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Gauge Independent Phase Structure of Gauged Nambu-Jona-Lasinio and Yukawa Models

We investigate the critical behavior of the gauged NJL model (QED plus 4-fermion interaction) and the gauged Yukawa model by use of the inversion method. By calculating the gauge-invariant chiral condensate in the inversion method to the lowest order, we derive the critical line which separates the spontaneous chiral-symmetry breaking phase from the chiral symmetric one. The critical exponent for the chiral order parameter associated with the second order chiral phase transition is shown to take the mean-field value together with possible logarithmic correction to the mean-field prediction. All the above results are gauge-parameter independent and are compared with the previous results obtained from the Schwinger-Dyson equation for the fermion propagator.