Collectivity in large and small systems formed in ultrarelativistic collisions

TL;DR

This paper reviews the development of collective flow phenomena in ultrarelativistic collisions, highlighting new observables and their role in probing the properties of the quark-gluon plasma and initial state conditions.

Contribution

It provides a comprehensive tutorial on modern flow measurements and the diverse aspects of collectivity in quantum chromodynamics, expanding beyond traditional flow harmonics.

Findings

Enhanced understanding of higher flow harmonics and multiparticle cumulants.

Introduction of new observables like flow decorrelation and symmetric cumulants.

Demonstration of collectivity as an emergent phenomenon in QCD.

Abstract

Collective flow of the final-state hadrons observed in ultrarelativistic heavy-ion collisions or even in smaller systems formed in high-multiplicity pp and p/d/$^3$He-nucleus collisions is one of the most important diagnostic tools to probe the initial state of the system and to shed light on the properties of the short-lived, strongly-interacting many-body state formed in these collisions. Limited, in the initial years, to the study of mainly the directed and elliptic flows -- the first two Fourier harmonics of the single-particle azimuthal distribution -- this field has evolved in recent years into a much richer area of activity. This includes not only higher Fourier harmonics and multiparticle cumulants, but also a variety of other related observables, such as the ridge seen in two-particle correlations, flow decorrelation, symmetric cumulants and event-plane correlators which measure correlations between the magnitudes or phases of the complex flows in different harmonics, coefficients that measure the nonlinear hydrodynamic response, statistical properties, such as the non-Gaussianity of the flow fluctuations, etc. We present a Tutorial Review of the modern flow picture and the various aspects of the collectivity -- an emergent phenomenon in quantum chromodynamics.