Gluon polarization contribution to the spin alignment of vector mesons from holography

Abstract

We investigate the behaviour of vector mesons $ρ$, $ϕ$, and $J/Ψ$ in both non-rotating and rotating thermal media using the soft-wall holographic QCD model with four flavours. By incorporating anisotropic backgrounds derived from the Einstein-Maxwell-dilaton action, we incorporate rotational effects via a $U(1)$ gauge field, and the induced polarization of gluons is described by a rotation dependent dilation field. Spectral function analysis reveals that $ρ$ and $ϕ$ mesons exhibit broad peaks at lower temperatures, indicating their presence in the medium, while these peaks disappear at higher temperatures. Rotation delays this melting process, increasing the dissociation temperature. In contrast, the $J/Ψ$ meson, owing to its heavy charm quark content, demonstrating its resilience to thermal effects. We further explore the global spin alignment of these mesons in the event plane frame. For the $ϕ$ meson, the averaged $ρ_{00}$ over the full range of azimuthal angle shows weak temperature dependence at low transverse momentum ($p_T$) but significant suppression at high $p_T$, aligning with experimental observations. Rotation enhances $ρ_{00}$ at high $p_T$, a phenomenon attributed to angular momentum transfer via spin-orbit coupling. The $J/Ψ$ meson, however, displays insensitivity to temperature and rotation up to $p_T=5$ GeV, with a very small suppression observed at higher $p_T$, likely due to its heavy quark nature. Although $ρ$ meson spin alignment is not yet experimentally measured, it exhibits behaviour qualitatively similar to the $ϕ$ meson, with thermal fluctuations dampening alignment and rotation enhancing it.