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

Abstract

High-energy heavy-ion collisions contain enormous angular momentum, $|\vec{J}|$, which is $\mathcal{O}(10^3-10^6\hslash )$ 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 $\vec{J}$.

Comments: 7 pages, 7 figures