Thermodynamics of resonances and blurred particles
  Exact and approximate expressions for thermodynamic characteristics of heated matter, which consists of particles with finite mass-widths, are constructed. They are expressed in terms of Fermi/Bose distributions and spectral functions, rather than in terms of more complicated combinations between real and imaginary parts of the self-energies of different particle species. Therefore thermodynamically consistent approximate treatment of systems of particles with finite mass-widths can be performed, provided spectral functions of particle species are known. Approximation of the free resonance gas at low densities is studied. Simple ansatz for the energy dependence of the spectral function is suggested that allows to fulfill thermodynamical consistency conditions. On examples it is shown that a simple description of dense systems of interacting particle species can be constructed, provided some species can be treated in the quasiparticle approximation and others as particles with widths. The interaction affects quasiparticle contributions, whereas particles with widths can be treated as free. Example is considered of a hot gas of heavy fermions strongly interacting with light bosons, both species with zero chemical potentials. The density of blurred fermions is dramatically increased for high temperatures compared to the standard Boltzmann value. The system consists of boson quasiparticles (with effective masses) interacting with fermion -- antifermion blurs. In thermodynamical values interaction terms partially compensate each other. Thereby, in case of a very strong coupling between species thermodynamical quantities of the system, like the energy, pressure and entropy, prove to be such as for the quasi-ideal gas mixture of quasi-free fermion blurs and quasi-free bosons. 
D.N. Voskresensky