Schwinger - Dyson equation and NJL approximation in massive gauge theory   with fermions
  We consider massive $SU(N)$ gauge theory with fermions. Gauge bosons become massive due to the interaction with the scalar field, whose vacuum average provides the spontaneous breakdown of gauge symmetry. We investigate Dyson - Schwinger equation for the fermion propagator written in ladder approximation and in Landau gauge. Our analysis demonstrates that the chiral symmetry breaking in the considered theory is the strong coupling phenomenon. There are the indications that there appears the second order phase transition between chirally broken and symmetric phases of the theory at the value of coupling constant $\alpha_c = (1+\gamma)\times \frac{\pi}{3}\times \frac{1}{2 C_2(F)}$, where $0<\gamma<1$, and $\gamma$ depends on the scale, at which the fluctuations of the scalar field destroy the gauge boson mass. In the broken phase near the critical value of $\alpha$ the Dyson - Schwinger equation is approximated well by the gap equation of the effective Nambu - Joina - Lasinio model with the value of cutoff around gauge boson mass $M$ and the effective four - fermion coupling constant $\frac{4 \pi \alpha}{M^2}\times \frac{2C_2(F)}{N}$. The dynamical fermion mass $m$ may be essentially smaller than $M$. 
M.A. Zubkov
hep-ph