Domain Wall in the Linear Sigma Model

We discuss the role of the axial U(1)_A symmetry in the chiral phase transition using the U(N_f)_R×U(N_f)_L linear sigma model with two massless quark flavours. We expect that above a certain temperature, the axial U(1)_A symmetry will be effectively restored as well as SU(N_f)_R×SU(N_f)_L. Then we can construct a string-like static solution of the η string and a kink-like classical solution of the domain wall during the chiral phase transition. We give out the possible signals for detecting the domain wall in ultrarelativistic heavy-ion collisions.