Dynamical symmetry restoration for a higher-derivative four-fermion model in an external electromagnetic field

A four-fermion model with additional higher-derivative terms is investigate d in an external electromagnetic field. The effective potential in the lead ing order of large-N expansion is calculated in external constant magnetic and electric fields. It is shown that, in contrast to the former results co ncerning the universal character of "magnetic catalysis" in dynamical symme try breaking, in the present higher-derivative model the magnetic field res tores chiral symmetry broken initially on the tree level. Numerical results describing a second-order phase transition that accompanies the symmetry r estoration at the quantum level are presented.