Phase Transitions in NJL and super-NJL models

An elementary method of determination of the character of the hot phase transition in 4d four-fermion NJL-type models is applied to non-supersymmetric and supersymmetric versions of simple NJL model. We find that in the non-susy case the transition is usually of the second order. It is weakly first order only in the region of parameters which correspond to fermion masses comparable to the cut-off. In the supersymmetric case both kinds of phase transitions are possible. For sufficiently strong coupling and sufficiently large susy-breaking scale the transition is always of the first order.