Collision geometry fluctuations and triangular flow in heavy-ion collisions

TL;DR

This paper introduces the concept of triangular flow in heavy-ion collisions, linking initial geometric fluctuations to observed azimuthal correlations, and demonstrates its significance in understanding collision dynamics.

Contribution

It defines participant triangularity and triangular flow, and shows their relation to experimental azimuthal correlation data, providing new insights into collision geometry and collective behavior.

Findings

Triangular flow correlates with initial participant triangularity.

Fourier coefficients in data match model predictions.

Triangular flow influences ridge and away-side features.

Abstract

We introduce the concepts of participant triangularity and triangular flow in heavy-ion collisions, analogous to the definitions of participant eccentricity and elliptic flow. The participant triangularity characterizes the triangular anisotropy of the initial nuclear overlap geometry and arises from event-by-event fluctuations in the participant-nucleon collision points. In studies using a multi-phase transport model (AMPT), a triangular flow signal is observed that is proportional to the participant triangularity and corresponds to a large third Fourier coefficient in two-particle azimuthal correlation functions. Using two-particle azimuthal correlations at large pseudorapidity separations measured by the PHOBOS and STAR experiments, we show that this Fourier component is also present in data. Ratios of the second and third Fourier coefficients in data exhibit similar trends as a function of centrality and transverse momentum as in AMPT calculations. These findings suggest a significant contribution of triangular flow to the ridge and broad away-side features observed in data. Triangular flow provides a new handle on the initial collision geometry and collective expansion dynamics in heavy-ion collisions.