Orateur
Description
In non-central heavy-ion collisions, a large orbital angular momentum is produced. A part of the orbital angular momentum can polarize the quarks and anti-quarks, hence the vector mesons, inside the medium. Recently, STAR measured the global spin alignment of $\phi$(1020) and $K^{*}$(892) mesons in Au+Au collisions from the RHIC Beam Energy Scan I (BES I) program [1]. The global spin alignment, quantified by the $\mathrm{00}^{th}$ coefficient of the spin density matrix, $\rho_{00}$, is measured by a fit to the acceptance and efficiency corrected $\phi$ meson yield versus polar angle ($\theta^{*}$) between the daughter kaon in the parent’s rest frame and the orbital angular momentum direction. In this talk, we present an alternative approach to extract $\rho_{00}$ by utilizing the $\langle \mathrm{cos}^2 \theta^{*} \rangle$ as a function of pair-invariant mass instead of analyzing the $\phi$ meson yields in $\mathrm{cos} \theta^{*}$ bins. We use a data-driven method to correct for acceptance and efficiency. We report new analysis from this method and discuss physics implications.
[1] M. Abdallah et al. (STAR Collaboration), Nature 614, 244–248 (2022).
