New critical properties of the Nambu-Jona-Lasinio model with nonzero chemical potential

Abstract
It is shown for the first time that the massive-fermion state in the Nambu-Jona-Lasinio model with nonzero chemical potential μ is described by two different phases, the transition between which is second-order. It is proved that both first-and second-order phase transitions, depending on the values of the model parameters, restore the chiral symmetry of the model. Two tricritical points exist in the (μ, M) phase diagram (M is the dynamical fermion mass at μ=0).