Refractive Index of Light in the Quark-Gluon Plasma with the   Hard-Thermal-Loop Perturbation Theory
  The electric permittivity and magnetic permeability for the quark-gluon plasma (QGP) is calculated within the hard-thermal-loop (HTL) perturbation theory. The refractive indices in the magnetizable and non-magnetizable plasmas are calculated. In a magnetizable plasma, there is a frequency pole $\omega_{mp}$ in the magnetic permeability and the refractive index. The refractive index becomes negative in the range $\omega\in[k,\omega_{mp}]$, where $k$ is the wave number, but no propagating modes are found. In a non-magnetizable plasma, the magnetic permeability and the refractive index are always positive. This marks the main distinction of a weakly coupled plasma from a strongly coupled one, where the negative refraction is shown to exist in a holographic theory. 
Juan Liu, M. J. Luo, Qun Wang and Hao-jie Xu
hep-ph