  Considered as a function of the quark mases, two-flavor QCD depends on three
parameters, including one that is CP violating. As the masses vary to
unphysical values, regions of both first- and second-order phase transitions
are expected. For non-degenerate quarks, non-perturbative effects leave
individual quark mass ratios with a renormalization scheme dependence. This
complicates matching lattice results with perturbative schemes and clarifies
the tautology with attacking the strong CP problem via a vanishing up quark
mass.
