FRG analysis of dense two-color QCD within the linear sigma model

We investigate the phase structure, hadron masses, and topological susceptibility in the two-flavor and two-color QCD (QC₂D) medium, particularly focusing on the U(1)A axial anomaly effects. To this end, we employ the linear sigma model, and hadron fluctuations are incorporated through the functional renormalization group method. We establish in detail an effective potential that respects symmetries of QC₂D at finite quark chemical potential, μq: SU(2)L× SU(2)R chiral, $U(1)$ baryon-number, parity and time-reversal symmetries. We find that the U(1)A anomaly couplings for mesons at finite temperature are enhanced with increasing μq, while that of the baryons are not too sensitive to μq. Despite the anomaly enhancement, we find that the topological susceptibility at larger μq is always suppressed regardless of the temperature, following chiral restoration. We also find that mass degeneracies of the chiral partners are well realized at higher temperatures and densities by the chiral restoration. Our findings are expected to provide useful information on properties of the U(1)A anomaly in medium for sign-problem-free lattice simulations of QC₂D.