Forward-backward multiplicity fluctuation and longitudinal harmonics in high-energy nuclear collisions

TL;DR

This paper introduces a new analysis method for studying forward-backward multiplicity fluctuations in high-energy nuclear collisions, revealing the role of longitudinal harmonics and charge-dependent correlations in particle production.

Contribution

The paper develops a novel decomposition method to separate centrality effects from dynamical fluctuations, applied to models to identify long-range correlations and charge-dependent structures.

Findings

Long-range FB correlations are described by a few longitudinal harmonics.

Higher-order harmonics are damped in AMPT compared to HIJING.

Charge-dependent short-range structures are observed in pseudorapidity correlations.

Abstract

An analysis method is proposed to study the forward-backward (FB) multiplicity fluctuation in high-energy nuclear collisions, built on the earlier work of Bzdak and Teaney. The method allows the decomposition of the centrality dependence of average multiplicity from the dynamical event-by-event (EbyE) fluctuation of multiplicity in pseudorapidity. Application of the method to AMPT and HIJING models shows that the long-range component of the FB correlation is captured by a few longitudinal harmonics, with the first component driven by the asymmetry in the number of participating nucleons in the two colliding nuclei. The higher-order longitudinal harmonics are found to be strongly damped in AMPT compare to HIJING, due to weaker short-range correlations as well as the final-state effects present in the AMPT model. Two-particle pseudorapidity correlation reveals interesting charge-dependent short-range structures that are absent in HIJING model. The proposed method opens an avenue to elucidate the particle production mechanism and early time dynamics in heavy-ion collisions. Future analysis directions and prospects of using the pseudorapidity correlation function to understand the centrality bias in $p$+$p$, $p$+A and A+A collisions are discussed.