A Decade of Collectivity in Small Systems

TL;DR

This review examines the evidence of collective phenomena in small collision systems like pp and p-Pb, challenging traditional assumptions and analyzing theoretical models and their scaling with system size.

Contribution

It provides a comprehensive review of small system collectivity data, evaluating existing models and their ability to explain observed phenomena across different system sizes.

Findings

Collective signatures are observed in small systems previously thought to lack such phenomena.

Current models struggle to fully describe collectivity across all system sizes.

Data challenges some dynamical explanations of collectivity.

Abstract

Signatures of collectivity, including azimuthally anisotropic and radial flow as well as characteristic hadrochemical dependencies, have been observed since long in (ultra)relativistic nucleus-nucleus collisions. They underpin the interpretation of these collision systems in terms of QGP formation and close-to-perfect fluidity. Remarkably, however, essentially all these signatures of collectivity have been identified within the last decade in collision systems as small as pp and p-Pb, where collective phenomena had been assumed to be absent traditionally. Precursor phenomena may have been found even in ep and $e^+e^-$ collisions. This article provides a complete review of all data on small system collectivity. It reviews model simulations of these data where available. However, in the absence of a phenomenologically fully satisfactory description of collectivity across all system sizes, we focus in particular on the theoretical basis of all dynamical frameworks of collectivity invoked in heavy ion collisions, and their expected scaling with system size. Our discussion clarifies to what extent all dynamical explanations are challenged by the available data.