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

Abstract

High-energy heavy-ion collisions contain enormous angular momentum, $|\stackrel{P\vec}{J}|$, which is $O({10}^{3}--{10}^{6}\ensuremath{\hbar})$ 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 $\stackrel{P\vec}{J}$ is transferred to the overlapping collision region, which is indispensable for measuring observables such as vorticity-driven hadron spin alignment with $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{J}$. Experiments estimate the orientation of $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{J}$ of the participant nucleons within the collision overlap region, ${\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{J}}_{\mathrm{part}}$, by using that of the forward- and backward-going spectating nucleons ${\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{J}}_{\mathrm{spec}}$. Using two models, we study the decorrelation between ${\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{J}}_{\mathrm{part}}$ and ${\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{J}}_{\mathrm{spec}}$, 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 ${\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{J}}_{\mathrm{part}}$. $\sqrt{{s}_{\mathrm{NN}}}$-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 $\stackrel{P\vec}{J}$.