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

TL;DR

This study uses holographic QCD to analyze how rotation and temperature affect the polarization and spin alignment of vector mesons like rho, phi, and J/Ψ in thermal media, revealing rotation's role in delaying meson melting and influencing spin states.

Contribution

It introduces a holographic model incorporating rotation and anisotropic backgrounds to study meson polarization and spin alignment in hot, rotating media, providing new insights into their thermal and rotational stability.

Findings

Rotation delays meson melting, increasing dissociation temperatures.

High transverse momentum phi mesons show significant spin alignment suppression.

J/Ψ mesons are resilient to thermal and rotational effects up to high p_T.

Abstract

We investigate the behaviour of vector mesons $\rho$, $\phi$, and $J/\Psi$ 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 $\rho$ and $\phi$ 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 $J/\Psi$ 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 $\phi$ meson, the averaged $\rho_{00}$ 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 $\rho_{00}$ at high $p_T$, a phenomenon attributed to angular momentum transfer via spin-orbit coupling. The $J/\Psi$ 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 $\rho$ meson spin alignment is not yet experimentally measured, it exhibits behaviour qualitatively similar to the $\phi$ meson, with thermal fluctuations dampening alignment and rotation enhancing it.