Understanding anisotropy generated by fluctuations in heavy-ion collisions

TL;DR

This paper presents an analytical model for initial fluctuations in heavy-ion collisions, linking geometric anisotropies to flow phenomena and validating results against Monte Carlo simulations.

Contribution

It introduces a simple analytical model for initial fluctuations based on independent flux tubes, providing insights into anisotropy moments and correlations.

Findings

Analytical calculations match Monte Carlo simulations.

Model explains the origin of flow anisotropies.

Deviations in data could reveal additional fluctuation sources.

Abstract

Event-by-event fluctuations are central to the current understanding of ultrarelativistic heavy-ion collisions. In particular, fluctuations in the geometry of the early-time collision system are responsible for new phenomena such as triangular flow, which have solved important puzzles in existing data. We propose a simple model where initial fluctuations stem from independent flux tubes randomly distributed in the transverse plane. We calculate analytically the moments of the initial anisotropies (dipole asymmetry, eccentricity, triangularity), which are the sources of anisotropic flow, and their mutual correlations. Our analytic results are in good agreement with calculations from commonly-used Monte-Carlo codes, providing a simple understanding of the fluctuations contained in these models. Any deviation from these results in future experimental data would thus indicate the presence of non-trivial correlations between the initial flux tubes and/or extra sources of fluctuations that are not present in current models.