Effect of flow fluctuations and nonflow on elliptic flow methods

TL;DR

This paper analyzes how flow fluctuations and nonflow effects influence elliptic flow measurements, providing equations to unify different methods and explain the observed spread in results.

Contribution

It introduces a framework to relate various elliptic flow estimates, accounting for fluctuations and nonflow, and shows how to convert between different flow measurements.

Findings

Different flow measurement methods yield estimates between two-particle and multiparticle methods.

Flow fluctuations and nonflow effects cannot be fully separated without additional assumptions.

Provided equations allow convergence of measurements to a unique mean elliptic flow value.

Abstract

We discuss how the different estimates of elliptic flow are influenced by flow fluctuations and nonflow effects. It is explained why the event-plane method yields estimates between the two-particle correlation methods and the multiparticle correlation methods. It is argued that nonflow effects and fluctuations cannot be disentangled without other assumptions. However, we provide equations where, with reasonable assumptions about fluctuations and nonflow, all measured values of elliptic flow converge to a unique mean v_{2,PP} elliptic flow in the participant plane and, with a Gaussian assumption on eccentricity fluctuations, can be converted to the mean v_{2,RP} in the reaction plane. Thus, the 20% spread in observed elliptic flow measurements from different analysis methods is no longer mysterious.