Effectiveness of nonflow suppression using multi-particle correlators

TL;DR

This paper critically examines the effectiveness of multi-particle correlators in estimating flow harmonics, revealing limitations especially in small systems and comparing with alternative methods.

Contribution

It demonstrates through toy models that multi-particle correlators can deviate from true flow values and provides analytic explanations for these deviations.

Findings

Multi-particle correlators can produce estimates further from true flow in small systems.

Toy models show non-flow effects cause significant deviations in flow estimates.

Comparison with maximum-likelihood estimation highlights limitations of traditional correlators.

Abstract

As flow estimators, multi-particle correlators, particularly the higher-order ones, are generally regarded as effective tools for suppressing non-flow contributions. In this work, however, using two well-known toy models that simulate non-flow effects, we demonstrate that multi-particle correlators can, especially in small systems, yield estimates that deviate even further from the underlying flow harmonics than those obtained from other conventional approaches. The two toy models considered here are designed to mimic non-flow effects arising from particle decay and global momentum conservation, such that the {\it apparent} harmonic coefficients become significantly different from the {\it input} values. We provide an analytic explanation for the observed behavior of flow estimates based on multi-particle correlators. Specifically, in the toy model mimicking particle decay, we elucidate the oscillations observed in $v_2\{2\}$ and $v_2\{4\}$. For the other toy model simulating momentum conservation, we show that multi-particle cumulants introduce a deformation in the collective flow that is unique to multi-particle correlators. Additionally, we compare these results with those obtained using the maximum-likelihood estimation method, a recently proposed flow estimator that serves as a viable alternative to traditional techniques.