Spin alignment of K* induced by strange-baryon density inhomogeneity

The difference between the spin alignments of K* and those of φ at low collision energies is a puzzle raised by the recent experiments. Unlike the φ meson, K*, carrying a unit strange charge, should react to strange chemical potential μS. In this paper, I shall first show that μS is not small in a baryon-rich medium for keeping strange neutrality, and then derive the spin alignment induced by the gradient of μS using linear response theory, with the transport coefficients expressed, without any approximation, in terms of the K*'s in-medium spectral properties by employing the Ward-Takahashi identity. It turns out that such an effect applies mainly to the particles whose longitudinal and transverse modes diverge, and induces only the local spin alignment in a static medium. The magnitudes of these coefficients will be further estimated using the quasiparticle approximation.