Abstract:
The square of the four-momentum of a photon in vacuum is zero. However, in an unmagnetized plasma, it is equal to the square of the plasma frequency. Further, the electron-photon coupling vertex is modified in a plasma to include the effect of the plasma medium. I calculate the cross sections of three processes in a plasma---Compton scattering and electron-positron pair annihilation and production. At high plasma densities, the cross sections are found to change significantly. Such high plasma densities exist in several astrophysical sources
