Thermal Dilepton Polarization under Rotation or Magnetic Field in Heavy-ion Collisions

Abstract

Dilepton (Virtual photon) polarization is characterized by anisotropic coefficients $λ_θ$, $λ_ϕ$, and $λ_{θϕ}$, which are expected to be influenced by vorticity and magnetic fields. This work investigates thermal dilepton production in a quark-gluon plasma via the quark-antiquark annihilation process $q\bar{q} \to γ^* \to l^+l^-$. Virtual photon polarization can be induced by both the spin polarization of quarks and the anisotropy of their momentum distribution in the medium. By employing the modified quark propagator under an external field, we derive the electromagnetic spectral function in a hot medium. Based on the spin-projection decomposition of the spectral function, the spin density matrix elements of the virtual photon and the anisotropy coefficients for the emitted dileptons are determined. Due to the distinct effects of vorticity and magnetic fields on the quark propagator, the resulting invariant mass spectra of dilepton polarization exhibit characteristic differences. Furthermore, our study reveals the response of dilepton polarization signals to external fields of varying strengths, suggesting dilepton polarization as a complementary and sensitive probe for both vorticity and magnetic fields in relativistic heavy-ion collisions.