Holographic spin alignment of $J/ψ$ meson in magnetized plasma

Abstract

We study the mass spectra and spin alignment of vector meson $J/ψ$ in a thermal magnetized background using a generalized theoretical framework based on gauge/gravity duality. Utilizing a soft wall model for the QGP background and a massive vector field for the $J/ψ$ meson, we delve into the meson's spectral function and spin parameters $(λ_θ,\, λ_\varphi,\,λ_{θ\varphi})$ for different cases, assessing their response to variations in magnetic field strength, momentum, and temperature. We initially examine scenarios where a meson's momentum aligns parallel to the magnetic field in helicity frame. Our results reveal a magnetic field-induced positive $λ_θ^\text{H}$ for low meson momentum, transitioning to negative with increased momentum. As a comparison, we also study the case of momentum perpendicular to the magnetic field and find the direction of magnetic field does not affect the qualitative behavior for the $eB$-dependence of $λ_θ^\text{H}$. Moreover, we apply our model to real heavy-ion collisions for three different spin quantization directions. Further comparisons with experimental data show qualitative agreement for spin parameters $λ_θ$ and $λ_\varphi$ in the helicity and Collins-Soper frames.