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We investigate the axial U(1) anomaly of two-flavor QCD at
temperatures 190–330 MeV. In order to preserve precise chiral symmetry
on the lattice, we employ the M¨obius domain-wall fermion action as
well as overlap fermion action implemented with a stochastic
reweighting technique. Compared to our previous studies, we reduce the
lattice spacing to 0.07 fm, simulate larger multiple volumes to
estimate finite size effect, and take more than four quark mass
points, including one below physical point to investigate the chiral
limit. We measure the topological susceptibility, axial U(1)
susceptibility, and examine the degeneracy of U(1) partners in meson
and baryon correlators. All the data above the critical temperature
indicate that the axial U(1) violation is consistent with zero within
statistical errors. The quark mass dependence suggests disappearance
of the U(1) anomaly at a rate comparable to that of the SU(2)L ×
SU(2)R symmetry breaking.
This talk is based on a work in JLQCD collaboration, arXiv:2011.01499.