Non-Zero Magnetic Screening Mass in QED and QCD at One Loop Level in Non-Equilibrium

Using the Schwinger-Keldysh closed time path integral formalism we show that the magnetic screening mass in QED and QCD at one loop level is non-zero as long as the single particle distribution function f(\vec{k}) is non-isotropic, {i.e.} it depends on the direction of the momentum. For isotropic distribution functions such as those corresponding to thermal equilibrium the magnetic screening mass at one loop level is found to be zero which is consistent with finite temperature field theory. The non-zero magnetic screening mass in non-isotropic non-equlibrium situations has fundamental importance in that it acts as a natural cut-off to remove infrared divergences in the magnetic sector. Thus it allows one to avoid infrared problems which previously made it difficult to use a transport theory approach using perturbative QCD or QED scattering kernels to study the thermalization of a QED or QCD plasma.