Oblique photons, plasmons, and current-plasmons in relativistic plasmas: manifestly covariant theory and topological implications*

We present a covariant theory for electromagnetic excitations in relativistic plasmas [1], revealing novel properties of photons and plasmons. Unlike the transverse photons in vacuum, both photons and plasmons in relativistic plasmas are oblique electromagnetic waves. Applying Wigner's little group method for elementary particle classification, we show that covariant compressibility distinguishes these modes. Our theory predicts a third oblique electromagnetic eigenmode: the current-plasmon. It also enables the study of photon topology in plasmas. While plasmas eliminate the vacuum photon's Dirac point by imparting an effective mass, they create a tilted Dirac-Weyl point by reviving the longitudinal photon, which only exists virtually in vacuum. These results open new avenues for investigating topological properties of relativistic plasmas. [1] arXiv:2404.05636.