Schwinger–Dyson equation and NJL approximation in massive gauge theory with fermions

We consider massive SU(N)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 αc=(1+γ)×π3×12C2(F), where 0<γ<10<γ<1, and γγ 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 αα the Dyson–Schwinger equation is approximated well by the gap equation of the effective Nambu–Jona-Lasinio model with the value of cutoff around gauge boson mass MM and the effective four-fermion coupling constant 4παM2×2C2(F)N. The dynamical fermion mass mm may be essentially smaller than MM.