Thermal properties of light mesons from holography

The thermal properties of light mesons, including the temperature dependence of their masses (both screening and pole masses) and thermal widths, are studied in a two-flavor ($N_f=2$) soft-wall AdS/QCD model. By solving the spatial correlation functions, we extract the screening masses ($m_{\rm{scr}}$) from their poles. The screening masses of pseudo-scalar ($\pi$) and axial-vector ($a_1$) mesons increase almost monotonously with the increase of temperature. The screening masses of scalar ($\sigma$) and vector ($\rho$) mesons decrease at low temperature and increase at high temperature. The pole masses ($m_{\rm{pole}}$) and the thermal widths ($\Gamma$) are extracted from the temporal correlation functions and the corresponding spectral functions. The results indicate that the pole masses have local minima at low temperature and increase at high temperature. The thermal widths increase rapidly above the chiral crossover temperature $T_{cp}$, indicating the dissociations of mesons at high temperature. Furthermore, the degeneration of the chiral partners ($\pi$ and $\sigma$, $\rho$ and $a_1$) above $T_{cp}$ is observed from the screening and pole masses, revealing the chiral symmetry restoration at the hadronic spectrum level. Finally, we numerically verify that the spectral functions in the temporal regime are strongly related to the quasi-normal modes with complex frequencies $\omega_0=m_{\rm{pole}}-i \Gamma/2$.