior in small QCD systems at high energies, highlighting measurements of azimuthal anisotropy and their implications for understanding the underlying physics.

Contribution

It presents new PHENIX measurements of anisotropy in d+Au and $^3$He+Au collisions, aiding in distinguishing between hydrodynamic and CGC models.

Findings

Observation of long-range angular correlations in small systems.

Comparison of experimental data with hydrodynamic and CGC models.

Enhanced understanding of collectivity origins in small QCD systems.

Abstract

Features of collectivity observed in high energy A+A collision systems at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC) imply a strongly coupled quark gluon plasma (QGP) that flows. One defining feature of collectivity is long-range angular correlations which are characterized experimentally by measuring the azimuthal anisotropy of particles well separated in rapidity. Evidence indicating long-range angular correlations include the appearance of the so-called near-side "ridge" in correlation functions and $p_T$ dependent flow components ($v_N$). These features have been well measured at RHIC for Au+Au collisions at $\sqrt{s_{NN}} $= 200 GeV and at the LHC for Pb+Pb collisions at $\sqrt{s_{NN}}$ = 2.76 TeV. However, evidence of collectivity has recently been observed at the LHC in p+p high multiplicity collisions at $\sqrt{s_{NN}}$ = 7.0 TeV and p+Pb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV and then at RHIC in d+Au and $3^He$+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV. In this talk, we present the recent PHENIX results for d+Au and $^3He$+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV. Precise measurements of anisotropy help distinguish between theoretical models based on relativistic hydrodynamics or Color Glass Condensate (CGC). Comparisons of these measurements to various theoretical models are shown.