The smallest fluid on earth

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.

Early time behavior of spatial and momentum anisotropies in a kinetic approach to nuclear collisions

“…will yield an odd function of p 1 in the integrand of the collision kernel: this vanishes in the integration over p 1 , thus giving no contribution to v n (t). 6 We mentioned above that the terms from expansion (18) of the form (1/E)∂ k t C[f ] 0 involve contributions of higher order in the inverse Knudsen number Kn −1 . Careful accounting based on iterating Eq.…”

“…The use of fluid dynamics is however more questionable in such systems with few degrees of freedom [6]. Moreover, since the overall system lifetime is shorter, the preand post-equilibrium stages become comparatively more important in the evolution.…”

Development of transverse flow at small and large opacities in conformal kinetic theory

“…Based on the tremendous success in quantfying properties of the QGP produced in heavy-ion collisions [6][7][8][9], different groups have performed hydrodynamic calculations for small systems [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24], which also provide a reasonable description of the experimentally observed collective flow in proton-nucleus and proton-proton collisions [25][26][27]. However, in constrast to nucleus-nucleus collisions, such calculations are subject to much larger uncertainties, where in addition to the poorly constrained initial state geometry [19,28,29], one may question the theoretical justification for employing a hydrodynamic description for a system, which features a very short lifetime and consists of very few degrees of freedom.…”

(3+1)-D viscous hydrodynamics CLVisc at finite net baryon density: identified particle spectra, anisotropic flows and flow fluctuations across BES energies

“…The framework of relativistic viscous hydrodynamics has provided a powerful tool to study the bulk observables, such as high-order anisotropic flows, flow fluctuations [35,36], flow correlations [37][38][39][40][41][42][43], and even collective flows in small collision systems [44][45][46][47][48][49][50][51][52][53], at RHIC top energies and at the LHC energies, where the net baryon density is nearly vanishing. However, in heavy-ion collisions at the BES low energies, the assumption of vanishing baryon chemical potential is not valid any more.…”