Nambu-Goldstone mechanism at finite temperature in the imaginary-time   and real-time formalism
  In the imaginay-time formalism of thermal field theory, and also in the real-time formalism but by means of some redefined physical propagators for scalar bound states by diagonalization of four-point function matrices, we reexamine the Nambu-Goldstone mechanism of electroweak symmetry breaking in a one-generation fermion condensate scheme, based on the Schwinger-Dyson equation in the fermion bubble diagram approximation, and compare the obtained results. We have reached the conclusion that in both the formalisms, the Goldstone theorem of spontaneous electroweak symmetry breaking is rigorously true for the case of mass-degenerate two flavors of fermions and only approximately valid at low energy scales for the mass-nondegenerate case, in spite of existence of some difference between the two formalisms in the imaginary parts of the denominators of the propagators for scalar bound states. When the two flavors of fermions have unequal nonzero masses, the induced possible fluctuation effect for the Higgs particle is negligible if the momentum cut-off in the zero temperature loops is large enough. All the results show physical equivalence of the two formalisms in the present discussed problems. 
Bang-Rong Zhou (Graduate School, Academia Sinica, Beijing)