Spin alignment of vector mesons in a plasma at finite density

Abstract

We study the spectral functions and spin alignment of $J/ψ$ and $ϕ$ mesons at finite density in a soft-wall holographic model. The quark-gluon plasma background is described by a charged black hole geometry, and the vector mesons are treated as probe bulk vector fields. The present analysis extends the zero-density study of Phys. Rev. D 110 (2024) 056047 (Ref.~\cite{XLS2024}) to finite density; in the zero-density limit our results reproduce the corresponding findings. We derive the relation between the production rates in different spin channels and the corresponding in-medium spectral functions, and examine their dependence on the chemical potential, meson momentum, and temperature. We analyze the spin alignment induced by the motion of the vector meson relative to the thermal bath. At $T=0.15~\mathrm{GeV}$, the $J/ψ$ spectral function exhibits a clear resonance peak, indicating that the $c\bar{c}$ pair can still form a quasistable bound state. As the chemical potential increases, this peak becomes lower and broader, signaling enhanced dissociation in the medium. For the $ϕ$ meson at the same temperature, no pronounced peak is observed, indicating substantial melting; its spectral function thus better characterizes the distribution of unstable $s\bar{s}$ pairs in the thermal environment. At $T=0.15~\mathrm{GeV}$, the helicity-frame spin alignment parameter $ρ_{00}$ shows a positive deviation from $1/3$ for the $J/ψ$ and a negative one for the $ϕ$ meson. A finite chemical potential alters these deviations, reducing the magnitude for the $J/ψ$ while increasing it for the $ϕ$ meson. The calculations are performed within a bottom-up soft-wall model; the results should be read as model-based estimates rather than model-independent predictions.