Measurement of global spin alignment of vector mesons at RHIC

TL;DR

This study measures the spin alignment of various vector mesons in heavy-ion collisions at RHIC, revealing a non-zero alignment for charged $K^*$ mesons and highlighting unexpected differences that challenge current theoretical understanding.

Contribution

First measurement of spin alignment for $K^{* ext{±}}$ mesons in heavy-ion collisions, showing unexpected differences compared to neutral $K^{*0}$ and no significant system size dependence.

Findings

First non-zero spin alignment observed for $K^{* ext{±}}$.

$K^{* ext{±}}$ $ ho_{00}$ larger than $K^{*0}$ by 3.9$\sigma$.

No significant system size dependence in $K^{*0}$ $ ho_{00}$.

Abstract

We report the measurements of spin alignment ($\rho_{00}$) for $K^{*0}$, $\overline{K^{*0}}$, $K^{*+}$, and $K^{*-}$ vector mesons in RHIC isobar collisions (Zr+Zr and Ru+Ru) at $\sqrt{s_{\mathrm {NN}}}$ = 200 GeV. We observe the first non-zero spin alignment for $K^{*\pm}$ in heavy-ion collisions. The $K^{*\pm}$ $\rho_{00}$ is about 3.9$\sigma$ larger than that of $K^{*0}$. The observed difference and the ordering between $K^{*\pm}$ and $K^{*0}$ are surprising, and require further inputs from theory. When comparing between the isobar and Au+Au collisions, no significant system size dependence in $K^{*0}$ $\rho_{00}$ is observed within uncertainties.