| Summary: | Abstract Based on the Nambu–Jona–Lasinio (NJL) model, we develop a framework for calculating the spin alignment of vector mesons and applied it to study $$\phi $$ ϕ mesons in a magnetic field. We calculate mass spectra for $$\phi $$ ϕ mesons and observe mass splitting between the longitudinally polarized state and transversely polarized states. The $$\phi $$ ϕ meson in a thermal equilibrium system is preferred to occupy the state with spin $$\lambda =0$$ λ = 0 than those with spin $$\lambda =\pm 1$$ λ = ± 1 , because the former state has a smaller energy. As a consequence, we conclude that the spin alignment will be larger than 1/3 if one measures along the direction of the magnetic field, which is qualitatively consistent with the recent STAR data. Around the critical temperature $$T_{C}=150~\hbox {MeV}$$ T C = 150 MeV , the positive deviation from 1/3 is proportional to the square of the magnetic field strength, which agrees with the result from the non-relativistic coalescence model. Including the anomalous magnetic moments for quarks will modify the dynamical masses of quarks and thus affect the mass spectra and spin alignment of $$\phi $$ ϕ mesons. The discussion of spin alignment in the NJL model may help us better understand the formation of hadron’s spin structure during the chiral phase transition.
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