Nonperturbative Flavor Breaking in Topological Susceptibility at Chiral
  Crossover
  We demonstrate that the QCD topological susceptibility nonperturbatively gets
a significant contribution signaled by flavor-nonuniversal quark condensates at
around the pseudo-critical temperature of the chiral crossover. It implies a
remarkable flavor breaking in the axial anomaly as well as the QCD theta vacuum
in high temperature QCD, which are almost flavor universal in the vacuum. A
nontrivial flavor breaking is triggered by nonperturbative thermal loop
corrections at around the chiral crossover, which is different from the trivial
flavor violation just scaled by the quark mass ratio, observed at
asymptotically high temperatures. This critical flavor violation cannot be
dictated by the chiral perturbation theory with that lattice QCD usually
compares, or the dilute instanton gas approximation based on that its
astrophysical implications have conventionally been made. This would give an
impact on the thermal history and the cosmological evolution of QCD axion
including the estimate of the relic abundance as a dark matter candidate.
