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