Decorrelation of participant and spectator angular momenta in heavy-ion collisions

TL;DR

This paper investigates how the angular momentum orientations of participant nucleons and spectator nucleons in heavy-ion collisions decorrelate due to conservation laws and fluctuations, affecting the interpretation of experimental observables.

Contribution

It introduces a study of the decorrelation between participant and spectator angular momenta using models, highlighting energy-dependent effects crucial for experimental analysis.

Findings

Decorrelation between participant and spectator angular momenta varies with collision energy.

Energy-dependent decorrelation impacts the measurement of vorticity-driven phenomena.

Decorrelations are significant enough to require correction in experimental interpretations.

Abstract

High-energy heavy-ion collisions contain enormous angular momentum, $|\vec{J}|$, which is $\mathcal{O}(10^3-10^6\hbar)$ in the range of collision energy, $\sqrt{s_\mathrm{NN}}$, spanned experimentally by the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC). A fraction of $\vec{J}$ is transferred to the overlapping collision region, which is indispensable for measuring observables such as vorticity-driven hadron spin alignment with $\hat{J}$. Experiments estimate the orientation of $\hat{J}$ of the participant nucleons within the collision overlap region, $\hat{J}_\mathrm{part}$, by using that of the forward- and backward-going spectating nucleons $\hat{J}_\mathrm{spec}$. Using two models, we study the decorrelation between $\hat{J}_\mathrm{part}$ and $\hat{J}_\mathrm{spec}$, driven both by angular-momentum conservation and event-by-event fluctuations, as well as by the decorrelation between the orientation of the elliptic overlap region and the $\hat{J}_\mathrm{part}$. $\sqrt{s_\mathrm{NN}}$-dependent decorrelation is observed in both of these cases and is large enough to be an important corrective factor used when experimentally observing phenomena driven by $\vec{J}$.