The smallest fluid on earth

TL;DR

This paper explores whether small systems, like proton collisions, can exhibit fluid-like behavior similar to larger quark-gluon plasmas, involving advances in understanding initial conditions and quantum chromodynamics effects.

Contribution

It provides new insights into fluid behavior in small collision systems by analyzing initial geometry fluctuations and quantum effects, expanding the understanding of fluid dynamics applicability.

Findings

Small systems show signs of fluid-like behavior.

Proton shape fluctuations influence initial geometry.

Quantum chromodynamics effects impact observable fluid signatures.

Abstract

High energy heavy ion collisions create quark gluon plasmas that behave like almost perfect fluids. Very similar features to those that led to this insight have also been observed in experimental data from collisions of small systems, involving protons or other light nuclei. We describe recent developments aimed at understanding whether, and if so how, systems that produce relatively few particles (orders of magnitude less than in typical heavy ion collisions) and are only one to a few times the size of a proton, can behave like fluids. This involves a deeper understanding of fluid dynamics and its applicability, improvements of our understanding of the initial geometry of the collisions by considering fluctuations of the proton shape, as well as advancements in the calculation of initial state effects within an effective theory of quantum chromodynamics, which can affect the observables that are used to study fluid behavior. We further address open questions and discuss future directions.