Fusion rates in nuclear plasmas
  Energy and momentum of the elementary excitations become independent variables in medium: energy and momentum statistical distributions are not identical. The momentum distribution and not the energy distribution is relevant for barrier penetration. The deviations of the momentum distribution from the Maxwell-Boltzmann energy distribution can be expressed in terms of the imaginary part of the self-energy of the quasi-particle. It is possible to obtain an effective Tsallis' distribution for the kinetic energy. These effects are different from static or dynamical screening and can have important consequences for reaction rates in stars. 
M. Coraddu, M. Lissia, G. Mezzorani, and P. Quarati