Speaker
Description
A significant global spin alignment ($\rho_{00}$) signal for $\phi$-mesons was observed by the STAR collaboration in Au+Au collisions using the data from the first phase of the Beam Energy Scan at RHIC (BES-I) [1]. Conventional physical mechanisms which contribute to $\rho_{00}$ fail to explain the observed signal; however, it may be attributable to the presence of a $\phi$-meson strong force field [2] or to non-zero $\rho_{00}$ in the helicity frame induced by the relative motion of $s\bar{s}$ pairs to the thermal background in heavy-ion collisions [3]. Recently, a quark recombination model with quark-antiquark spin correlations was developed and is able to simultaneously explain lambda polarization ($P_{\Lambda}$) and $\phi$-meson $\rho_{00}$ measurements, in addition to predicting non-zero off-diagonal spin density matrix elements [4]. Therefore, measurements of off-diagonal matrix elements of $\phi$-mesons may provide a probe of $s\bar{s}$ spin correlations. Off-diagonal spin density matrix elements could also be important when measuring Chiral Magnetic Effect (CME) observables involving vector meson decay products, as these elements represent a possible physics background [5]. Previous $\rho_{00}$ measurements in [1] use a 1D angular distribution in $\theta^*$ (polar angle of a daughter kaon in the $\phi$-meson's rest frame with respect to the orthogonal of the harmonic plane), which cannot account for possible contributions to $\rho_{00}$ from off-diagonal spin density matrix elements. In this talk, we address this by measuring $\phi$-meson $\rho_{00}$ and off-diagonal spin density matrix elements using both angular dimensions $\theta^*$ and $\beta$ of a daughter kaon in the $\phi$-meson's rest frame, where $\beta$ is the azimuthal angle within the reaction plane, measured relative to the beam axis. We will present differential measurements of $\phi$-meson global $\rho_{00}$ and off-diagonal spin density matrix elements with respect to rapidity and transverse momentum, using data from the second phase of RHIC BES (BES-II) in Au+Au collisions at $\sqrt{s_{NN}}=19.6$ GeV collected by STAR.
[1] STAR Collaboration., Nature $\textbf{614}$, 244–248 (2023)
[2] X.L. Sheng et al., Physical Review C $\textbf{108}$, 054902 (2023).
[3] X.L. Sheng et al., Physical Review D \textbf{110}, 056047 (2024).
[4] J.P. Lv et al., Physical Review D \textbf{109}, 114003 (2024).
[5] Z. Wang et al., Physical Review C \textbf{111}, 014910 (2025)
