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

Abstract

We investigate the behavior of vector mesons ρ, ϕ, and J/Ψ in both nonrotating and rotating thermal media using the soft-wall holographic QCD model with four flavors. 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, demonstrates 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 ρ00 over the full range of azimuthal angles shows weak temperature dependence at low transverse momentum (pT) but significant suppression at high pT, aligning with experimental observations. Rotation enhances ρ00 at high pT, a phenomenon attributed to angular momentum transfer via spin-orbit coupling. The J/Ψ meson, however, displays insensitivity to temperature and rotation up to pT=5 GeV, with a very small suppression observed at higher pT, likely due to its heavy quark nature. Although ρ meson spin alignment is not yet experimentally measured, it exhibits behavior qualitatively similar to the ϕ meson, with thermal fluctuations dampening alignment and rotation enhancing it. Published by the American Physical Society 2025